FOOD QUALITY IN MODIFIED ATOMOSPHERE HEAT PUMP ...

FOOD QUALITY IN MODIFIED ATOMOSPHERE HEAT

PUMP DRYING

TIAN MIN

(B. Eng. HUST)

A THESIS SUBMITTED

FOR THE DEGREE OF MASTER OF ENGINEERING

DEPARTMENT OF MECHANICAL ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2005

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Acknowledgement

i

ACKNOWLEDGEMENT

The author wishes to express her heartfelt appreciation to A/Prof. M. N. A.

Hawlader and A/Prof. Conrad O. Perera for sharing their expertise and deep

knowledge in their interesting field of work giving the fullest supervision and

continuous encouragement throughout all stages of this project.

I would like to thank Ms. B.A. Roslina, Mr. S. Anwar, Mr. K. H. Yeo, Mr. Y.L.

Chew of Thermal process lab, Mr. T. T. Tan of Energy Conservation Lab, Ms. Lee

Chooi Lan of the Food Science and Technology Lab, Ms. Zhong Xiang Li of

Materials Lab, Mr. Shang Zhenghua of Chemical Engineering Lab and those who

have helped me one way or another.

The thesis is dedicated to my parents for their encouragement and moral

support during the period of study.

Table of Contents

ii

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ........................................................................................ I

TABLE OF CONTENTS ...........................................................................................II

SUMMARY ............................................................................................................... IV

LIST OF TABLES .................................................................................................... VI

LIST OF FIGURES .................................................................................................VII

NOMENCLATURE.................................................................................................. IX

CHAPTER 1 INTRODUCTION................................................................................1

1.1 BACKGROUND .....................................................................................................11.2 OBJECTIVE ..........................................................................................................31.3 SCOPE .................................................................................................................4

CHAPTER 2 LITERATURE REVIEW ...................................................................5

2.1 WORKING PRINCIPLE OF DIFFERENT DRYING METHODS ....................................52.2 PERFORMANCE OF HPD ......................................................................................82.3 QUALITY OF DRIED PRODUCTS .........................................................................10

2.3.1 Colour ......................................................................................................102.3.2 Shrinkage .................................................................................................132.3.3 Density and porosity ................................................................................152.3.4 Texture .....................................................................................................182.3.5 Nutrients...................................................................................................182.3.6 Ginger flavour..........................................................................................20

CHAPTER 3 MATERIAL AND METHODS ........................................................23

3.1 DRYING APPARATUS AND CONDITIONS ............................................................233.2 SAMPLE PREPARATION .....................................................................................25

3.2.1 Fruits........................................................................................................253.2.2 Ginger ......................................................................................................26

3.3 DRYING KINETICS & ENERGY PERFORMANCE OF HEAT PUMP DRYER .............283.4 QUALITY TESTS FOR FRUITS .............................................................................29

3.4.1 Colour measurements ..............................................................................293.4.2 Shrinkage, density and porosity...............................................................303.4.3 Texture analysis .......................................................................................313.4.4 Rehydration studies..................................................................................31

Table of Contents

iii

3.4.5 Vitamin C test...........................................................................................323.5 GINGEROL ANALYSIS ........................................................................................32

3.5.1 Reagents and materials............................................................................323.5.2 Instruments and Conditions .....................................................................323.5.3 Preparation of calibration curve for 6-gingerol ......................................323.5.4 Extraction of gingerol from dried ginger samples...................................33

CHAPTER 4 DRYING KINETICS AND ENERGY ANYLYSIS........................34

4.1 METHOD OF ANALYSIS .....................................................................................344.1.1 Analyses of drying process.......................................................................344.1.2 Evaluation of energy performance...........................................................37

4.2 DRYING KINETICS .............................................................................................404.3 COP, ENERGY EFFICIENCY & SMER ...............................................................50

CHAPTER 5 FOODS QUALITY ............................................................................55

5.1 COLOUR ............................................................................................................555.2 SHRINKAGE, DENSITY AND POROSITY ..............................................................605.3 REHYDRATION ..................................................................................................675.4 TEXTURE...........................................................................................................725.5 VITAMIN C........................................................................................................755.6 PUNGENT PRINCIPLES ANALYSIS ......................................................................76

CHAPTER 6 CONCLUSIONS ................................................................................80

CHAPTER 7 RECOMMENDATIONS...................................................................83

REFERENCES...........................................................................................................84

APPENDICES..........................................................................................................100

APPENDIX A: DETERMINATION OF VITAMIN C .......................................................100APPENDIX B: DATA OF HEAT PUMP DRYING EXPERIMENT .....................................102APPENDIX C: DATA OF COLOUR .............................................................................104APPENDIX D: DATA OF REHYDRATION ...................................................................106

Summary

iv

SUMMARY

Most fruits consist of water, carbohydrates, proteins and fraction of lipids.

These compounds are heat sensitive and tend to degrade easily under microbial attract.

In order to extend their shelf-life, drying is widely used to preserve them. Among a

good number of drying technologies, the unique advantage of heat pump drying make

it a technology of choice, where temperature and relative humidity can be controlled

independently. In order to make best use of heat pump dryer, inert gas was used to

improve the drying process in this work. The effect of nitrogen and carbon dioxide on

drying kinetics, energy efficiency and dried product quality of heat pump dried foods

were investigated extensively.

Apple, guava, potato, papaya and ginger were selected to conduct drying

experiments because of their particular characteristics. They were cut into 1cm cubes

or 3mm thick slices and dried under mild conditions: the drying temperature was set

at 45°C, circulating air velocity was 0.7m/s and relative humidity was around 20%.

Lemon juice and clean peel were used as natural inhibitors to prevent oxidation.

. Experimental results showed that inert gas did contribute to some

improvement on both dried food quality and drying kinetics. For the dried products,

there are less browning and less colour changes; lower shrinkage and more porous

structures were observed, which resulted in faster rehydration rate; vitamin C

retention of nitrogen dried guava was as high as 1.64 times that obtained in normal air

drying; and ginger flavour (6-gingerol) was even better than that obtained by freeze

Summary

v

drying. For drying kinetics, the effective diffusivity in nitrogen atmosphere drying of

guava was increased by 73% compared with that found for normal air drying.

List of Tables

vi

LIST OF TABLES

Table 1-1: Factors that influence food quality during drying ........................................1

Table 2-1: Empirical models of shrinkage related to moisture content .......................15

Table 2-2: Mathematical models of porosity for fruits and vegetables .......................17

Table 4-1: Page’s equation parameters ........................................................................44

Table 4-2: Diffusivity of Guava and Papaya in MAHPD............................................47

Table 4-3: The energy results of normal air drying processes. .................................51

Table 4-4: The energy results of nitrogen drying process ...........................................52

Table 4-5: The energy results of carbon dioxide drying process.................................53

Table 5-1: Colour values for dried apple, guava and potato samples ..........................56

Table 5-2: Colour values for pre-treated dried apples .................................................58

Table 5-3: Colour values of apples resulting from three drying methods ...................58

Table 5-4: Moisture Removal from samples dried in MAHPD over 18 hours ............61

Table 5-5: The density and porosity of guava and papaya ..........................................62

Table 5-6: Firmness of apples dried by different methods ..........................................73

Table 5-7: Firmness of dried papaya and guava ..........................................................74

Table 5-8: Retention of Vitamin C in dried products ..................................................75

List of Figures

vii

LIST OF FIGURES

Figure 2-1: The schematic diagram of basic heat pump dryer.......................................6

Figure 2-2: Browning mechanism ...............................................................................11

Figure 2-3: Typical variation of density/porosity with water content. ........................16

Figure 2-4: Vitamin C Chemical Structure, C6H8O6 ...................................................19

Figure 2-5: Structure of gingerol homologues.............................................................21

Figure 3-1: Schematic of heat pump dryer ..................................................................24

Figure 3-2: Flow chart of experiments procedure on fruits .........................................27

Figure 3-3: Schematic diagram of peeling the skin of ginger......................................28

Figure 4-1: Effect of inert gas on drying of papaya.....................................................41

Figure 4-2: Effect of inert gas on drying of guava.......................................................41

Figure 4-3: Effect of inert gas drying of ginger ...........................................................42

Figure 4-4: Drying rate of papaya................................................................................43

Figure 4-5: Drying rate of guava .................................................................................43

Figure 4-6: Drying rate of ginger.................................................................................44

Figure 4-7: Variation of ln(MR) with t/L2 of guava (a) N2 (b) CO2 ............................45

Figure 4-8: Variation of ln(MR) with t/L2 of papaya (a) N2 (b) CO2 ..........................47

Figure 4-9: Variation of ln(MR) with t/L2: (a) normal air; (b) N2; (c) CO2 for ginger 49

Figure 4-10: Comparison of COP for HPD and MAHPD ...........................................54

Figure 4-11: Comparison of energy efficiency of HPD and MAHPD ........................54

Figure 5-1: h* values of dried apple, guava, potato samples.......................................57

Figure 5-2: Total colour change: (a) Guava; (b) Papaya .............................................60

Figure 5-3: Structure of 8 hours normal air HPD dried material (SEM) .....................64

List of Figures

viii

Figure 5-4: SEM of dried papaya: ...............................................................................67

Figure 5-5: Rehydration behaviour of samples dried by MAHPD with 2N ................68

Figure 5-6: Rehydration behaviour of samples dried by MAHPD with 2CO .............69

Figure 5-7: Rehydration of nitrogen dried papaya.......................................................69

Figure 5-8: Rehydration capability of dried papaya ....................................................70

Figure 5-9: Rehydration behaviour of HPD and MAHPD dried papaya.....................71

Figure 5-10: Rehydration curve of freeze dried and vacuum dried papaya.................72

Figure 5-11: Chromatograms 6-gingerol: (a) Standard (b) Sample .............................77

Figure 5-12: The calibration graph of 6-gingerol determined by HPLC assay ...........77

Figure 5-13: Comparison of 6-gingerol content in different dried ginger samples .....78

Nomenclature

ix

NOMENCLATURE

a* Redness

A Availability

AA Ascorbic Acid

b* Yellowness

C* Chroma

CIE

COP

PCO

Commission international de l’Eclairage

Coefficient of performance

Maximum COP

Dr Shrinkage ration

h* Hue angle

H Specific enthalpy

HPD

HPLC

Heat pump dryer/ drying

High performance liquid chromograph

k Constant in shrinkage relationship

L* Ligteness

em The mass flow rate in external condenser (kg/s)

im The mass flow rate in internal condenser(kg/s)

om The total mass flow rate(kg/s)

M Moisture evaporation rate(kg/s)

M Moisture content(kg/kg)

eqM Equilibrium moisture content(kg/kg)

MAHPD Modified atmosphere heat pump dryer/drying

Nomenclature

x

MR

PPO

Moisture ratio(kg/kg)

Polyphonenol oxidase

iX

fX

Initial dry base moisture content of sample(kg/kg)

Final dry base moisture content of sample (kg/kg)

∆X fi XX (kg/kg)

RH

S

Relative humidity

Entropy(kJ/kg.K)

SEM Scanning electron microscopy

SMER Specific moisture extraction rate(kg/kWh)

V

0V

Volume of sample(m3)

Original volume of sample(m3)

W Total input power(kW)

cW Input power of compressor(kW)

FW Input power of internal fan(kW)

Constant in shrinkage relationship

Porosity

Second law energy efficiency

Chapter 1 Introduction

1

CHAPTER 1

INTRODUCTION

1.1 Background

Drying refers to a process in which moisture is removed from a solid using heat

as the energy input. In many agricultural countries, large quantities of food products

are dried to improve shelf-life, reduce packaging costs, lower shipping weights,

enhance appearance, retain original flavour and maintain nutritional value

(Sokhansanj and Jayas, 1987). Product loses its moisture content during drying, which

results in increased concentration of nutrients in the remaining mass. Hence, proteins,

fats and carbohydrates are present in larger amounts per unit weight in dried food than

in their fresh counterparts. However, the mechanism of drying is a complex

phenomenon involving combined heat and mass transfer and, in most cases, resulting

in products with modified properties. Depending on the drying conditions, food

products may undergo various degree of browning, shrinkage, loss of nutrients and so

on. According to Chou and Chua (2001), the degradation of food occurs mainly in

three areas, which is shown in Table 1-1.

Table 1-1: Factors that influence food quality during drying [Chou and Chua, 2001]

Chemical Physical NutritionalBrowning reactionLipid Oxidation

Colour lossGelatinization

RehydrationSolubilityTexture

Aroma loss

Vitamin lossProtein loss

Microbial survival


2

Foods like fruits and vegetables are especially high in water, carbohydrate and

vitamins. These compounds are easily altered in the high temperature drying

condition and result in degradation in food quality (Sokhansanj and Jayas, 1987).

Consequently, the products desirability for consumption is affected. Empirically, if

the type of applied drying method and conditions change, the same raw material may

end up as a completely different product. Physicochemical changes that occur during

drying seem to affect the quality properties of the dehydrated product very much. The

increasing need for producing high quality and convenient products at a competitive

cost in industry requires for a suitable employment of drying methods in practice

(Saravacos, 1993). In order to find a trade-off between quality and cost, much

research effort has been put into this area.

Usually, the goals set for a drying process are three-fold:

Product quality: To avoid the undesirable changes and yield the desired

quality.

Economic considerations: To balance the initial investment and running

cost, making it operate at optimum conditions

Environmental concerns: To minimize energy consumption during drying

and reduce the impact of industrial waste on environment.

So far, taking all the three considerations mentioned above into account, it is

generally agreed that heat pump drying (HPD) is one of the most promising

technologies. The ability of heat pump to convert the latent heat of vapour

condensation into the sensible heat of an air stream passing through the condenser

makes them attractive. HPD can be operated over a wide range of temperatures,


3

providing very good conditions for heat sensitive materials to dry, as it enables

independent control of temperature and RH. This technology requires far less energy,

as the system can recover the latent heat in a closed loop, and be conducted

independent of ambient weather conditions (Perera and Rahman, 1997; Hawlader et

al., 1998; Uddin et al., 2004). Strømmen et al. (2002) found that HPD consumes 60-

80% lower energy than other dryers operating at the same temperature. Further more,

it was reported that onion slices dried by HPD confirmed energy saving of the order

of 40% with better product quality (Rossi et al., 1992). For decades, it has been used

in wood kilns to dehumidify air and control lumber quality (Rosen, 1995). A great

number of heat pumps are installed in Finland, Norway and Canada for drying of

wood and fish (Strømmen and Kramer, 1994). The current trend is developing the

applications on heat sensitive food and biological material drying. When substituting

normal air with some inert gas, O’Neill et al. (1998) noted that dried apple cubes

resulted in porous products, leading to quick rehydration. Perera (2001) observed that

MAHPD dried apples showed excellent colour and retention of vitamin C, and the

overall quality of the dried product was very high. Hence, modified air heat pump

drying (MAHPD) seems to offer a great potential in this area.

1.2 Objective

From the previous literature review, there is obviously lack of detailed

information on modified atmosphere heat pump drying. In order to provide a better

understanding and have a clearer insight, the objective of this project is to investigate

the drying kinetics of modified atmosphere heat pump dryer and the quality of its

dried products.


4

This study undertakes to investigate various drying methods of preservation of

perishable fruits and vegetables. Apple, guava and potato are easily oxidized in the air

after being peeled off, and hence, require investigation on browning. Guava and

papaya are rich in vitamin C, which is an index of nutrient loss, were used to detect

the vitamin C retention. Ginger is widely used for cooking and it was chosen to

investigate flavour retention.

The focus was put upon heat pump drying for which the drying media were

normal air, carbon dioxide and nitrogen while other conditions were kept the same.

The differences between final products were compared with those resulting from

freeze drying and vacuum drying.

1.3 Scope

This thesis is divided into seven chapters. Chapter 1 includes an introduction to

the problem. Chapter 2 reviews previous literature on heat pump drying. Chapter 3

describes material and methods. Chapter 4 deals with the drying kinetics and energy

efficiency. Chapter 5 includes results and discussion on food quality. Conclusions

drawn from the study are stated in Chapter 6. Chapter 7 gives recommendations for

improvement of equipment and future research.

Chapter 2 Literature Review

5

CHAPTER 2

LITERATURE REVIEW

Drying is probably the oldest method of food preservation. From ancient time,

people used solar drying techniques to preserve fish, meat, fruit and vegetables

(Brennan, 1994). Dried foods were the main supply of troops and travellers for

centuries. However, heating and loss of water during drying cause stresses in the

internal structure and chemical reaction with oxygen, consequently, food materials

tend to change its colour, lose volume and sometimes increase hardness (Mayor and

Sereno, 2004). These lead to optical, sensory and nutrient characteristics different

from their fresh counterparts and may lose appeal to some consumers.

Generally, the examined properties of dried products are classified into two

major categories: the engineering side and the quality side (Krokida and Maroulis,

2000). In order to get a general idea and find a right direction to move on, a large

number of literatures have been reviewed in this section, which dealing with drying

methods, energy efficiency as well as the quality of dried food, which includes colour,

shrinkage, density, porosity, rehydration, nutrient and flavour.

2.1 Working Principle of Different Drying Methods

There is an increased demand for convenient foods, including ready to eat and

instant foods containing minimum concentration of synthetic chemicals. This creates


6

challenges for the food industry and dryer manufactures with regard to development

of new technologies to process difficult or sensitive materials into final products with

high quality and improved properties. Three drying methods have been in use, such

as heat pump drying, freeze drying and vacuum drying, to maintain quality and retain

nutrients.

Figure 2-1: The schematic diagram of basic heat pump dryer

Figure 2-1 shows a simplified schematic diagram of a heat pump drying system.

The solid arrow line stands for the refrigerant cycle. The dashed arrow line stands for

air path. Basically, heat pump has four components: an evaporator, a condenser, a

compressor and an expansion valve. A refrigerant is compressed to relatively high

pressure and temperature before entering the condenser, where it rejects the heat to

the surrounding medium. It is then throttled by the expansion valve to a low pressure

and absorbs heat at the evaporator for vapourization. When associating heat pump to

drying process, air, the drying media, is heated at the condenser. Then, the hot air

flows into the drying chamber, where its humidity content increases and its

temperature decreases, because water is evaporated from the drying materials. Humid

air flowing out of the drying chamber abandons moisture at the evaporator due to

ExpansionValve

Air

Air

Condenser

Evaporator

Compressor

Drying

Chamber


7

condensation and cools down. As heat supply and moisture removal take place at

different location, the drying temperature and relative humidity can be controlled

independently in a heat pump dryer. Currently, the maximum drying temperature in a

heat pump assisted drying is limited to 120°C ([online] available at:

http://tfe81.wtb.tue.nl/education/4P570/HP.pdf). This is due to the lack of proven high

temperature working fluids and the non-availability of suitable compressors.

Freeze drying removed water from a frozen sample by sublimation under

reduced pressure. To facilitate sublimation, the drying temperature is reduced to -

10°C and pressure below the triple poin, 4.58 mm/Hg. According to Pikal (1990), it

can be divided into three stages: freezing, primary drying (in which sublimation

occurs) and secondary drying (where unfrozen water is removed). The processing

time is usually long, typically 3 to 5 days.

Certain biological materials, such as pharmaceuticals and foodstuffs, which are

heat-sensitive, may be freeze dried. There is increasingly the trend in the

biotechnology and pharmaceutical industries for preparation and storage of many

therapeutic proteins and labile enzymes (Liapis, 1995; Carpenter et al., 1997). As a

rule, freeze dried products have the best quality among all dry products. The porous,

non-shrunken structure results in a quick rehydration, which is good for some food

products. However, the cost of freeze drying of food has been found to beat least one

order-of-magnitude higher than conventional drying system such as a spay dryer

(Chou and Chua, 2001).


8

Vacuum drying is performed at a low pressure which is an advantage because

the boiling point of water is lower under reduced pressure. This enables the products

to be effectively dried at lower temperatures. During the process, the internal pressure

of the food was greater than the ambient pressure in the drying chamber and, hence,

managed to prevent shrinkage and maintained the puffy cubic shape. However, the

time and energy cost is intensive.

2.2 Performance of Heat Pump Drying

Ever since the dawn of civilization, energy consumption has been increasing.

Every indication shows that the energy consumption will continue its upward trend as

the world population increases. Drying is an industrial process requiring large

amounts energy, especially when drying of temperature-sensitive materials, because

of the low-temperature thermal energy use. A very low exergetics efficiency resultes,

when high-grade thermal energy source (fuel or electricity) is used to accomplish this

task.

Exergy is defined as the theoretical maximum amount of work that can be

obtained from the system at a prescribed state (P, T, h, s, u, v) when operating with a

reservoir at the constant pressure and temperature P0 and T0. Heat pumps are devices

that make the best use of the exergy of high-grade energy sources and offer the

possibility of providing a heating output which is several times the amount of high-

grade energy input. These advantages increase as the target temperature for heating

approaches the ambient temperature. Therefore, it is natural to consider heat pump


9

dryers to be high efficiency devices compared with others, especially for low-

temperature drying.

The coefficient of performance (COP) is the most commonly used parameter to

evaluate the efficiency of a heat pump, which is defined as follow:

inputPoweroutputheatUsefulCOP (2-1)

The maximum theoretical heat pump efficiency is the efficiency when running on a

Carnot cycle:

evaporatorcondenser

condenser

TTT

PCO

(2-2)

For a heat pump, COP is large than 1, which means favourable performance.

When analyzing drying, a more appropriate efficiency parameter is the specific

moisture extraction rate (SMER), which means the mass of water evaporated per unit

of energy input:

)/(, kWhkginputEnergy

evaporatedwaterofAmountSMER (2-3)

The SMER varies as the maximum air temperature in the dryer, the relative

humidity of air, the evaporation and condensation temperatures, and the efficiency of

a refrigeration system. A typical SMER value achieved by a heat pump is 2.5 kg/kWh

in a range of 1 to 4 kg/kWh, much higher than conventional drying for which values

ranging from 0.5 to 1 kg/kWh (Perera and Rahman,1997). However, it is should be

noted that the energy a compressor consumed is electricity. It is higher grade energy


10

than heat. Therefore, when comparing the economics, the relative costs of thermal

energy and electricity should be considered.

2.3 Quality of Dried Products

2.3.1 Colour

Some foods are easily-oxidized in the air, their colour changes a lot during

dehydration. It is not only due to evaporation of the surface water but also due to

certain reactions, such as enzymatic browning, non-enzymatic browning and

caramelization reactions (Kudra and Strumillo, 1998). If it is necessary to retain light

colour of products, enzymic browning is a problem in drying industry for a great

number of commodities, for example, fruits like apples, bananas and grapes,

vegetables like potatoes, mushrooms and lettuce (Shewfelt, 1986; Hall, 1989). These

reactions usually impair the sensory properties of products due to associated changes

in colour, flavour and texture, besides nutritional properties, which are undesirable

(Martinez and Whitaker, 1995). Hence, the regulation of colour is important for

improving quality of dried products.

Browning of foods results from both enzaymatic and non-enzymatic oxidation of

phenolic compounds as well as from Maillard reaction that occurs when mixtures of

amino acids and reducing sugars are heated (McEvily et al., 1992). However,

sometimes it is difficult to ascertain whether the mechanism has been enzymatic or

nonenzymatic unless the enzymes in the food that are responsible for the enzymatic

browning is inactivated. Some researchers thought that the colour developed non-


11

enzymatically from intermediates formed through enzyme-mediated oxidations,

which took place before the enzymes were inactivated (Wedzicha, 1984).

Enzymatic browning requires four different components: oxygen, enzyme,

copper and a substrate (Langdon, 1987). The polyphonenol oxidase (PPO) group of

enzymes catalyzes the oxidation of phenolic compounds in the plants to o-quinones.

Immediately, the quinones condense and react nonenzymatically with other phenolic

compounds, amino acids, etc., to produce dark brown, black or red pigments of

indeterminate structures (Sapers and Hicks, 1989), which are better illustrated in

Figure 2-2. PPOs are found in almost all higher plants, including papaya, potato and

apple as well as seed such as cocoa (Martinez and Whitaker, 1995).

Figure 2-2: Browning mechanism

Maillard reaction limits the shelf life of various dehydrated fruits and vegetables,

citrus products, and juice (Handwerk and Coleman, 1988). This non-enzymatic

browning result from (Wedzicha, 1984; Namiki, 1988):

The reaction of carbonyl groups and amino compound.

Caramelization or pyrolysis of food carbohydrates

Ascorbic acid browning

Browning

QuinonePhenolics

OH O

PPOReaction Nonenzymic

Reaction

OH OO2


12

Lipid browning

Some researchers found that low pH values helped to decrease the activity of

PPO due to less tight binding of Cu to activate the enzyme allowing the acid

molecules to remove the Cu and, hence, reduce browning (Martinez and Whitaker,

1995). Consequently, exclusion of oxygen and/or application of low pH environment

can ease browning. So far, sulfating agents are wildly used in drying industry, which

are good colour preservative of fresh fruits and vegetables, but they are considered

harmful for certain asthmatics and are, therefore, unacceptable to such consumers.

Many studies have shown Ascorbic Acid (AA) is able to reduce browning (Son et al.,

2001; Özoğlu and Bayındırlı, 2002; Choi et al., 2002). Lemon is such a kind of fruit

with plenty of ascorbic acid. According to Leong and Shui (2002), lemons were found

to have 49.6mg AA per 100g of fresh juice. They also found relatively high levels of

AA in lemon peels (129mg/100g). But limited reports on using it as natural inhibitors

were available.

Colour is the sensation experienced by an individual when energy in the form of

radiation within the visible spectrum falls upon the retina of the eye (Krokida and

Maroulis, 2000). It may be affected by several factors: the spectral energy distribution

of the light, the conditions under which the colour is being viewed, the spectral

characteristics of the object, the sensitivity of the eye. Thus, in order to measure the

colour of material objectively, CIE (Commission International de I’Eclairage)

(Wyszecki and Stiles, 2000) system is widely adopted.


13

Many research works have been done on a number of products in the colour

development. The effect of various drying methods and conditions on colour

degradation has been measured. For example, Krokida and Maroulis (2000)

investigated the drying process of banana using five methods (conventional, vacuum,

microwave, osmotic and freeze drying) and various drying conditions extensively. By

changing drying methods, they found the lighteness parameter (L*) decreased

significantly during air, vacuum, osmotic and microwave drying, while increased

slightly in freeze drying; the redness (a*) value increased significantly during air

drying, followed by microwave and vacuum drying, then freeze drying, while keeping

constant for osmotic drying; the yellowness parameter (b*) showed a similar

behaviour to redness parameter (a*). When changing the conditions of conventional

and vacuum drying, L* is not affected by temperature and air relative humidity while

a* increases as drying temperature increases and relative humidity decreases, b*

increases as drying temperature decreases and relative humidity increase. Usually, the

increase of chroma parameters (a* and b*) means the samples experienced an

extensive browning. Freeze drying removes water by sublimation of ice and prevent

enzymatic browning reaction, resulting in relative stability of colour parameter (L*,

a*, b*). Hence, a conclusion that freeze drying yields the best colour preservative

method but conventional drying is the worst can be drawn.

2.3.2 Shrinkage

Solid and semi-solid food systems are highly heterogeneous materials. When

water is evaporated, segregation of components occurs, resulting in a network of cell

walls. A contracting stress may be developed in this process, leading to damage or


14

disruption of the cellular walls, even collapse of the cellular tissue, which are

associated with the reduction of the external volume (Mattea et al., 1989). This

phenomenon is usually referred as shrinkage.

In case of food materials, two types of shrinkage are observed: isotropic

shrinkage and anisotropic shrinkage (Rahman, 1995). Most fruits and vegetables

undergo isotropic shrinkage. That means they shrink uniformly in all dimensions.

When shrinkage is not uniform, an unbalance stress is formed and the material cracks.

Several authors reported cracking of food materials: soybean (Mensah et al., 1984),

pasta (Akiyama and Hayakawa, 2000), corn (Fortes and Okos, 1980).

Shrinkage affects mass and heat transfer parameters, such as diffusivity and

permeability. White and Bell (1999) reported structural collapse decreased the

glucose loss rate constant in the food system composed by glucose and glycine.

Consequently, it decreases the rehydration capability of the dried products. Mcminn

and Magee (1997b) found lower rehydration capacity of air dried potatoes

corresponded to more shrunk samples. Shrinkage is also a relevant factor to be

accounted for establishing drying models. Lozano et al. (1980) explained shrinkage

on the basis of the ratio between the bulk volume of the product and the initial bulk

volume (bulk shrinkage coefficient). Reeve (1943) and Craft (1944) started pioneer

studies of shrinkage at microscopic level on carrots, potatoes and several fruits. Wang

and Brennan (1995) observed structural changes in potato by light microscopy. A

linear relationship between percentage change of volume and moisture content was

found. Ramos et al. (2002) studied grape microscopic shrinkage, quantifying several

parameters directly related to cellular dimensions. General empirical shrinkage model


15

have been suggested for fruits and vegetables during drying (Suzuki et al., 1976;

Lozano et al., 1980; Madamba et al., 1994; Ratti, 1994; Zogzas et al., 1994; Rovedo et

al., 1997; Lou, 1997; Xiang, 2001) as a function of water content of products,

including linear models and non-linear models. Table 2-1 gives some examples of

these models.

The degree of shrinkage can be controlled by applied drying method and drying

conditions, subsequently, influence density and porosity of dried products (Krokida

and Maroulis, 1997). Choosing a proper drying method is very important for industry

to yield desired products. Using microscopy to observe macroscopic shrinkage and

relating microstructure to texture and physical properties is an interesting field of

research (Ramos et al., 2003).

Table 2-1: Empirical models of shrinkage related to moisture content

Model Food products

21 kXkDr Apple (Lozana et al., 1980)

XDr 1 Apple, carrot, potato (Zogzas et al., 1994)3

42

321 XkXkXkkDr Apple, carrot, potato (Ratti,1994)

)exp( 321 tkkkDr Potato and squash (Rovedo et al., 1997)q

r XX

D

0

Guava, mango, and honeydew (Xiang,2001)

2.3.3 Density and porosity

The density and porosity are important physical properties characterizing the

quality of dry and intermediate moisture foods (Schubert, 1987; Zogzas et al., 1994).

However, there are a number of density definitions of practical interest which need to


16

be considered (Rahman, 1995; Rahman et al., 1996). In most case, the following

definitions are used:

Particle density: is defined as the current particle mass divided by the particle

volume, disregarding the volume of all pores (Lewis, 1987). Typically, the particle

density increases as the material loss water, as shown in Figure 2-3. Several

researchers observed this phenomenon in diversified fruits and vegetables: apple,

banana, grape, pear, carrot, potato and garlic (Lozano et al., 1980, 1983; Vagenas et

al., 1990; Zogzas et al., 1994; Krokida and Maroulis, 1997). Peculiarly, apples and

carrots have an inverted tendency for lower values of water content (Lozano et al.,

1980, 1983). Krokida and Maroulis (1997) reported that particle density was not

affected by drying method except osmotic dehydration.

Figure 2-3: Typical variation of density/porosity with water content.

Bulk density (or apparent density): is defined as the particle mass divided by

the particle volume, including the volume of all pores. It also increases as water

content decreases, as shown in Figure 2-3, but was strongly affected by dehydration

process. The bulk density of freeze drying samples is the lowest, while for

conventional air drying is the highest (Krokida and Maroulis, 1997). They also report

Den

sity

/Por

osity

Water content


17

that it decreased significantly as the pressure was decreased in both vacuum drying

and freeze drying, and developed a model for vacuum drying.

Porosity: is defined as the ratio between volume of pores and the total volume

of product (Lewis, 1987). During drying, the product porosity increase as the water

and volatiles are removed (Figure 2-3). Krokida et al. (1997) stated that porosity

depends on initial water content, composition and volume, and compared with freeze,

microwave and vacuum drying, air-dried products have low porosity. Materials

influence porosity as well. Carrots and potatoes developed almost negligible porosity

while apples increased a lot in air drying (Zogzas et al., 1994). These authors correlate

porosity with water content and density at the same time to derive mathematical

models, which are presented in Table 2-2.

Table 2-2: Mathematical models of porosity for fruits and vegetables

Model Food Products

XbbXaaXaaa

2154

321

exp)exp()exp(

1

Apple, carrot and potato cubes

X<1.5g/g (Zogzas et al., 1994)

XX ws

b

1

)/()/1(1 0 Avocado, prune and strawberry slices

(Tsami and Katsioti, 2000): the volume-shrinkage coefficient

Banana, apple, carrot and potato cylinders(Krokida and Maroulis, 1997)

)exp()exp(

154

321

XaaXaaa

Apple, pear, carrot, potato, sweet potato and

garlic cylinders (Lozano et al., 1980)1.5<X<7.45g/g


18

2.3.4 Texture

Texture is another important parameters connected to product quality, which is

related to the structure of foods (Ramana and Taylor, 1994). Textural characteristics

also depend on chemical and biophysical characteristics of the products (Moshenin,

1986; Bourne, 1992; Thiagu et al., 1993). The properties are usually related to

mechanical tests. By applying static or dynamic loading, the viscoelastic behaviour of

a material is investigated. The viscoelasticity is strongly related to complex quality

characteristics perceived by people as mouth feeling. The rheological behaviour of

dehydrated products has been studied through both compression and relaxation

stresses (Bagley, 1987; Mohan Rao, 1984; Katz and Labuza, 1981).

A compression test is one of the most common techniques for estimation of

the texture. The simplest approach is to measure the maximum applied force or stress

at fracture of the material. The quantification of difficult terms such as hardness and

chewness, has been made by a methodology called Texture Profile Analysis.

2.3.5 Nutrients

Fruits serve as indispensable sources of vitamins and minerals to human diet

and also provide some protein, fat and energy. The value of fruits in human diet is

increasingly being realized in recent years. Virtually, all human’s dietary vitamin C is

obtained from fruits and vegetables (Jayaraman and Das Gupta, 1999). They are also

important suppliers of calcium, phosphorus and iron.

Most fruits contain more that 80% water and are, therefore, easily perishable.

In order to extend their shelf period, several process technologies have been employed


19

to preserve them. Drying is probably the oldest and most widely used method. As is

true in other food preservation methods, drying destroys certain nutrients.

Carbohydrates and fibre contents of food are not affected by drying. Vitamins A and

C could be degraded by exposure to air, and vitamin C is also degraded by heat.

Mineral losses may occur with soaking, but these losses are usually minimal,

compared with their fresh counterpart. However, loss of moisture content results an

increase in the concentration of nutrients in the remaining mass. Hence, it is quite

understandable that to determine the efficacy of different dehydration techniques and

compare the effect such techniques have on these nutrients.

Among all the nutrients in fruits, vitamin C (L-threo-2-hexenono-l, 4-lactone) is

one of the most vulnerable to chemical degradation. It can easily be oxidized to

dehydroascorbic. Fennema (1985) suggested that if vitamin C, being the most

unstable among the nutrients, is retained well, the other nutrients are generally

assumed to be well retained. Thus, vitamin C may be used as a quality index for the

degradation of vitamin C rich product (Uddin et al, 2002).

Figure 2-4: Vitamin C Chemical Structure, C6H8O6

CH2OH

HO

HOO

OH

O


20

Vitamin C is a water soluble vitamin. Also known as Ascorbic acid (3-oxo-L-

gulofuranolactone), Anti-scorbutic Acid, Hexuronic Acid, Cevitannic Acid, L-

xyloascorbic Acid, Ascorbyl Palmitate, Ascorbyl Nicotinate. It was isolated from

fruits, paprika and adrenal glands in 1922 by Albert Szent-Gyorgi (Availabe at

http://www.pdrhealth.com/drug_info/nmdrugprofiles/nutsupdrugs/vit_0264.shtml). It

can be prepared by synthesis from glucose, or extracted from plant sources such as

rose hips, blackcurrants or citrus fruits. It is easily oxidized in air. Man is one of the

few mammals unable to manufacture AA in his liver. The chemical structure is shown

in Figure 2-4.

Vitamin C has many functions in human body. It accumulates iron in bone

marrow, spleen and liver; control blood cholesterol levels; fights bacterial and viral

infection; helps healing and so on.

2.3.6 Ginger flavour

Ginger is the rhizome of Zingiber officinale Roscoe (Zingiberaceae). Although

ginger is technically a rhizome rather than a root (which means it is a tuber that grows

horizontally under the ground), its name comes from the Sanskrit word for ‘horned

root’. Ancient Greek, Roman, Indian, Persian and Chinese cultures had a fondness for

its culinary and medicinal properties. They have used it to add flavour, tenderize meat,

stimulate the appetite and to calm the stomach. In Europe, ginger has been studied for

its antibacterial, antifungal, pain relieving, anti-ulcer, anti-tumour and other properties.

Recently, clinical trials showed that ginger rhizome could relieve appetite loss,


21

motion sickness and prevent vomiting, even tend to boost the pumping action of heart

(Stewart et al. 1991)

Pungency is an important quality characteristic of ginger. The rhizome has

been valued for this since antiquity (Denniff and Whiting, 1976). Purseglove (1972)

noted that the main pungent principles of ginger rhizome are gingerol homologues

and their dehydrated products –shogaol homologues, which may result from thermal

process or long term storage. The structure of gingerol is shown in Figure 2-5.

Balladin et al. (1996; 1998) found the quantities of the main pungent principles

(gingerol and shogaol) extracted from fresh, non-steam-distilled solar-dried and

steam-distilled solar dried ginger rhizomes increased but the oleoresin quality

decreased. The reason they explained it was that some gingerols dehydrated and

produced shogaols.

Figure 2-5: Structure of gingerol homologuesNote: when n=4 is 6-gingerol.

From the literature review, it is noted that a great number of works have been

done on drying, both energy engineering side and quality side. However, limited

research on modified air drying is found. In order to provide a better understanding

and have a clearer insight, both the drying kinetics and quality of several foods dried

32 CHCH n

HOO

HO

OCH 3


22

in different gases were studied extensively in this project. The results of quality tests

were also compared with those of freeze drying and vacuum drying samples.

Chapter 3 Material and Methods

23

CHAPTER 3

MATERIAL AND METHODS

This chapter provides details about the test set-ups, experimental procedures

and materials used in the drying experiments. Except drying processes, the other

procedure for apple, guava and papaya were far different from that of ginger. Hence,

they were described in separated sections.

3.1 Drying Apparatus and Conditions

A heat pump dryer, shown in Figure 3-1, was used in the experiment, which

operates in a closed loop. There were two condensers. One was external, the other

was internal. According to the desired temperature, different amount of refrigerant

was piped into the internal one. Status of refrigerant is monitored at several key points

by thermocouples and pressure gauges. The blower circulated the air/inert gas flow,

enabling it condensed water vapour at internal evaporator and heated at the condenser.

A two-story rotating tray was placed inside the drying chamber, which makes the

every sample get uniform heat. A vacuum pump and a gas cylinder were connected to

the chamber, which are needed when modifying the atmosphere.

In this project, about 100 grams of samples were fed in each batch. When

using inert gas, the chamber was evacuated first before nitrogen or carbon dioxide

was pumped in. This procedure was repeated three times at the beginning of each

experiment, making sure that the normal air was substituted as completely as possible.


24

Figure 3-1: Schematic of heat pump dryerNote: air/ inert gas path; refrigerant path

The drying temperature inside the chamber was measured by T type

thermocouple with the accuracy of ±0.1°C, which was inserted in the middle position

of the inlet cross section. The flow velocity was measured in advance and set at a

particular value. A humidity sensor was placed inside to measure the relative

humidity. The weight of the samples was continuously monitored by a load cell. All

Sight Glass

Evap

orat

or

Inte

rnal

Con

dens

er

RH1, T1

Drying ChamberN2/C

O2

Cyl

inde

rBlower

Vacuum Pump

FilterDryer

ExternalCondenser

Liquid Receiver

CompressorExpansion Valve

Check Valve

Sight Glass

P6 T6

P10

T10

P12

T12

T7

P7


25

the data were collected by HP 34970A data logger and downloaded to a personal

computer. In the stable stage, a relative humidity (RH) of around 10%, air velocity of

0.7m/s and a temperature slightly fluctuating by about 0.5°C at 45°C was maintained

in the drying chamber. The drying time was varied according to different goals set for

this study.

For freeze drying, the basic principle is the removal of water from a specimen

frozen during the process, shrinkage is eliminated or minimized, and near perfect

preservation will result . Samples were first placed in a freezer at a temperature of -

50°C for 24 hours. All the moisture inside were frozen to ice during this period. Virtis

BT4K XL freeze dryer was used. A full vacuum was applied and pressure in the

drying chamber was maintained below 4.6mm Hg. Under this condition, ice will

begin to sublimate into water vapour without going through a liquid state.

Vacuum drying is a very efficient way. Unlike drying at atmospheric pressure,

it lowers the vapour pressure of the water and provides a greater temperature

difference between the heating medium and product. This results in fast drying and

more efficient heat recovery. The drying process for samples was applied at a pressure

of 0.15 bar and at a temperature of 45ºC for 24 hours. A vacuum dryer, model She

Lab 1430D, was used in this experiment,

3.2 Sample Preparation

3.2.1 Fruits

Fuji Apples from China, large white Guavas from Thailand, solo papaya from

Malaysia and Russet Burbank Potatoes from USA were purchased from a local


26

supermarket. The materials were cut into 1±0.1cm cubes (only those without skin

were used in experiments, others were discarded), then immediately immersed to

distilled water at room temperature to prevent contact with oxygen, and wash away

phenolic compounds and free enzymes from the cut surfaces, avoiding rapid oxidation.

The preparation was done as soon as possible, which was no longer than 10 minutes.

Lemon juice used as a natural inhibitor of browning was only applied on

normal air drying of apples. When squeezing juice, the seeds were removed, because

some seeds themselves have good antioxidant properties (Guo, Yang, Wei, Li, Xu and

Jiang, 2003). The squeezed lemon juice was diluted with distilled water to

concentrations of 10%, 30%, and 50%. A second soaking-agent was prepared by

blending 10g of lemon peel with 150 ml of distilled water, then filtering. The filtrate

was used as the soaking agent. Approximate 100 grams of sample used for air-drying

were immersed in 150ml of the soaking solution for 30 minutes before air drying. No

pre-treatment with inhibitors was used when drying with inert gases ( 2CO , 2N ). The

whole procedure is summarized in Figure 3-2.

3.2.2 Ginger

Fresh ginger was purchased from Shop N Save supermarket, Singapore.

Because mechanical methods cannot be controlled properly to maintain a uniform

peeling, a chemical method was adopted. About 200 grams fresh ginger was

immersed in 1 litre of 1.5% sodium hydroxide solution, in a 2 litre beaker and

maintained at 90oC by a hot plate, for 5 minutes. A magnetic stirrer keeps the solution

homogeneous. The schematic of the whole device could be seen clearly in Figure 3-3.


27

Figure 3-2: Flow chart of experiments procedure on fruits

Apple/ Guava/Potato/Papaya

Cut into 1cm cubes

Put samples into water, washing awayphenolic compounds and free enzymes

(less than 10 mins)

HPD

Vacuum dryer

Freeze dryer

2CO 2N

Only apple soakedwith lemon juiceor peel

4 hours / 8 hours / 18 hours drying

Normal Air

Frozen at-50°C for

24 hrs

24 hours’drying

24 hrs’dryingReduce to bone dry mass in desiccator

Quality Test

Colour Porosity Texture Rehydration


28

After the chemical treatment of removing skin, the ginger pieces were rinsed

five times with distilled water. The loose skin was gently washed away. In order to

make sure that traces of alkali were neutralized, the rinsed ginger pieces were dipped

in 0.5% citric acid (w/v) solution for two minutes, then rinsed again in distilled water

five times. The peeled ginger pieces were immediately sliced longitudinally into 3mm

flakes.

Figure 3-3: Schematic diagram of peeling the skin of ginger

3.3 Drying Kinetics and Energy Performance of Heat Pump Dryer

As the weight of all kind of samples was continuously monitored and stored in a

computer during drying by HPD and MAHPD, drying curves can be obtained from

these data. Dried samples were stored in vacuum desiccators with silica gels at the

bottom to reach bone dry mass. It took about 10 days. During this period, the weight

was monitored regularly until the difference between recent 2 recordings was not

larger than 0.1 gram. With bone dry mass, moisture content (dry base) can be

calculated. By plotting this against time or other item, important parameter such as

drying constant, effective diffusivity can be found.

Heater

Stirrer

Ginger

Beaker

NaOH (15%)


29

Only papaya cubes were used for energy test. Power input of compressor and

internal fan were measured manually by digital multi-meter (HIOKI). The feed load

of 100 gram, 150gram and 200gram were tested. Pressure values were recorded when

the system reached stable stage.

3.4 Quality Tests for Fruits

3.4.1 Colour measurements

CIE colour values were measured using a Minolta CM-3500d

Spectrophotometer (Tokyo, Japan). For each cube of sample, the lighteness (L*),

redness (a*), and yellowness (b*) were captured. Five randomly selected samples of

each batch were analyzed and the mean and standard deviation (SD) were recorded.

Chroma or strength of colour (C*) and hue angle (h*) were also calculated, and these

values are related to a* and b* in the following form:

C* = 22 (b*)(a*) (3-1)

h* = tan-1(a*/b*) (3-2)

High values of hue angle with lower values of chroma indicate less browning. The

mean values and standard deviations were also calculated. Total colour change DE*

as defined by the following equation, was calculated and compared with their fresh

counterpart:

222* *** baLDE (3-3)


30

3.4.2 Shrinkage, density and porosity

Shrinkage was related to moisture content in this experiment. Moisture content

based on bone dry mass is defined as:

massdryBonemassdryBonemassTotal

X

(3-4)

Bone dry mass was obtained by putting it in vacuum desiccator. Initial moisture

content ( iX ) and final moisture content ( fX ) were calculated. Their moisture

changes were compared.

A Scanning Electron Microscope (SEM)，model Joel JSM 5800-LV, was

used to investigate the surface porosity of the dried food samples. The SEM captures

detailed 3-dimensional images at magnifications ranging from 15 to 200,000 with a

resolution of 5 nanometers. In this study, low vacuum condition was used and the

magnification was restricted to 70 or 100. Five samples were examined for each

drying method, but only clear pictures were selected to show the surface porosity.

Mercury porosimeter (Micromeritics, AutoPore) can detect a capillary

diameter range from 0.003 m to 360 m . Its working principle based on Washburn’s

equation, assumes the pore is cylindrical and the opening is circular in cross-section

(Webb, 2001). As mercury is non-wetting to most solids, the pore diameter could be

determined once the pressure applied is known. Both low pressure and high pressure

analysis were used for pore information.


31

3.4.3 Texture analysis

A stable Micro System texture analyzer (TA.XT2i, Stable Micro System,

Godalming, Surry, UK) was used to test the hardness of the samples. Due to the size

of samples, a 2mm diameter stainless steel cylinderial probe was chosen. Only

compression tests were conducted in this study. Load size is 2kg. According to the

recommendation in the software, pre-test speed is set at 4m/s, test speed is 5m/s and

after-test speed is 5m/s. Perforating distance is 2mm. The resistance that the sample

exerted was recorded. The three surfaces in coordinate axes of each sample cube were

tested and the average for one cube was recorded. Ten samples from each batch were

tested. The mean value and standard deviation of 5 readings were reported.

3.4.4 Rehydration studies

Water bath which can maintain the water temperature at a certain value was

used in rehydration experiments. Sixty and eighty degree C water were used in this

study. Each bone dried sample was weighed first, placed in a tea drainer and then

immersed in the water bath for various lengths of time. At certain intervals, the

samples were removed from water; excess water from their surfaces was gently wiped

off using tissues and weighed. The accuracy of balance is 0.0001 gram. Each

experimental run was performed triple times. Their mean values were recorded as the

weight of absorbed water for a given rehydration period.


32

3.4.5 Vitamin C test

Vitamin C test adopted indophenol method (using Tillman’s reagent-titration

or photometric): titrate indophenol solution (16.2mg/L), monitoring colour changes

due to 2, 6-dichloroindophenol reduction, which is recommended by AOAC (1984).

The detail information could be found in Appendix A. The concentration in fresh

fruits was set to 100 percent compared with that in their dried counterparts.

3.5 Gingerol Analysis

3.5.1 Reagents and materials

6-gingerol, lot No.PKH3775, was purchased from Wako Pure Chemical

Industries (Osaka, Japan). The purity was 99.6%. Methanol, HPLC grade, was used.

All other reagents were of analytical grade and were used without further purification.

3.5.2 Instruments and Conditions

Hewlett-Packard series 1100 HPLC system was used. The column was

Hewlett-Packard 799160D-574 Hypersil 0DS 5 UM 200×4.6mm. For the mobile

phase system: methanol and water were used in this study (60:40, v/v).

3.5.3 Preparation of calibration curve for 6-gingerol

6-gingerol aqueous solutions were prepared in concentrations of 82.8, 62.1,

41.4 and 20.7 mLg / for the calibration curve. The mobile phase system used in this


33

study was composed of methanol: water (60:40, v/v). The flow rate was 1.0mL/min.

The eluted substances were detected at 281nm using a UV detector.

3.5.4 Extraction of gingerol from dried ginger samples

All the samples were reduced to bone dry mass before use, because the

insufficiently dried ginger would result in extracting undesirable water-soluble

components such as starches and sugars (Balladin et al., 1996). According to the

analytical procedure introduced by Yong et al. (2002), a sample of 0.1 gram each of

bone dried materials resulting from freeze drying, vacuum drying, normal air HPD,

nitrogen HPD and carbon dioxide HPD were grounded in a mortar then adjusted to

50mL volume with pH 4.0 acetate buffer solution, because it was reported that the

gingerol solutions had better physical and chemical stability between pH 4.0~5.0

(Young et al., 2002). This solution was shaken first to make sure that all the soluble

components were dissolved, then filtered by HPLC filter unit (FP Point 45, Schleicher

and Schuell) and 10 L was injected into the column.

Chapter 4 Drying kinetics and energy analysis

34

CHAPTER 4

DRYING KINETICS AND ENERGY ANYLYSIS

Methods of analyzing drying kinetics and energy performance of heat pump

dryer were introduced first. Raw data were fitted into mathematical models to obtain

drying constant and effective diffusivity. Parameters of refrigerant were used to

calculate the COP, energy efficiency and SMER of heat pump dryer.

4.1 Method of Analysis

4.1.1 Analyses of drying process

Drying commonly refers to the process of thermally removing volatile

substances (moisture) to yield a solid product. It includes a combination of heat and

mass transfer: energy transfer from the surrounding environment to evaporate the

surface moisture (Process 1) and internal moisture transfer to the surface then

subsequently evaporated (Process 2). Hence, in Process 1, it depends much on the

external conditions, such as temperature, air humidity and flow, area of exposed

surface and pressure. While in Process 2, it is governed by the movement of internal

moisture, which is a function of the physical nature the material.

In existing literature, a generalized drying rate curve usually includes a

constant drying rate followed by falling rate regions (Mujumdar and Menson 1995;

van Arsdel and Copley 1963). However, not all materials have this pattern. When the

surface contains free moisture, the drying rate is constant. As the free moisture


35

depletes, some shrinkage might occur and dry spots appear on the surface. The first

falling rate period begin. After the surface is totally dried, it is the second falling rate

stage. Moisture concentration gradients between the deeper parts and the surface are

the main controlling force.

Crank (1975) thought if only radial diffusion is considered and neglect the

effects of temperature and total pressure gradients, Fick’s second law of the unsteady

state diffusion could describe the transport of water during food dehydration process.

In this chapter, a simple diffusion model based on it is considered for the transport

mechanisms of the falling rate region and can be given by the following equation:

2

2

xX

DtX

(4-1)

where X= dry base free moisture content (g water/g dry mass); t=times(s); x=length

(m); D=diffusion coefficient for moisture in solid (m2/s).

Considering sample cube as a flat plate of thickness L drying on both sides

and under the given boundary conditions of

1. Uniform initial amount moisture content at the beginning:

0)(;0;0 XtXLxt (4-2)

2. Equilibrium content at the surface:


36

eqXtXLxt )(;,0;0 (4-3)

The following solution exists in the absence of any external resistances (Rao and

Rizvi, 1986):

2

22

122

0

)12(exp)12(

18)(

L

tDn

nXX

XtXMR eff

neq

eq

(4-4)

where MR is moisture ratio of free water still to be removed at time t to the total free

water initially available; eqX and X0 represent the equilibrium and initial moisture

content respectively; effD is the effective diffusivity of water (m2/s), n is the number

of terms in the series, L is the sample thickness. In Eq. (4-4), it is assumed that the

sample temperature is constant during drying.

This solution is applied to thin-layer drying of different foods by a great

number of researchers (Jayas et al., 1991; Vaccarezza and Chirife, 1978). Equilibrium

moisture content of a sample being dried is decided by the structure and type of

material, and the moisture content of the air. For a long time drying, where t is large

and L is small, the higher order terms are neglected by setting n=1. Hence, the Eq. (4-

4) could be further simplified to a linear equation:

)()8ln()(

ln)ln( 2

2

20 L

tDXX

XtXMR

eq

eq

(4-5)


37

Shrinkage of the material is also neglected in this model, so L is a constant

during the whole process. As a consequence, the effective diffusivity, which

incorporates factors that were not considered separately but would affect the drying

characteristics, can be determined by plotting experimental drying data in light of

ln(MR) versus time. In this chapter, the drying kinetics of guava, papaya and ginger is

investigated under controlled laboratory conditions. This simple equation (4-5) was

applied to fit the drying data by non-linear regression to evaluate the influence of inert

gas on the diffusivity quantitatively.

Another most useful empirical models is Page’s equation (Page, 1949), which

is an empirical modification of simple exponential model. It is written in the form:

)exp()(

0

B

eq

eq KtXX

XtXMR

(4-5)

where K is the drying constant, B is the Page’s parameter and t is the process time (s).

It may be linearized as:

)ln(ln)]ln(ln[ tBKMR (4-6)

4.1.2 Evaluation of energy performance

All energy conversion processes are governed by the first and second laws of

thermodynamics. In a process, the energy usually changes from one form to another.

The first law involves the conservation of energy. It indicates that there will be no

energy neither created nor destroyed in any conversion process. In other words, all


38

energy can be accounted for during the energy conversion process. Hence, it deals

with the quantity of energy. While the second law of thermodynamics deals with the

quality of energy. Quality of energy is defined by its utility, or its ability to cause a

change. Not all energy has the same quality. The work-producing potential of the

substance is defined as availability. Availability is consumed in the conversion

process. The availability output of the process is always less than the availability input

because of the irreversibility. The ratio of these two is usually referred to as the

second law efficiency.

As shown in the schematic figure, the heat pump dryer consists of two

condensers: one is used as internal heat source; the other one is placed out of drying

chamber to adjust desirable temperature. The refrigerant is HCFC-22 (R22). In this

study, only internal condenser was considered to provide useful energy and taken into

account in COP and energy efficiency computation. All the numbers in the following

analysis refer to the point label in Figure 3-1.

As both internal and external condensers are used to control drying temperature,

so there are two paths to flow after the refrigerant discharged from compressor. The

compression process is assumed to be adiabatic, hence,

)( 6100 hhmWc (4-7)

Then, the total mass flow rate from the discharge line can be calculated as:


39

6100 hh

Wm c

(4-8)

However, only the internal condenser provides the useful heat input in this drying

system, therefore, the mass flow rate to internal condenser should be known.

Applying mass balance equation and energy balance, the following array is obtained:

0

01267

mmmmTmTmT

ei

ei

(4-9)

im can be obtained by solving this set of equations.

As R22 passes through the condenser, it loses a substantial amount of energy.

Applying the first law of thermodynamics,

)( 76 hhmQ ic (4-10)

Associated with this heat transfer, the substance will reduce its flow availability (also

called exergy, the maximum work can be perform) by

)]()[()( 7607676ssThhmaamA iffic (4-11)

Finally, for the heat pump system, the coefficiency of performance is calculated as


40

c

c

WQ

COP (4-12)

The second law efficiency of this system is:

WAc (4-13)

where Fc WWW and FW is the input power of internal fan.

And specific moisture evaporating rate is defined as:

WM

SMER

(4-14)

where Mis the moisture evaporating rate.

4.2 Drying Kinetics

The results obtained from experiments and analyses are presented in this

section. Figure 4-1 shows the dimensionless drying curves of papaya resulting from

different drying media. By using the inert gas, either nitrogen or carbon dioxide, it

was found that the moisture evaporated faster than in normal air. The same

phenomena were observed in drying of guava and ginger, which are shown in Figure

4-2 and Figure 4-3. The difference is much clearer in Figure 4-2. Material structure

may contribute to it.


41

00.10.20.30.40.50.60.70.80.9

1

0 100 200 300 400 500

Drying time (min)

Dim

ensi

onle

ssm

ois

ture

rati

o

NitrogenCarbon DioxideNormal Air

Figure 4-1: Effect of inert gas on drying of papaya

00.1

0.20.30.40.5

0.60.70.8

0.91

0 100 200 300 400 500

Drying time (min)

Dim

ensi

onle

ssm

oist

ure

rati

o

Carbon DioxideNitrogenNormal Air

Figure 4-2: Effect of inert gas on drying of guava


42

00.10.20.30.40.50.60.70.80.9

1

0 100 200 300 400 500

Drying time(min)

Dim

ensi

onle

ssm

oist

ure

rati

o

Normal AirCarbon DioxideNitrogen

Figure 4-3: Effect of inert gas drying of ginger

By plotting the drying rate against moisture content in Figure 4-4, Figure 4-5

and Figure 4-6, the difference can be seen more clearly. The drying rates were found

to the highest at the beginning of drying when moisture content was the highest, with

nitrogen displaying the highest drying rate. In the early period, the drying rates

declined not so rapidly as in the final period. The moisture content influenced the

drying rate. High moisture content resulted in fast drying rate while high gradient of

depleting moisture content led to a slow declined gradient of drying rate.

Table 4-1 shows Page’s parameters for papaya, guava and ginger. As expected,

nitrogen yeilded the highest drying constant, which represents the water diffusion

velocity in the material. It consists with the following analyzing part.


43

-0.005

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0 2 4 6 8 10

Moisture content(g/g dry)

Dry

ing

rate

(g/g

dry

min

)

NitrogenCarbon DioxideNormal Air

Figure 4-4: Drying rate of papaya

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0 2 4 6 8 10

Moisture content(g/g dry)

Dry

ing

rate

(g/g

dry

min

)


Figure 4-5: Drying rate of guava


44

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0 2 4 6 8 10

Moisture content (g/g dry)

Dry

ing

rate

(g/g

dry

min

)Normal AirNitrogenCarbon Dioxide

Figure 4-6: Drying rate of ginger

Table 4-1: Page’s equation parameters

Material Method K BNormal Air 0.0077 0.8977

Nitrogen 0.0166 0.7656PapayaCarbon Dioxide 0.0117 0.8303

Normal Air 0.0022 1.1373Nitrogen 0.0046 0.9555Guava

Carbon Dioxide 0.0036 1.0496Normal Air 0.0023 1.0714

Nitrogen 0.0031 0.9720GingerCarbon Dioxide 0.0027 1.0574

Effective diffusivity of guava, papaya and ginger were also analysed in this

work by plotting data. For guava and papaya, only falling rate periods were observed

in the drying curve, as the drying of heat sensitive material was considered a slow

process. The diffusivity of guava and papaya in MAHPD using different gas were

calculated according to equation (4-4), assuming the thickness L was constant

throughout the drying process.


45

Nitrogen Drying

y = -0.6247x + 0.1677R2 = 0.9892

-3-2.5

-2-1.5

-1-0.5

00.5

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5t/L^2 (min/mm^2)

ln(M

R)

(a)

Carbon Dioxide Drying

y = -0.5209x + 0.082

R2 = 0.9932-3

-2

-1

0

1

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5t/L^2 (min/mm^2)

ln(X

/X0)

(b)

Figure 4-7: Variation of ln(MR) with t/L2 of guava (a) N2 (b) CO2

In Figure 4-7 and Figure 4-8, the experimental values of ln(MR) were plotted

against 2/ Lt . The slope of the curve is a measure of the effective diffusivity. It was


46

found that guava and papaya had its own pattern in inert gas drying. In Figure 4-7, the

variation nearly follows a straight line. Therefore, it was considered that the drying of

guava took place in one falling rate period, which means that the liquid diffusion was

the main driving force controlling the whole drying process. However, it is slightly

different for papaya drying. Experiment value deviated from the about 2. This point

was considered to be the beginning of the second falling rate period, where the

effective diffusivity dropped a little. This difference might be due to the different

micro structure of material. It was observed that papaya cubes formed a hard shell

during drying while there was no such phenomenon observed in drying of guava. The

hard shell might have blocked the moisture transfer to the hot air, resulting in a

slowing transfer rate. The results of other drying methods are summarized in Table 4-

2, including correlation coefficients ( 2R ) and improvement.

Nitrogen Drying

y = -0.4559x - 0.0798R2 = 0.9858

y = -0.374x - 0.2033R2 = 0.9848

-2.5

-2

-1.5

-1

-0.5

00 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

t/L^2 (min/mm^2)

ln(M

R)

(a)


47


y = -0.4457x - 0.0361

R2 = 0.9936

y = -0.3829x - 0.1146

R2 = 0.9851

-2.5

-2

-1.5

-1

-0.5

0

0.5

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5t/L^2(min/mm^2)

ln(M

R)

(b)

Figure 4-8: Variation of ln(MR) with t/L2 of papaya (a) N2 (b) CO2

Table 4-2: Diffusivity of Guava and Papaya in MAHPD

Material Drying MediumEffective Diffusivity

(First/Second)( sm /10 210 )

R2IncreasedPercentage

(First Stage)

Normal Air 6.09 0.9926 0Nitrogen 10.55 0.9892 73.20%Guava

Carbon Dioxide 8.79 0.9932 44.33%Normal Air 6.61 0.9943 0

Nitrogen 7.69/6.32 0.9858/0.9848 16.34%PapayaCarbon Dioxide 7.53/6.47 0.9936/0.9851 13.92%

The diffusivity of water in ginger slices under MAHPD was evaluated (Figure

4-9). Correlation coefficients ( 2R ) is included. The effect of inert gas on ginger was

similar to that of guava. The variation nearly followed a straight line. Therefore, it

was considered that the liquid diffusion was the main driving force controlling the

whole drying process of ginger, with a constant diffusivity.


48

Normal Air Drying

y = -0.0467x + 0.1717

R2 = 0.987D=7.892e-11

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

0 10 20 30 40 50 60

t/L^2(min/mm^2)ln

(MR

)

(a)

Nitrogen Drying

y = -0.0681x + 0.1464

R2 = 0.9847D=11.512e-11

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

0 10 20 30 40 50

t/L^2(min/mm^2)

ln(M

R)

(b)


49


y = -0.0538x + 0.1157R2 = 0.9847

D=9.085e-11-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

0 10 20 30 40 50

t/L^2(min/mm^2)

ln(M

R)

(c)

Figure 4-9: Variation of ln(MR) with t/L2: (a) normal air; (b) N2; (c) CO2 for gingerNote: D is measure in m2/s

Although the method described here was a simplified one and the diffusivity

measured was a lumped value, it would affect the drying characteristics. From these

data, it can be found that nitrogen drying had the highest effective diffusivity,

followed by carbon dioxide drying, while normal air drying yielded the lowest. The

possible reason is probably due to the different physical properties of different gas.

For nitrogen, the specific heat (1.04kJ/kg.ºC) is higher than that of normal air (1.00

kJ/kg·ºC) while their densities were nearly the same (0.9737:1). Hence, their masses

were the same at the same volume. Therefore, in a fixed volume system, nitrogen

would absorb more heat from the heat exchanger and release more when passing over

the samples in the chamber. For carbon dioxide, though the specific heat (0.85

kJ/kg.ºC) is lower than that of air, its density is much higher (1.539) than air.


50

Therefore, CO2 had a larger mass at fixed volume compared to air. It also had a larger

heat transfer capacity than air.

4.3 COP, Energy Efficiency & SMER

Table 4-3, Table 4-4 and Table 4-5 indicate the energy parameters of different

drying processes in heat pump dryer. Taking the data of normal air (100 gram) as a

sample, the calculation procedure is as following:

1. The input power of compressor: 0.6kW;

2. The total mass flow rate of compressor discharge line is:

skghh

Wm c

o /06.04454356.0

610

3. The mass flow rate to internal condenser is:

skgm

mmmm

i

ei

ei

/052.0

06.006.05.310.698.25

4. The useful heat input is:

kWhhmQ ic 82.1)410445(052.0)( 76

5. Availability:

kWssThhmA ic 43.0)]70.179.1(297)410445[(052.0)]()[( 76076

6. COP of heat pump:

03.36.0

82.1

c

c

WQ

COP

7. The input power of internal fan: 0.1kW

8. The total input power is:

kWWWW Fc 7.01.06.0


51

9. Energy efficiency:

%4.617.0

43.0

WAc

10. Average moisture evaporating rate:

hkgM /015.0

11. SMER:

hkWkgSMER ./021.07.0

015.0

Table 4-3: The energy results of normal air drying processes.

Feed Load(g)/ M(kg/s)

Status T(°C) P(Mpa) h(kJ/kg) s(kJ/kg.K) Results

6 69.0 1.62 445 1.79 im=0.052kg/s7 25.8 1.76 410 1.69 COP=3.0310 32.0 0.14 435 - ＝39.4%

100M=0.015

12 31.5 1.57 - - SMER=0.0216 68.1 1.62 445 1.79 im=0.053kg/s7 26.5 1.76 410 1.69 COP=3.0910 32.0 0.14 435 - =40.1%

150M=0.016

12 31.4 1.58 - - SMER=0.0236 67.2 1.62 444 1.79 im=0.055kg/s7 28.1 1.76 411 1.70 COP=3.0210 32.0 0.14 435 - ＝49.3%

200M=0.010

12 31.2 1.58 - - SMER=0.014

In each drying method, it was noted that the enthalpy and entropy of

refrigerant did not vary much. The only changed parameter was the mass flow rate in

internal condenser, which increased as feed load increased. Since there was more

moisture needed to be evaporated and the refrigerant temperature was nearly the same,

there was certainly more amount of refrigerant flowing to the internal condenser in to


52

supply the decreased quantity of heat. Hence, it is the most important factor that

influences COP and energy efficiency.

Table 4-4: The energy results of nitrogen drying process



6 68.1 1.62 445 1.79 im=0.051

7 25.0 1.76 410 1.69 COP=2.9810 32.0 0.14 435 - ＝38.6%

100M=0.015

12 31.2 1.57 - - SMER=0.0226 67.3 1.62 445 1.79 im=0.0527 26.0 1.76 410 1.69 COP=3.0310 32.0 0.14 435 - ＝39.4%

150M=0.016

12 31.5 1.57 - - SMER=0.0236 67.5 1.59 443 1.79 im=0.0557 27.0 1.76 411 1.70 COP=2.9310 33.5 0.14 437 - ＝41.4%

200M=0.011

12 30.5 1.57 - - SMER=0.015

It was also found that the imin nitrogen drying and carbon dioxide drying

were lower than normal air drying at the same feed load. This phenomenon could be

linked to effective diffusivity. The effective diffusivity in inert gas is higher than in

normal air, so less heat is needed, subsequently, lower mass flow rate to internal

condenser.

Note that the energy efficiency increased with im, but it was not true for

SMER and COP. SMER values were nearly the same in different drying media. It

was observed that the evaporated moisture could not be condensed properly by the

system when the feed load reached 200 gram. Much of it condensed on the internal

surface of the drying chamber. The RH inside increased significantly, which was

about 78%. Therefore, it slowed down the drying rate and resulted in a decreased


53

SMER. The feed load of 150 gram seemed to be a suitable value. In this case, COP

was also the highest, no matter in HPD or MAHPD. And in this experiment, the

value of SMER was extremely low, which was not consistent with the data in

literature review. The possible reason was that there was no collection and discharge

device in this lab dryer. Hence, it is only suitable for low feed load.

Table 4-5: The energy results of carbon dioxide drying process



6 67.0 1.62 444 1.79 im=0.0537 26.0 1.76 410 1.69 COP=3.0010 31.8 0.14 435 - ＝32.6%

100M=0.015

12 30.8 1.58 - - SMER=0.0226 67.5 1.62 444 1.79 im=0.0547 26.6 1.76 410 1.69 COP=3.0610 31.8 0.14 435 - ＝33.2%

50M=0.017

12 30.6 1.58 - - SMER=0.0246 68.0 1.59 443 1.79 im=0.0557 27.0 1.76 411 1.70 COP=2.9310 33.2 0.14 437 - ＝41.4%

200M=0.010

12 30.3 1.57 - - SMER=0.014

Comparison of COP for HPD and MAHPD is made in Figure 4-11. It is found

that COP decreases when inert gas is used. The same trend was also observed in

energy efficiency, referring to Figure 4-12. This was because there was less

refrigerant flowing to the internal condenser when using inert gas. Therefore, the

useful energy amount reduced but the total input power was not changed.

Although COP and energy efficiency of MAHPD was lower than HPD, the

difference was not significant. This was due to the design deficiency of the dryer. If

the heat of external condenser can be fully used, it should not be a limitation.


54

Comparison of COP

2.85

2.9

2.95

3

3.05

3.1

3.15

100 150 200

Feed Load

CO

P


Figure 4-10: Comparison of COP for HPD and MAHPD

Comparison of Energy Efficiency

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

100 150 200

Feed Load

Ener

gyEf

fici

ency

Normal Air

Carbon DioxideNitrogen

Figure 4-11: Comparison of energy efficiency of HPD and MAHPD

Chapter 5 Foods quality

55

CHAPTER 5

FOOD QUALITY

Food quality is an important criterion to judge the performance of dryers. In this

study, the quality tests were conducted in colour, density, porosity, rehydration,

texture, nutrient and flavour, and the results are presented in this chapter.

5.1 Colour

The L*, a*, b*, C*, h* values of samples (apple, guava and potato) dried in N2,

CO2 and normal air for different lengths of time are given in Table 5-1.

Comparing the data, it is possible to deduce that there is an improvement in

colour with longer drying time, e.g. 18 hours compared to 8 and 4 hours. L* and h*

were selected to be the index of browning. Samples dried for longer times, had

increased L* and h* values, and generally decreased a*, b* and C* values. As it was

reviewed in Chapter 2, the main cause of browning is PPO, which is heat-sensitive. It

is suggested that longer time drying deactivate this enzyme group more effectively.

Hence, there more lightness and less browning products resulted.

The h* values of samples dried for 18hrs in MAHPD using nitrogen and

carbon dioxide showed no significant difference at a confidence level of 95%.

However, comparing these samples dried using normal air drying, there was a

significant difference in h* values for guava and potato, as shown in Figure 5-1. For

apple, although h* values were not significant different, L* value was increased much


56

and C* value was decreased much in MAHPD. Introducing carbon dioxide or

nitrogen into the drying chamber reduces the level of oxygen in the atmosphere, thus

modifying its composition. This can positively affect the colour and, hence,

improving the appearance.

Table 5-1: Colour values for dried apple, guava and potato samples

Drying Method Test Apple Guava PotatoL* 52.211.88 50.361.37 40.203.62a* 10.331.00 7.110.53 3.88 0.38b* 33.660.63 38.541.95 11.181.81C* 35.220.85 39.191.97 11.841.82

Nitrogen(4h)

h* 72.951.33 (a) 79.550.63 70.681.73L* 57.343.95 53.982.72 55.863.30a* 8.13 2.67 7.25 1.16 2.76 0.34b* 30.642.59 26.871.40 16.900.34C* 31.753.16 27.851.50 17.130.67

Nitrogen(8h)

h* 75.403.49(a) 74.922.11 80.720.95L* 64.932.00 64.061.79 70.942.62a* 2.93 0.27 0.100.36 -0.250.10b* 26.370.61 20.752.24 16.970.62C* 26.540.64 20.752.24 16.970.62

Nitrogen(18h)

h* 83.670.47(b) 89.760.97(c) 90.870.34(d)L* 66.231.35 62.371.62 66.232.38a* 3.55 0.92 0.25 0.29 -0.140.34b* 26.011.80 21.612.63 16.700.87C* 26.261.88 21.612.62 16.700.87

CO2(18h)

h* 82.311.65(b) 89.250.81(c) 90.501.15 (d)L* 60.580.85 61.571.29 63.541.74a* 5.59 0.96 2.48 0.88 0.850.18b* 31.750.99 27.192.74 14.900.56C* 32.251.10 27.312.71 14.930.57

Normal Air(18h)

h* 80.031.46(b) 84.681.96 86.760.64Note: Values following ±sign are standard deviation of 5 replicates

Means followed by the same letter in a column are not significantly different(p<0.05)

The L*, a*, b*, C*, h* values of apples treated with lemon juice or peel as

natural inhibitors are presented in Table 5-2. From the data, it can be seen that there is

less browning with increasing concentration of lemon juice, as shown by h* value


57

increasing significantly. This could be attributed to its ability to reduce quinones,

generated by PPO-catalysed oxidation of polyphenols, back to phenolic compounds

before they undergo further reaction to form pigments. The optimum pH for PPO

activity is between pH 5 and 7 (Özdemir, 2003). Adjusting the pH to 4 or below has

been reported to effectively reduce browning (Martinez and Whitaker, 1995). That is

why higher concentration of lemon juice results in better colour. Comparing the use of

lemon juice with the use of inert gas, 30% lemon juice pre-treatment gave similar

results. And the peel of lemon could be used in place of 30% or 50% lemon juice,

little difference was observed.

7476

788082848688

9092

Apple Guava Potato

hva

lue

Nitrogen

CarbonDioxide

NormalAir

Figure 5-1: h* values of dried apple, guava, potato samples

A colour comparison between the MAHPD and vacuum dried samples is shown

in Table 5-3. Freeze drying yielded the highest h* and L* value, lowest a*, b* and C*

values, which gave a nice appearance. It removes water from a frozen sample by

sublimation under reduced pressure. Hence, it also reduced the oxygen content. Most

freeze-drying is done at -10°C, or lower, at absolute pressures of about 2 mm Hg or


58

less. It is generally agreed that freeze drying yields the best products in terms of

quality such as colour (Liapis and Bruttini, 1995). However, it is also a very

expensive way of drying.

Table 5-2: Colour values for pre-treated dried apples

Gas Conc.(v/v)

L* a* b* C* h*

--- 40.61 ±4.83 11.44±0.81 28.76±1.90 30.96±1.91 68.29±1.48(a)

10% 46.64 ±2.89 12.02±1.00 31.29±1.74 33.52 ±1.96 69.01±0.74(a)30% 51.18±3.6 8.75±1.48 29.74±1.54 31.36±1.87 73.60±1.81(b)50% 52.80±1.22 7.40 ±0.72 24.42±1.34 25.52±1.49 73.17±0.74(b)

Normal Air

peel 62.23 ±4.63 9.32±3.12 32.96±3.17 34.30±3.88 74.51±3.21(b)N2 --- 54.05 ±4.17 10.78±1.75 32.40±1.66 34.17±1.95 71.64±2.34

CO2 --- 57.92±4.08 10.26 ±2.07 30.89±3.13 32.56±3.59 71.77±1.88Note: Values following sign are standard deviation of 5 replicates

Means followed by same letter are not significantly different (p<0.05)

Table 5-3: Colour values of apples resulting from three drying methods

Methods L* a* b* C* h*Freeze drying 67.23±1.40 0.4±0.23 16.94±1.84 16.95±1.84 88.64±0.69

Vacuum drying 60.02±2.37 6.93±0.86 26.17±0.81 27.08±0.95 75.19±1.45CO2(18Hrs) 66.23±1.35 3.55±0.92 26.01±1.80 26.26±1.88 82.31±1.65N2(18Hrs) 64.93±2.00 2.93±0.27 26.37±0.61 26.54±0.64 83.67±0.47

Note: Values followed sign are standard deviation of 5 replicates

Although the degree of browning can be expressed by the absolute values of

L*, a*, b*, C*, it is more reasonable to compare the colour of dried products with the

fresh food, since people prefer its appearance can be kept as much the same as its

original. Therefore, the total colour changes of dried guava and papaya resulting from

5 drying methods are compared in Figure 5-2. From both Figure 5-2(a) and 5-2(b), it

is clear that inert gas have a smaller colour change than normal air. It is known that

enzymic browning requires four different components: oxygen, enzyme, copper and a

substrate (Langdon, 1987). Hence, a reduction of oxygen content in the drying


59

chamber could improve the colour. Normal air consists of approximate 20% oxygen

in volume while that in inert gas for this experiment is much lower. This result is

consistent with the previous study (Hawlader et al., 2004). It is also noted that, for

guava, the nitrogen dried products are even better than vacuum dried. Freeze drying

gave the smallest change. In contrast, for papaya, the DE* of freeze drying is the

largest. The possible reason is that freeze drying increased the lightness (L*) too

much, subsequently affected the DE* value. And papaya is dark colour fruit; while

guava is light colour one. Hence, compared with their original colour, freeze dried

papaya seemed far too light, which was not desirable.

Total color difference (DE*) of guava

05

1015202530354045

Freeze N2 Vacuum CO2 Normalair

DE*

valu

e

(a)


60

Total Color difference (DE*) of Papaya

05

10152025303540

Nitrogen CO2 Normalair

Vacuum Freeze

DE*

valu

e

(b)

Figure 5-2: Total colour change: (a) Guava; (b) Papaya

5.2 Shrinkage, Density and Porosity

During drying, shrinkage of the samples was observed visually. It is noted that

different material lead to different degrees of shrinkage under the same drying

conditions. Among the samples of apple, guava and potato, the latter shrunk much

less. This can be related to the moisture removal from each food.

It has been reported by several researchers that shrinkage bears a simple

relation to moisture removed as stated in the literature review. In this study, it was

observed that potatoes had much less moisture removed than apple and guava, as

shown in Table 5-4. In general, potato has higher percentage of solid, 35% to 40%,

compared to fruits which have about 15%. Therefore, when it was dried, there

certainly less water was removed, which resulted in lower degree of shrinkage.


61

Another important fact influencing shrinkage is case-hardening. This

phenomenon was observed in drying of papaya, which has been stated in the previous

chapter. When the external RH is lower, the surface moisture is removed very fast that

and, once surface moisture disappears, the transport of internal moisture to the surface

is controlled by diffusion coefficient often leading to case hardening of a material.

Since the shell formed, it supports the original shape of dried product, preventing its

further distortion, even when the internal structure collapses.

Table 5-4: Moisture Removal from samples dried in MAHPD over 18 hours

Method Moisture content Apple Guava PotatoXi 9.00 8.92 4.06Xf 0.25 0.27 0.05

NitrogenHPD

∆X 8.75 8.65 4.01Xi 9.00 8.92 4.06Xf 0.35 0.36 0.08

Carbon DioxideHPD

∆X 8.65 8.56 3.98Xi 9.00 8.92 4.06Xf 0.28 0.32 0.12

Normal AirHPD

∆X 8.72 8.60 3.94Note: M expresses moisture per g of dry matter

The initial mass for each material is around 100g

Shrinkage influences the product porosity, as it alters the air space of the

internal structure. The density and porosity are important physical properties

characterizing the quality of dry and intermediate moisture foods (Schubert, 1987;

Zogzas et al., 1994).

Porosity characterizes the open structure of a dehydrated material and is

strongly affected by the material moisture content, drying method and conditions

(Krokida and Maroulis, 2000). In addition, internal porosity development is


62

dependent upon the type of product (Zogzas, Maroulis, and Marinos-Kouris, 1995).

Usually, porosity is linked to density.

However, there are a number of definitions of density of practical interest

which need to be considered (Rahman, 1995; Rahman et al., 1996). In this paper, the

definition is followed as stated in the literature review. Hence, the relationship

between density and porosity can be expressed as:

densityParticledensityBulk

Porosity 1 (5-1)

Table 5-5: The density and porosity of guava and papaya

Material Drying Methods Bulk Density Particle Density PorosityNormal Air 0.9953 1.1832 15.88%

Nitrogen 0.9414 1.2121 22.33%Carbon Dioxide 0.8801 1.2392 28.98%

Vacuum 0.7589 1.0914 30.46%

Guava

Freeze 0.3543 0.5400 34.39%Normal Air 1.0460 1.5583 32.88%

Nitrogen 0.6915 1.2613 45.18%Carbon Dioxide 0.7259 1.2715 42.91%

Vacuum 0.6254 1.2593 48.19%

Papaya

Freeze 0.1098 0.1631 32.69%

Table 5-5 summarizes the results for guava and papaya. For guava, the

porosity increased in the sequence of normal air drying, nitrogen drying, carbon

dioxide drying, vacuum drying and freeze drying. On the contrary, the porosity of

freeze dried papaya is smaller than that of normal air, and the vacuum dried samples

have the largest porosity. For vacuum dried papaya, a puff effect was observed

visually: there were bubbles under a hard shell. Reduced pressure may contribute to it.

It is the possible reason that the vacuum dried papaya had the largest porosity. For


63

freeze dried papaya, some pore diameters may be smaller than 0.003 m , which is out

of the measuring capability of mercury porosimeter, so they can not be detected and

taken into account. With the help of SEM, more information was found.

(a)

(b)


64

(c)

Figure 5-3: Structure of 8 hours normal air HPD dried material (SEM)(a) Potato, (b) Guava, (c) Apple

SEM measures the surface porosity. Figure 5-3(a) shows the oval starch

granules that distinctively characterize the potato cell structure. This indicates that

potato contains a higher percentage of solid, which may be hindrance to moisture

transfer. In guava structure, Figure 5-3(b), two types of pores can be identified,

namely open pore and closed pores. For better rehydrability, more open pores are

desired because closed pore have no capillary action, which means an inability to

imbibe sufficient water. As these are soft tissues and structures, careful material

handling was needed while cutting the sample. A more desirable structure is seen in

the top right hand corner of Figure 5-3(c); a regular honeycomb shape. The darkened

areas are the pores that have shrunk and their water content removed. If a material

undergoes different drying methods, it has a different porosity. This can be seen in


65

Figure 5-4 (a) to (e), which is papaya dried by normal air HPD, MAHPD, freeze dryer

and vacuum dryer, respectively.

(a)

(b)


66

(c)

(d)


67

(e)

Figure 5-4: SEM of dried papaya:(a) freeze drying; (b) normal air drying; (c) vacuum drying; (d) Carbon dioxide drying;

(e) Nitrogen drying

5.3 Rehydration

Rehydration ability interconnects closely with porosity. Usually, the more

porous the products are, the faster the products rehydrate. The rehydration rate

changes with water temperature. The rates of rehydration of MAHPD dried apple,

guava and potato at 60°C and 80 °C were investigated and compared. These resultls

are shown in Figures 5-5 and 5-6.

Theoretically, if there are no adverse effects on the tissue structure, the

product will absorb water to the same extent as its initial state (Senadeera et al., 2000).

However, rehydration ability is reduced as the product gets drier, possibly because of

the increased loss of porosity and the collapse of cellular structure. As shown in


68

Figure 5-5, it is found that all samples rehydrating at 80°C absorbed water at a higher

rate as compared to those rehydrating at 60°C. This may due to the water molecules

have higher kinetic energy at higher temperature. They could move quicker into the

capillary paths through the pores of the dried products.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1 2 3 4 5 6 7 8 9 10

Time (mins)

Wei

ghto

fwat

erab

sorb

ed(g

ram

) Apple 60°C

Apple 80°C

Guava 60°C

Guava 80°C

Potato 60°C

Potato 80°C

Figure 5-5: Rehydration behaviour of samples dried by MAHPD with 2N

It was also noted that the rehydration patterns for apple and guava samples

followed a nonlinear curve, in which the rate of rehydration decreased after 4 minutes.

However, the behaviour for potato was linear. This may be due to a more uniform

distribution of pores in the potato which leads to rehydration at a constant rate. On the

other hand, apple and guava, having bigger pores may initially rehydrate more easily,

and thus a steeper gradient is observed during the first 4 minutes of rehydration.


69

00.10.20.30.40.50.6

0.70.80.9

1 2 3 4 5 6 7 8 9 10

Time (mins)

Wei

ghto

fabs

orbe

dw

ater

(gra

ms) Apple

80℃Apple60℃Guava80℃Guava60℃Potato80℃Potato60℃

Figure 5-6: Rehydration behaviour of samples dried by MAHPD with 2CO

0.0000

1.0000

2.0000

3.0000

4.0000

5.0000

6.0000

0 2 4 6 8 10

Time (mins)

Moi

stur

eco

nte

nt(d

.b)

40

60

80\

Figure 5-7: Rehydration of nitrogen dried papaya


70

The same phenomena were observed in papaya. Three rehydrating

temperatures: 40°C, 60°C and 80°C were tried. In Figure 5-7, the rehydration of

nitrogen dried papaya at different temperatures for the first 10 minutes were shown. It

is noted that it had a faster rehydration rate at higher temperature as well.

However, the rehydration capability seems not to be affected by temperature.

In Figure 5-8, the rehydration capability of dried products resulting from all the

drying methods only increased slightly at higher temperature; porosity may have

contributed to this. Although the temperature was different, the space to contain water

is nearly the same for the samples resulting from each drying method. And the

capability of dried samples increased in the sequence of their porosity. The higher

porosity one resulted in a slightly higher rehydration capability.

Rehydration Capability of dried Papaya

0.0000

1.0000

2.0000

3.0000

4.0000

5.0000

6.0000

7.0000

Normal Air Carbon Dioxide Nitrogen

4060

80

Figure 5-8: Rehydration capability of dried papaya


71

Rehydration of dried papaya at 40 degree

0.0000

1.0000

2.0000

3.0000

4.0000

5.0000

6.0000

7.0000

0 20 40 60 80 100

Time (mins)

Moi

stur

eco

nten

t(d.

b)


Figure 5-9: Rehydration behaviour of HPD and MAHPD dried papaya

The same effect that porosity plays on rehydration can be also observed in

Figure 5-9. During the whole process, nitrogen dried samples has the highest

rehydration rate and the sequence among these three samples never changed.

Therefore, it can be concluded that the more porous samples absorb more water at a

faster rate.

However, this conclusion can not be applied to vacuum dried samples. Figure

5-10 shows the rehydration behaviour of papaya from vacuum dryer and freeze dryer.

It is found that the vacuum dried samples rehydrated slower than freeze dried samples

for the first 10 minutes, while it is faster after that period. This may be due to the puff

effect on the vacuum dried papaya, which enables it to possess a higher porosity than

freeze dried samples. But its open pores are not as many as in freeze dried papaya.


72

That can be seen from their SEM. Hence, although it absorbed less water at the

beginning, it had large space to hold them.

Comparison of Freeze dried and Vacuum dried papaya rehydrationat 40 degree

0.0000

1.0000

2.0000

3.0000

4.0000

5.0000

6.0000

0 50 100 150 200

Time (min)

Moi

stur

eco

nten

t(d.

b)

FreezeVacuum

Figure 5-10: Rehydration curve of freeze dried and vacuum dried papaya.

5.4 Texture

Texture is the response of the tactile senses to physical stimuli that result from

contact, between some parts of the body and food products (Bourne, 2002). The

sensory acceptability factors of foods are extremely important because people like to

attain great enjoyment from eating food. For dried foods, firmness is one of the main

considerations of consumers, neither a too hard nor too soft, being satisfying.

Generally, samples dried over a longer period are harder than those dried over a

shorter period because of greater loss of moisture. Hence, the drying time was fixed in

order to consider the influence of other factors on texture.


73

Table 5-6: Firmness of apples dried by different methods

Methods Concentration (v/v) Firmness--- 210.15±30.35

10% 351.90±55.5130% 484.81±96.9850% 576.35±60.78

Normal Air(4 hrs)

Peel 295.04±58.04--- 117.78±19.94(ad)

10% 136.57±13.75(ab)30% 140.28±33.15(ab)50% 213.18±27.95

CO2(4 hrs)

Peel 117.47±19.62--- 116.16±21.11(cd)

10% 136.45±26.99(cd)30% 136.70±31.22(cd)50% 352.68±113.08

N2(4 hrs)

Peel 121.31±21.89Vacuum (18 hrs) --- 365.68±52.43Freeze (18 hrs) --- 75.62±18.08

Note: Values following sign are standard deviation of 10 replicatesMeans followed by same letter are not significantly different (p<0.05)

Table 5-6 shows the results for apples dried in HPD for 4 hours using normal

air and inert gas. Freeze dried and vacuum dried samples are also included. It is worth

mentioning that, in the same batch, sample firmness varied a lot due to samples being

taken from different parts of the apple. The part near the core was the firmest.

Therefore, it resulted in a large standard deviation. As freeze dried samples is most

porous, its firmness was the lowest. In vacuum drying, the main mode of heat transfer

form is conduction, which is different from HPD and MAHPD. This may contribute

to the relatively high firmness. It was also observed that the firmness of normal air-

dried samples was much higher than that of nitrogen or carbon dioxide dried samples.

The inert gas may have contributed to this change. As stated in last section, the

samples dried by MAHPD were more porous in structure, which means more air

space in the solid. It could bear less force and resulted in less firm products.


74

Another interesting phenomenon is that when using normal air, firmness

increased as the concentration of lemon juice increased. Similar results were observed

in fresh-cut pear slices (Gorny et al., 2001). A possible reason for this observation is

that the citric acid has changed the structure of the fruits. This conclusion may require

further study.

The firmness value for papaya and guava are summarized in Table 5-7. It also

has a large variance. Secondly, compared with normal air dried papaya or guava

cubes, it is found that all other drying methods increased the firmness of dried

products, which is contradictory to the results for apple. Different factors may

contribute to this. For vacuum drying, conduction may increase firmness. It was

observed that the contact area was harder than the other sides. For freeze drying, some

researcher thought the freeze-thaw process causes the demethylation which results

firming (Main et al., 1986). Inert gases also increase firmness a lot, especially for

nitrogen dried guava. It is suggested there may be some reactions between the

chemicals inside these fruits and inert gas. However, it needs further research.

Table 5-7: Firmness of dried papaya and guava

Drying Methods

MaterialNormal Air Nitrogen Carbon Dioxide Freeze Vacuum

Papaya 169.66±19.03 317.69±42.63 360.30±18.9 266.26±30.84 1073.48±137.84Guava 302.21±21.32 3436.39±194.87 981.36±81.68 669.87±69.53 1525.77±91.40Note: Values following sign are standard deviation of 10 replicates


75

5.5 Vitamin C

Fresh fruits are rich sources of vitamin A and C besides minerals, which are

considered important for human body. Because of their perishable characteristic, it is

desired to store a long time with minimum loss of nutrients (Jayaraman and Das

Gupta, 1995). The present study showed that, as indicated in Table 5-8, freeze drying

retained the most vitamin C in both papaya and guava (88% and 63%, respectively),

and then vacuum drying is the second. Compared with normal air, carbon dioxide and

nitrogen have better retention. The improvements are considerably greater, though

they are different in guava and papaya. It is because the retention of vitamins depends

on the nature of foods (Jayaraman and Das Gupta, 1995).

As vitamin C is easy to deteriorate in oxidative atmosphere, the first reason for

the improvement in inert gas is that it reduced oxidation. Temperature also affects

vitamin C. Hence, freeze drying has the best results since it is a low-temperature

drying process. However, products retained slightly more vitamin C resulting from

carbon dioxide drying, 82% versus 80% in papaya and 41% versus 39% in guava.

The possible reason is that nitrogen drying has a higher drying rate than carbon

dioxide drying. Hence, the more heat presented the more vitamin C lost.

Table 5-8: Retention of Vitamin C in dried products

Drying methodsMaterialNormal Air CO2 N2 Freeze Vacuum

Papaya 75% 82% 80% 88% 86%Guava 25% 41% 39% 63% 58%


76

5.6 Pungent Principles Analysis

According to Leverington (1975), and Connell and Sutherland (1969), the

main pungent principles extracted from the rhizomes were 6-gingerol, 8-gingerol, and

10-gingerol, and in terms of pungency 6-gingerol was the most pungent compounds

(Govindarajan, 1979; 1982). Hence, 6-gingerol was chosen to be the index of dried

ginger quality.

Figure 5-11 show the chromatograms of 6-gingerol standard and sample. The

retention time was found to be 16 minutes in standard. Therefore, the area of peak

appeared around 16 minutes in each sample profile were taken to calculate

concentration of samples. The calibration graph for 6-gingerol (Figure 5-12) was

obtained over the range of 20.7 - 82.8 mLg / . The result, by linear regression

analysis, showed a very good linear relationship between peak area and concentration.

(a)


77

(b)

Figure 5-11: Chromatograms 6-gingerol: (a) Standard (b) Sample

Calibration Graph

y = 5.4562x + 8.41

R2 = 0.99790

100

200

300

400

500

0 20 40 60 80 100

Conc.(μg/mL)

Peak

Are

a

Figure 5-12: The calibration graph of 6-gingerol determined by HPLC assay

Results of ginger sample dried under different conditions are shown in Figure

5-13. It is observed that the amount of 6-gingerol used as an index of retention of


78

volatiles in ginger dried under different methods of drying increased in the order of

normal air HPD, freeze drying, nitrogen HPD, carbon dioxide HPD and vacuum

drying. However, the difference between nitrogen HPD and carbon dioxide HPD was

not significant at p<0.05. As all other condition were kept the same for heat pump

drying, it is clear that inert atmospheres contributed to greater retention of 6-gingerol

compared to normal HPD drying in air. This may be because of the nature of the

complex mixture of relatively unstable phenolic compounds in ginger, which

undergoes oxidation in air (Denniff et al., 1981). In normal air, oxygen content is

about 20% in volume. Modified environment reduced the oxygen level during drying

periods, which prohibited possible destruction of gingerols. Moreover, gingerol has

acidic methylene protons and are easily dehydrated to produce shogaols (Balladin et

al., 1998; Denniff et al, 1981).

Figure 5-13: Comparison of 6-gingerol content in different dried ginger samples(6-gingerol content is based on mg/g of dry matter)

Note: The values in the graph are the mean values of 3 replicates.The same superscripts over bars means the values are not significantlydifferent (p<0.05).

c cd

0

2

4

6

8

10

12

Normalair

drying

Freezedrying

Nitrogendrying

Carbondioxidedrying

Vacuumdrying

Con

tent

(mg/

g) ab


79

Gingerols and shogaols undergo the retro-aldol cleavages to form zingerone

and corresponding aldehydes; Zingerone and aldehydes under acidic condition are

condensed to form corresponding gingerols (Young et al., 2002). The reactions are

very complex and pH affects stability of gingerol. Therefore, aside from preventing

oxidation of phenolics, it is assumed that inert gas such as carbon dioxide may form

an acidic environment with the presence of moisture, which helps to reform gingerols,

so the retention of gingerol is slightly higher than in nitrogen HPD dried samples.

Vacuum drying showed the highest retention of 6-gingerol. The chamber of

vacuum dryer, which is made of aluminium, may contribute to this. It eliminated all

the light from ginger slices during drying. Whereas, the chambers of other dryers used

were made of glass, and allowed light to fall on the samples directly. As 6-gingerol is

sensitive to light, this probably accounted for the lower retention of gingerol in dried

products.

Freeze drying is said to be the best way to retain flavour (Liapis and Bruttini,

1995). However, it was not proven in this experiment. The result obtained was only

better than normal air drying, while worse than inert gas drying. Comparing its drying

conditions with those of other methods, freeze drying also reduced oxygen content

and pressure. In particular, for freeze drying, an absolute pressure of less than 4.6

mm of Hg had to be reached, and at such low pressures, perhaps the gingerols were

very volatile and were lost into the vacuum pump. However, the real reason is

uncertain, which needs further research.

Chapter 6 Conclusions

80

CHAPTER 6

CONCLUSIONS

This study focused on modified atmosphere heat pump dryer. Dried product

quality and energy efficiency were investigated. The materials tested include apple,

guava, potato, papaya and ginger. Results were compared with normal air drying,

freeze drying and vacuum drying.

For quality tests, the investigations covered colour, shrinkage, density, porosity,

rehydration, texture, nutrient and flavour. Experimental results showed that the use of

inert gas led to an improvement of product quality. The important findings of this

work are listed as follows:

In terms of optical property, for long time HPD drying of apple, guava and potato

resulted in higher value of h* and L*, which means less browning and more

lighteness. Compared with normal air drying, the improvement in carbon dioxide

and nitrogen atmosphere drying was significant. When using lemon juice as

natural inhibitor of oxidation, 30% (v/v) solution has similar effect as inert gas.

And this could be substituted by lemon peel. Total colour difference of dried

papaya and guava from fresh counterpart was calculated. Nitrogen dried products

had a smaller change than carbon dioxide. For papaya, the colour is even better

than freeze drying.


81

In structural properties, food such as potato containing less moisture shrunk less

during drying. This led to higher porosity, which also links to particle and bulk

densities. Lower shrinkage and more porous structure were also observed in

MAHPD products. The improvement of porosity is as high as 82.5%. SEMs were

also taken to identify the surface properties. From the picture, it is clearer to see

the difference.

In rehydration study, the more porous products had better rehydration ability. It

absorbed water faster. Higher temperature led to a faster rehydration, but it has

little effect on rehydration capacity.

In textural properties, the results are complex. Apple dried in MAHPD was not so

firm as HPD dried one, whereas, for guava and papaya, inert gas increased

firmness. This may be attributed to the properties of the materials.

In nutritional characteristics, vitamin C retention is generally considered to be an

index of nutrient preservation. If vitamin C was better retained, it is suggested

that the other nutrients will also be preserved. MAHPD provided a better

retention of vitamin C. The vitamin C content in CO2 dried guava is 41% of fresh

one, while only 25% remained in normal air dried guava.

In sensory properties, by using inert gases, ginger flavour (6-gingerol) retention is

much higher than normal air drying. Compared to normal air drying, nitrogen and

carbon dioxide drying led to an increase of flavour by 43.5% and 45.4%,

respectively, even better than freeze drying.


82

From the energy analysis, it was found that the drying constant, effective

diffusivity increased while COP and energy efficiency decreased a bit when using

inert gas. If the heat rejected by the external condenser could be utilized, it is

probably that the energy efficiency and COP would be increased. SMER did not

change much in this work because the system is very small.

All the analysis justifies an unshakable view that MAHPD did improve the

quality of dried food while not affect energy efficiency much. By using it, the

products owned less browning colour, lower shrinkage, more porosity, different

texture, better retention of nutrients and increased flavour, which are considered

desirable attributes that food industry would like to pursue.

Chapter7 Recommendations

83

CHAPTER 7

RECOMMENDATIONS

This work investigated the differences in final products dried by MAHPD and

suggested some possible improvements.

Further improvements can be made on the heat pump dryer. The current one is

small and manual controlled. A big size and computer controlled one is desired. As

the COP was not satisfactory, the rejected heat of the external condenser can be made

use of in certain way.

Shrinkage here was only visually observed because of the closed system of

MAHPD. It is difficult to measure the parameters needed. If an automatic measuring

tool can be fixed into this system, a model of shrinkage to moisture content can be

established for materials considered in this study.

.

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84

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Appendices

100

Appendices

Appendix A: Determination of Vitamin C

Introduction

This method again relies on the reduction of the dye 2, 6-dichlorophenol-indophenol

in aqueous solution by the vitamin C (ascorbic acid). Indophenol is blue in neutral and

alkaline solutions but pink in acid. Vitamin C is a reducing agent and will decolorize

the dye. Thus, taking a standard amount of the dye and measuring the vitamin C

content of the solution.

Materials

Indophenol solution (already prepared): This contains 0.0162g in 1L of water. Acetic

Acid (10%) (already prepared from 100ml of glacial acetic acid in 1L of water).

Standard vitamin C solution (already prepared): This contain 0.015g of vitamin C in

1L of acetic acid.

Method

Standardisation of the dye solution

Pipette 25ml of the dye solution into 250ml conical flask. Fill a burette with the

standard vitamin C solution and run into the dye solution. At first the dye will go

pink and then decolorize. Note the volume of vitamin C to decolourise the dye.

You are advised to do a rough titration first and then repeat at least twice until you

obtain two readings that are no more than 0.1ml different from each other.

Appendices

101

Determination of Vitamin C in samples

Get some juice from the sample and filter it. Take 2.0ml of juice and dilute to

50ml with the 10% acetic acid. Place this solution in the burette. Now take a 25ml

portion of the dye solution and titrate against the solution in the burette. Calculate

the value of vitamin C in the lemon.

Repeat the above but using the vitamin C supplement, take 25ml of stock provided

and dilute to 250ml accurately with 10% of acetic acid. Place this solution in the

burette and titrate with the dye as above.

Appendices

102

Appendix B: Data of Heat Pump Drying Experiment

Table B - 1: Heat pump drying of guava

Time (min) Normal Air(g) Nitrogen(g) Carbon Dioxide(g)0 100.00 100.20 99.8530 87.85 92.63 88.2160 84.68 80.85 79.2090 74.66 71.09 70.02120 67.57 61.49 63.82150 61.64 55.77 56.94180 61.89 48.45 51.07210 52.63 42.31 45.89240 50.04 35.83 38.41270 44.61 31.20 35.61300 41.27 29.26 31.62330 38.35 25.73 27.56360 36.27 23.63 26.87390 33.43 20.34 24.20420 30.51 17.90 20.74450 26.33 13.34 19.26480 24.33 11.32 17.34

Table B - 2: Heat pump drying of papaya

Time (min) Normal Air(g) Nitrogen(g) Carbon Dioxide(g)0 116.45 118.18 105.4930 92.61 99.94 91.2860 82.69 86.29 80.9790 77.00 74.48 71.80120 69.36 69.67 64.00150 60.31 60.81 56.96180 55.17 53.74 52.90210 53.42 52.24 47.08240 49.23 47.24 44.59270 42.63 43.84 41.28300 41.28 40.16 37.12330 38.38 37.50 34.99360 34.16 34.48 31.53390 32.77 31.91 30.52420 29.97 30.69 27.97450 26.15 27.94 25.76480 24.74 25.94 21.39

Appendices

103

Table B - 3: Heat pump drying of ginger

Time (min) Normal Air (g) Nitrogen (g) Carbon Dioxide (g)0 98.57 94.86 99.7530 90.05 82.32 90.1260 80.80 76.61 81.2190 72.89 68.11 68.23120 67.75 64.04 63.23150 61.47 60.31 59.35180 55.30 52.44 51.46210 49.87 47.28 45.34240 43.18 42.08 41.35270 38.29 37.56 35.35300 33.98 32.08 30.35330 28.98 27.57 27.04360 26.71 24.47 24.35390 23.81 23.26 23.21420 21.36 20.35 21.37450 19.35 18.33 17.65480 16.80 14.32 15.23

Appendices

104

Appendix C: Data of Colour

Table C - 1: Colour parameter of papaya

Methods L* a* b* C* h*18.8893 21.1287 15.243 26.0532 35.808122.2432 25.5217 19.465 32.0974 37.332123.1767 26.3373 20.8911 33.6169 38.421932.7482 29.491 28.6385 41.1082 44.1598

CO2

19.2954 15.9814 12.4425 20.2539 37.903166.7547 13.7352 27.4857 30.7265 63.447761.4565 16.856 29.6211 34.0813 60.357862.7233 15.5383 29.3245 33.1868 62.08261.9291 15.5355 28.6758 32.6136 61.5528

Freeze

64.7815 14.9637 27.8711 31.634 61.769223.811 22.349 22.1912 31.4949 44.7971

22.7651 17.5497 16.9359 24.3888 43.980322.1 18.4235 17.3034 25.2751 43.2041

15.6328 18.5799 17.4526 25.4913 43.208N2

30.0255 24.6429 25.0803 35.161 45.503931.2493 24.8649 30.5119 39.3604 50.822635.2169 25.9032 27.0019 37.4176 46.189724.987 28.4138 28.1244 39.9791 44.7067

29.7507 29.529 28.537 41.0649 44.0212Normal Air

21.555 24.8039 20.0951 31.9225 39.012954.6891 26.4679 35.6885 44.4322 53.438141.5239 28.2894 33.2584 43.6625 49.615755.9795 24.7579 34.7577 42.6738 54.537747.001 26.4327 44.2877 51.5761 59.1695

Vacuum

39.1117 29.4819 33.3018 44.4769 48.481731.5632 10.0204 14.9742 18.0176 56.210630.034 12.3522 14.3469 18.9317 49.2726

28.6124 9.6128 12.0233 15.3937 51.357329.817 12.1987 14.3926 18.8668 49.7165

Fresh

36.3371 10.0551 18.5458 21.0963 61.5346

Appendices

105

Table C - 2: Colour parameter of guava

Method L* a* b* c* h*53.8 10.46 35.07 36.59 73.3958.99 3.83 26.86 27.13 81.8953.24 7.46 31.67 32.53 76.7457.82 10.06 35.99 37.37 74.39

Normal Air

59.47 10.54 33.53 35.15 72.5551.93 8.24 33.36 34.36 76.1249.87 7.2 25.94 26.92 74.4956.31 5.16 25.43 25.95 78.5362.15 3.53 19.08 19.4 79.51

CO2

66.08 4.42 28.91 29.25 81.362.29 3.36 20.68 20.95 80.7653.63 3.9 24.08 24.4 80.7962.09 3.67 22.39 22.69 80.751.7 3.86 22.21 22.55 80.15

N2

58.72 4.96 24.7 25.19 78.6581.68 -0.17 8.62 8.62 91.1479.72 -0.37 7.06 7.07 92.9682.96 -0.43 8.88 8.89 92.7982.36 -0.34 7.92 7.93 92.42

Freeze

79.49 -0.52 9.44 9.45 93.1663.94 6.16 31.17 31.77 78.8357.04 7.64 31.14 32.07 76.2258.75 7.41 32.99 33.81 77.3556.86 7.75 32.51 33.42 76.6

Vacuum

53.45 6.69 29.39 30.14 77.1867.22 -1.13 6.19 6.29 100.3563.24 -0.64 5.03 5.07 97.2467.13 -0.86 5.64 5.7 98.6568.88 -0.57 4.38 4.42 97.42

Fresh

64.42 -0.71 5.46 5.51 97.43

Appendix

106

Appendix D: Data of Rehydration


0.0000

1.0000

2.0000

3.0000

4.0000

5.0000

6.0000

7.0000

0 20 40 60 80 100

Time (mins)

Moi

stur

eco

nten

t(d.

b)


Figure D - 1: Rehydration of dried Papaya at 40°C


0.0000

1.0000

2.0000

3.0000

4.0000

5.0000

6.0000

7.0000

0 20 40 60 80

Time (mins)

Moi

stur

e(d

.b)


Figure D - 2: Rehydration of dried papaya at 60 °C

Appendix

107

Rehydration of dried papaya at 80 Degree

0.0000

1.0000

2.0000

3.0000

4.0000

5.0000

6.0000

7.0000

0 20 40 60

Time (mins)

Moi

sture

cont

ent(

d.b)


Figure D - 3: Rehydration of dried papaya at 80 °C

Rehydration of vacuum dried papaya

0

1

2

3

4

5

6

0 50 100 150 200

Time(mins)

Moi

sture

cont

ent(

d.b)

40 Degree

60 Degree80 Degree

Figure D - 4: Rehydration of vacuum dried papaya

Appendix

108

Rehydration of freeze dried papaya

0

1

2

3

4

5

0 50 100 150 200

Time(mins)

Moi

stur

eco

nten

t(d.b

)

40 Degree60 Degree80 Degree

Figure D - 5: Rehydration of freeze dried papaya

Rehydration curve for vacuum-dried guava

00.5

11.5

22.5

33.5

4

0 50 100 150

Time of rehydration (min)

Moi

sture

cont

ent(

g/g

dry)

40°C 60°C 80°C

Figure D - 6: Rehydration curve for vacuum-dried guava

Appendix

109

Rehydration curve for normal air-dried guava

00.5

11.5

22.5

33.5

4

0 50 100 150


Moi

sture

cont

ent(

g/g

db)

40°C 60°C 80°C

Figure D - 7: Rehydration curve for normal air-dried guava

Rehydration curve for nitrogen-dried guava

0

0.51

1.5

2

2.53

3.5

0 50 100 150 200


Moi

stur

eco

nten

t(g/g

db)

40°C 60°C 80°C

Figure D - 8: Rehydration curve for nitrogen-dried guava

Appendix

110

Rehydration curve for carbon dioxide-dried guava

00.5

11.5

22.5

33.5

0 50 100 150 200


Moi

stur

eco

nten

t(g/

gdb

)

40°C 60°C 80°C

Figure D - 9: Rehydration curve for carbon dioxide-dried guava

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