ﺍﻟﺮﺣﻴﻢ ﺍﻟﺮﺣﻤﻦ ﺍﷲ ﺑﺴﻢEFFECT OF FRYING PROCESS ON PHYSICOCHEMICAL CHARACTERISTICS OF CORN AND SUNFLOWER OILS By Mohammed Salaheldin Mustafa Elhassan B.Sc. Agric. (Honours) Faculty of Agriculture University of Khartoum (2004) Supervisor Dr. Hassan Ali Mudawi A thesis submitted to the Graduate College, University of Khartoum for partial fulfillment for requirements for Master Degree of Food Science and Technology Department of Food Science and Technology Faculty of Agriculture University of Khartoum June 2008
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بسم اهللا الرحمن الرحيم
EFFECT OF FRYING PROCESS ON PHYSICOCHEMICAL CHARACTERISTICS OF CORN AND SUNFLOWER OILS
By Mohammed Salaheldin Mustafa Elhassan
B.Sc. Agric. (Honours) Faculty of Agriculture
University of Khartoum (2004)
Supervisor Dr. Hassan Ali Mudawi
A thesis submitted to the Graduate College, University of Khartoum for partial fulfillment for requirements for Master Degree
of Food Science and Technology
Department of Food Science and Technology
Faculty of Agriculture University of Khartoum
June 2008
أ
DEDICATION
To my Parents, my brother, my sisters and All those who taught me a letter
ب
ACKNOWLEDGEMENTS
I am indebted to "Alla" who granted me every thing including the
mind, health and patience to accomplish this work.
I wish to express my deepest gratitude to my supervisor
Dr. Hassan Ali Mudawi for his supervision, encouragement and
support, through the duration of this work.
I express my thanks to Dr. Ahlam Ahmed, Food Research Centre,
Dr. Asha Fagir, Faculty of Agriculture, University of Khartoum.
due Thanks to my cousin Rania Mustafa, Chemical Engineer, Mona
Tag elSir and Mr. Omer Carbose, Arabian Company for Plant Oils
Production, my friend Mr. Police 2nd lieutenant, Osman Abdel Rahman,
all my colleagues and my friends specially the eighth batch in the Faculty
of Agriculture and fourth batch of M.Sc. in Food Science, University of
Khartoum. Also I express my thanks to all staff of agricultural engineers
in administration of agriculture in shendi town.
My thanks also due to all those who helped me to achieve this
study.
ت
LIST OF CONTENTS
Page DEDICATION i ACKNOWLEDGEMENTS ii LIST OF CONTENTS iii LIST OF TABLES v LIST OF CURVES vi ABSTRACT vii ARABIC ABSTRACT viii CHAPTER ONE: INTRODUCTION 1 CHAPTER TWO: LIETERATURE REVIEW 4 2.1 Quality of frying oils 4 2.2 Oxidation of oils during frying 6 2.3 Measurement parameters of oils oxidation 9
2.3.1 peroxide value 9 2.3.2 Iodine value 11
2.4 Hydrolysis of oils 13 2.5 Measurement parameters of oils hydrolysis 14
2.5.1 Free Fatty Acids 14 2.6 Polymerization of oils 16 2.7 Measurement parameters of oils polymerization 18
2.7.1 The effectiveness on the oil polymer content during frying 18 2.8 The effectiveness on the oil fatty acid composition during frying 20 2.9 The effectiveness on the oil viscosity during frying 21 2.10 The effectiveness on the oil refractive index during frying 23 2.11 The effectiveness on the oil colour during frying 24 2.12 Factors affecting the quality of oil during frying 26
2.12a Replenishment of fresh oil 26 2.12b Frying time and temperature 27 2.12c Quality of frying oil 27 2.12d Compositions of foods 30 2.12e Types of fryer 31 2.12f Antioxidants 31 2.12g Dissolved oxygen contents in oil 34
CHAPTER THREE: MATERIALS AND METHODS 34 3.1 Materials 34
3.2.1 Raw materials 34 3.2.2 Chemicals and reagents and instruments 34
3.2 Preparation of raw materials 34 3.3 Frying processes 34 3.4 Analytical procedures 35
3.4.1 Oil characteristics 35 3.4.1.1 Refractive index 35
ث
3.4.1.2 Colour of oil 35 Page 3.4.1.3 Viscosity 36 3.4.1.4 Peroxide value (PV) 36 3.4.1.5 Free fatty acids (FFA) 37
3.5 Statistical analysis 38 CHAPTER FOUR: RESULTS AND DISCUSSIONS 39 4.1 Physicochemical properties of corn and sunflower oils 39 4.2 Changes in physical parameters of oils during frying 39
4.2.1 The effectiveness on the oil refractive index during frying 39
4.2.2 The effectiveness on the oil viscosity during frying 39 4.2.3 The effectiveness on the oil colour during frying 45
4.3 Changes in chemical parameters of oils during frying 45 4.3.1 The effectiveness on the oil peroxide value during
frying 45 4.3.2 The effectiveness on the oil free fatty acids value
during frying 52 CHAPTER FIVE: CONCLUSIONS AND RECOMMENDATIONS 53 5.1 Conclusions 53 5.2 Recommendations 54 References 55 Appendices
ج
LIST OF TABLES
Table Page
1 Physicochemical properties of corn and sunflower oils
40
2 The effectiveness on the oil refractive index during frying
41
3 The effectiveness on the oil viscosity during frying
43
4 The effectiveness on the oil colour during frying 46
5 The effectiveness on the oil peroxide value during frying
48
6 The effectiveness on the oil free fatty acids value during frying
50
ح
LIST OF FIGURES
Figure Page
1 The effectiveness on the oil refractive index during frying 42
2 The effectiveness on the oil viscosity during frying 44
3 The effectiveness on the oil colour during frying 47
4 The effectiveness on the oil peroxide value during frying 49 5 The effectiveness on the oil free fatty acids value during
frying 51
خ
ABSTRACT
This study has been conducted to explore the effect of frying on
the physical and chemical characteristics of corn and sunflower oils.
For the purpose of this study, corn and sunflower oils and potatoes
were obtained from the local market, the oils was reused for frying five
days consecutively
After each frying process a sample of oil was taken daily after it
cools. These samples were physically and chemically tested, all the
physicochemical characteristics were changed.
The colour of corn oil increased from 0.9 to 6.4, and the colour of
sunflower oil increased from 0.4 to 5.1. The refractive index of corn oil
Increased from 1.4750 to 1.4820, and in sunflower oil it increased from
1.4750 to 1.4810. Viscosity of corn oil Increased from 28.7 to 31.0,
Viscosity of sunflower oil Increased from 28.3 to 31.0.
The free fatty acids increased in corn oil, from 0.125 to 200.0
whereas in sunflower oil they increased from 0.110 to 0.150
In case of the peroxide value in the both oils the study showed that
it was not constant. It worth mentioning here that the frying process
followed in this study is the same that is usually used at homes.
د
لدراسةخالصة ا
فيزيائية على الخواص الكيميائية وال)التحمير( أجريت هذه الدراسة لمعرفة أثر القلي
.لزيت الذرة وزهرة الشمس
تم الحصول على زيت الذرة وزيت زهرة الشمس من السوق المحلي، وأجري تحمير
بطاطس لخمس مرات بمعدل مرة كل يوم، ومن ثم تم سحب عينة العلى كل منهما لشرائح
.بعد كل تحميرة بعد أن يبرد الزيت وأجريت التحاليل الفيزيائية والكيميائية عليها
0.4 اما لون زيت زهرة الشمس ازداد من 6.4 الي 0.9ره ازداد من لون زيت الذ
ومعامل انكسار زيت 1.4820 الي 1.4750ره ازداد من زيت الذاما معامل انكسار. 5.1الي
ره ازدادت من اللزوجه بالنسبه لزيت الذ. 1.4810 الي 1.4750زهرة الشمس ازداد من
.31.1 الي 28.3بالنسبه لزيت زهرة الشمس ازدادت اللزوجه من و31.1 الي 28.7
في زيت زهرة و0.200 الي 0.125ره من الحموض الدهنيه الحره في زيت الذازدادت
ت الدراسه انه غير بالنسبه لرقم البيروكسيد اوضح .0.150 الي 0.110لشمس ازدادت من ا
.ثابت اثناء التحمير
.والجدير بالذكر أن هذه التجربة أجريت على حسب ما يحدث في القلي المنزلي .
1
CHAPTER ONE
INTRODUCTION
Fat frying is one of the oldest and popular food preparations. Fried
foods have desirable flavor, colour, and crispy texture, which make deep-
fat fried foods very popular to consumers (Boskou et al., 2006). Frying
is a process of immersing food in hot oil with a contact among oil, air,
and food at a high temperature of 150 to 190°C. The simultaneous heat
and mass transfer of oil, food, and air during fat frying produces the
desirable and unique quality of fried foods. Frying oil acts as a heat
transfer medium and contributes to the texture and flavor of fried food.
Fat frying produces desirable or undesirable flavor compounds and
changes the flavor stability and quality of the oil by hydrolysis,
oxidation, and polymerization. Tocopherols, essential amino acids, and
fatty acids in foods are degraded during deep-fat frying. The reactions in
deep-fat frying depend on factors such as replenishment of fresh oil,
frying conditions, original quality of frying oil, food materials, type of
fryer, antioxidants, and oxygen concentration. High frying temperature,
the number of fryings, the contents of free fatty acids, polyvalent metals,
and unsaturated fatty acids of oil decrease the oxidative stability and
flavor quality of oil. Antioxidant decreases the frying oil oxidation, but
the effectiveness of antioxidant decreases with high frying temperature.
Lignan compounds in sesame oil are effective antioxidants in deep-fat
frying.
Frying time, food surface area, moisture content of food, types of
breading or battering materials, and frying oil influence the amount of
2
absorbed oil to foods (Moreira et al., 1997). The oil contents of potato
chips, corn chips, tortilla chips, doughnuts, French fries, and fried noodle
(ramyon) are 33 to 38%, 30 to 38%, 23 to 30%, 20 to 25%, 10 to 15%
(Moreira and others 1999a), and 14% (Choe et al., 1993), respectively.
The absorbed oil tends to accumulate on the surface of fried food during
frying in most cases and moves into the interior of foods during cooling
(Moreira and others 1997).
Foods fried at the optimum temperature and time have golden
brown colour, are properly cooked, and crispy, and have optimal oil
absorption (Blumenthal, 1991). However, underfried foods at lower
temperature or shorter fying time than the optimum have white or slightly
brown color at the edge, and have ungelatinized or partially cooked
starch at the center. The underfried foods do not have desirable deep-fat
fried flavor, good color, and crispy texture. Overfried foods at higher
temperature and longer frying time than the optimum frying have
darkened and hardened surfaces and a greasy texture due to the excessive
oil absorption.
Deep-fat frying produces desirable or undesirable flavor
compounds, changes the flavor stability and quality, color, and texture of
fried foods, and nutritional quality of foods. The hydrolysis, oxidation,
and polymerization of oil are common chemical reactions in frying oil
and produce volatile or nonvolatile compounds. Most of volatile
compounds evaporate in the atmosphere with steam and the remaining
volatile compounds in oil undergo further chemical reactions or are
absorbed in fried foods. The nonvolatile compounds in the oil change the
physical and chemical properties of oil and fried foods. Nonvolatile
3
compounds affect flavor stability and quality and texture of fried foods
during storage. Deep-fat frying decreases the unsaturated fatty acids of
oil and increases foaming, color, viscosity, density, specific heat, and
contents of free fatty acids, polar materials, and polymeric compounds.
Frying temperature and time, frying oil, antioxidants, and the type
of fryer affect the hydrolysis, oxidation, and polymerization of the oil
during frying. This review focuses on the chemical reactions of frying
oil and improvement of the oxidative stability and flavor quality of frying
oil during deep-fat frying (Choe and Min, 2007).
The Objective of this Study is:
- To explore the effect of Frying on the Physical and Chemical
Characteristics of Corn and Sunflower oils.
4
CHAPTER TWO
LITERATURE REVIEW
2.1 Quality of frying oils:
Free fatty acids increase the thermal oxidation of oils, and their
unsaturation rather than chain length led to significant effects on
thermooxidative degeneration; the addition of 0.53 mmol of tridecanoic,
palmitic, and oleic acids to virgin olive oil showed 15.0, 14.3, and 10.1 h
of induction period with a Rancimat (Metrohm) (Frega et al., 1999).
Stevenson and others (1984) and Warner and others (1994) reported that
the oxidation rate of oil increased as the content of unsaturated fatty acids
of frying oil increased. This explains why corn oil with less unsaturated
fatty acid is a better frying oil than soybean or canola oils with more
unsaturated fatty acids (Warner and Nelsen, 1996). The content of
linolenic acid is critical to the frying performance, the stability of oil, and
the flavor quality of fried food. Total polar compounds contents of
(PG), and tert-butylhydroquinone (TBHQ) slow down the oxidation of
oil at room temperature. However, they become less effective at frying
32
temperature due to losses through volatilization or decomposition. Tyagi
and Vasishtha (1996) reported the ineffectiveness of 0.01% BHA and
TBHQ during deep-fat frying of potato chips in soybean oil. The
decompositions of tocopherols in soybean oil, beef tallow, and palm oil
after 8-h frying of steamed noodles at 150°C were 12.5, 100, and 100%,
respectively. The retention of tocopherols in soybean oil decreased the
oxidation of soybean oil than beef tallow or palm oil without any
retention of tocopherols. Palm oil contained tocotrienols in addition to
tocopherols at 169 ppm, all of which was decomposed during 8-h frying
of steamed noodles. Soybean oil contained more unsaturated fatty acids
than beef tallow or palm oil.
Carotenes do not protect the oil from thermal oxidation in the
absence of other antioxidants. Carotenes are major compounds that react
with oil radicals in red palm olein. Tocotrienols regenerate carotenes
from carotene radicals. The combination of tocotrienols and carotenes
decreased the oxidation of oil synergistically during frying of potato
slices at 163°C.
Lignan compounds in sesame oil, sesamol, sesamin, and
sesamolin, are stable during heating and contribute to high oxidative
stability of roasted sesame oil during heating at 170°C. The blended
soybean oil with roasted sesame oil lowered the formation of conjugated
dienoic acids than soybean oil during frying at 160°C in spite of the
higher unsaturated fatty acids of the blended oil than soybean oil. As the
sesame oil contents in the blended oil increased, the formation of
conjugated diene decreased, possibly due to the antioxidants in sesame
33
oil. The addition of sesame oil and rice bran oil enhanced the oxidative
and flavor stability of high oleic sunflower oil, possibly due to the
avenasterol, which is stable at high temperature (Choe and Min, 2007).
Ascorbyl palmitate lowered dimers in oil during deep-fat frying.
Sterols and their fatty acid esters improved the oxidative stability of oil
during deep-fat frying. Silicone protected the oil from oxidation during
deep-fat frying (Choe and Min, 2007). By the formation of a protective
layer at the air-oil interface and the low convection currents of frying oil .
The combination of silicone and antioxidants (Choe and Min, 2007).
synergistically decreased the oxidation of oil during deep-fat frying.
Rosemary and sage extracts reduce oil deterioration during a 30-h
intermittent deep-fat frying of potato chips. the synergistic antioxidant
effects of rosemary, sage, and citric acid on palm olein during deep-fat
frying of potato chips. the hexane-extracts of burdock decreased the
formation of conjugated dienoic acids and aldehydes in lard significantly
(P< 0.05) at 160°C. The hexane-extract of burdock is a potential
antioxidant of oil for deep-fat frying (Choe and Min, 2007).
2.12.g Dissolved oxygen contents in oil:
Nitrogen or carbon dioxide flushing decreased the dissolved
oxygen in oil and reduced the oxidation of oil during deep-fat frying
.Carbon dioxide gives better protection from oxidation due to its higher
solubility and density than nitrogen. Przybylski and Eskin (1988)
suggested that a minimum of 15 min of nitrogen or 5 min of carbon
dioxide flushing prior to heating decreases the oxidation of oil during
deep-fat frying.
34
CHAPTER THREE
MATERIALS AND METHODS
3.1 Materials
3.1.1 Raw materials
Both refined, bleached and deodorized sunflower and corn oils
were purchased from local market. Potatoes used in the Frying
experiments were also purchased from local market.
3.1.2 Chemicals, reagents and instruments:
They are brought from University of Khartoum and Arabian
Company for Plant Oils Productions.
3.2 Preparation of raw materials:
Potato tubers were peeled, sliced to a thickness of 2 – 4 mm using
a manual slicer, dipped in water to reduce browning, then used in frying.
3.3 Frying processes:
Frying processes on the two oils were done simultaneously: 1.5 kg
oils were heated up to 180oC, two hundred grammes potato slices were
fried. The frying was carried out with sunflower and corn oils for five
consecutive days. At the end of each frying experiment, the oil was
allowed to cool; 100 gm oils were sampled from, in to a clean glass dark
container and stored in the deepfreeze until assessment.
35
3.4 Analytical procedures:
3.4.1 Oil characteristics:
3.4.1.1 Refractive index:
The refractive index (RI) was determined by Abbe 60 refracto-
meter as described by the A.O.A.C. method (2000). A double prism was
opened by means of screw head and few drops of oil were placed on the
prism. The prism was closed firmly by tightening the screw head. The
instrument was then left to stand for few minutes before reading in order
to equilibrate the sample temperature with that of the instrument (32 ±
2oC). The refractometer was cleaned between readings by wipping of the
oil with soft cloth, then with cotton moistened with petroleum ether and
left to dry.
3.4.1.2 Colour:
The colour intensity of oils was recorded using a Lovibond
Tintometer as units of red, yellow and blue according to the A.O.A.C.
method (2000).
Samples of oils were filtered through filter paper immediately
before testing. An appropriate cell (2" cell) was filled with oil and placed
in the tinomeer nearby the window for light. The instrument was
switched on and looked through the eye piece. The yellow colour was
adjusted to 25, then slides were adjusted until a colour match was
obtained from a combination of red and blue. The values obtained by
matching were recorded as red, yellow and blue.
36
3.4.1.3 Viscosity:
The viscosity of the oil samples was recorded using an Ostwald-U-
tube viscometer according to Cocks and Van Rede (1966). The
viscometer was suspended in the constant temperature bath (32 ± 2oC) so
that the capillary was vertical. The instrument was filled to the mark at
the top of the lower reservoir with the oil by means of pipette inserted
into the side arm, so that the tube wall above the mark is not wetted. The
instrument was then left to stand for few minutes before reading in order
to equilibrate the sample temperature with that of the instrument (32 ±
2oC). By means of the pressure on the respective arm of the tube, the oil
moved into the other arm so that the meniscus is 1 cm above the mark at
the top of the upper reservoir. The liquid was then allowed to flow freely
through the tube and the time required for the meniscus to pass from the
mark above the upper reservoir to that at the bottom of the upper
reservoir was recorded.
Calculation:
T – T0 V= T0 Where:
T : Flow-time of the oil T0: Flow-time of distilled water
3.4.1.4 Peroxide value (PV):
The PV of the oil samples was determined according to the
A.O.A.C. method (2000). One gram of the oil was accurately weighed
into 250 ml conical flask. Thirty ml of a mixture of glacial acetic acid
and chloroform (3:2) were added and the solution was swirled gently to
37
dissolve the oil. A 0.5 ml of 0.1N KI was added to the flask, and then the
contents of the flask were left to stand for one minute then adding 30 ml
of distilled water. The contents were titrated with 0.01N sodium
thiosulphae until the yellow colour almost disappeared. A 0.5 ml of 1%
starch solution was added, and the titration continued with vigorous
shaking until the blue colour completely disappeared. Volume of 0.01N
sodium thiosulphae required (a) were recorded. The same process was
repeated for blanks. Volume of 0.01N sodium thiosulphate required by
the blank (B) was recorded.
Calculation:
(b-a) x N x 1000 P.V. = S Where:
b : Volume of sodium thiosulphate required for blank (ml) a : Volume of sodium thiosulphate required for oil sample (ml) S : Weight of oil sample used (gm)
3.1.4.5 Free Fatty Acids (FFA):
FFA determination was carried out according the A.O.A.C.
method (2000). Five grams of the oil was weighed accurately into 250 ml
a conical flask. Fifty ml mixture of 95% alcohol and ether solvent (1:1)
were added. . The contents of the flask were then heated with caution
until the oil was completely dissolved the solution was neutralized after
addition of one ml of phenophtholin indicator. The contents of the flask
were then titrated with 01N KOH with continuous shaking until a pink
colour persisted for 15 seconds. The number of ml of 0.1N KOH required
(a) was recorded.
38
Calculation:
V (ml) x N x M.W. X 28.2 FFA= S Where:
V : Volume of KOH used (ml) N : Normality of KOH M.W. Molecular weight of oleic acid =28.2 S : Weight of oil sample used
3.5 Statistical analysis:
Triplicate of each sample was analyzed using statistical analysis
system. The analysis of variance was performed to examine the
significant effect in all parameters measured. Least significant difference
(LSD) was used to separate the means.
39
CHAPTER FOUR
RESULTS AND DISCUSSIONS
4.1 Physicochemical properties of sunflower and corn oils:
Table (1) shows the initial values of the physicochemical
properties of sunflower and corn oils used in experiments. The general
feature of the oils reflects good colors, acceptable levels of acidity and
peroxides.
4.2 Changes in physical parameters of oils during frying:
4.2.1 The effectiveness on the oil refractive index during frying:
Table (2) shows changes in refractive index (RI) of both oils
during frying of potato chips( corn oil, from 1.4750 in control sample to
1.4820 after the fifth frying process, sunflower oil, from 1.4750 in
control sample to 1.4810 after the fifth frying process). The RI exhibited
significant increase (P< 0.05) in both oils by frequent frying. Increase in
RI of oils by frying was reported recently by Tyagi and Vasishtha (1996)
in soybean oil.
4.2.2 The effectiveness on the oil viscosity during frying:
Table (3) shows changes in viscosity of corn and sunflower oils as
a result of frequent use in frying of potato chips(corn oil, from 28.7 in
control sample to 31.1 after the fifth frying process, sunflower oil, from
28.3 in control sample to 31.0 after the fifth frying process). The
viscosity increased significantly (P< 0.05) directly after second frying
time.
40
Table (1): Physicochemical properties of corn and sunflower oils
Parameter Corn oil Sunflower oil
Refractive index 1.4750 1.4750
Viscosity at 30 c 28.7 28.3
Yellow 25 25 colour
Red 0.9 0.4 Peroxide value (meq/kg) 1.0 1.0
Free fatty acids % 0.125 % 0.110 %
41
Table (2): The effectiveness on the oil refractive index* during frying
Frying times Corn oil Sunflower oil
0.0 1.4750e 1.4750d
1st 1.4760d 1.4760c
2nd 1.4763cd 1.4762c
3rd 1.4767bc 1.47675c
4th 1.4800b 1.4803b
5th 1.4820a 1.4810a
* values having different superscript letter(s) differ significantly (P< 0.05)
42
1.47
1.472
1.474
1.476
1.478
1.48
1.482
0 1 2 3 4 5
Frying time
Corn Sunflower
Fig. (1): The effectiveness on the oil refractive index during frying
43
Table (3): The effectiveness on the oil viscosity* during frying
Frying times Corn oil Sunflower oil
0.0 28.7e 28.3e
1st 28.8e 28.6e
2nd 29.3d 29.0d
3rd 29.9c 29.5c
4th 30.4b 30.1b
5th 31.1a 31.0a
* values having different superscript letter(s) differ significantly (P< 0.05)
44
26.527
27.528
28.529
29.530
30.531
31.5
0 1 2 3 4 5
Frying time
Corn Sunflow er
Fig. (2): The effectiveness on the oil viscosity* during frying
45
The increase in viscosity of both oils was more or less similar.
Increase in viscosity of oils by frying was found earlier by Thompson
et al. (1967) in cottonseed oil. Also increase in viscosity of oils by frying
was reported by Tyagi and Vasishtha (1996) in soybean oil.
4.2.3 The effectiveness on the oil colour during frying:
Table (4) shows changes in colour of corn and sunflower oils as a
result of frequent use in frying of potato chips (corn oil, from 0.9 in
control sample to 6.4 after the fifth frying process, sunflower oil, from
0.4 in control sample to 5.1 after the fifth frying process). The colour
increased significantly (P< 0.05) redness. The increase in corn oil was
more than sunflower oil. Increase in colour of oil by frying was reported
by Sonia and Badereldeen (1983), and was found earlier by Augustin
et al. (1987).
4.3 Changes in chemical parameters of oils during frying:
4.3.1The effectiveness on the oil peroxide value during frying:
Table (6) shows changes in peroxide value (PV) of corn and
sunflower oils as a result of frequent use in frying of potato chips(corn
oil, from 1.0 m.Eq/kg in control sample to11.9 m.Eq/kg after the third
frying process to 9.8 m.Eq/kg after the fourth frying process to 13.9
m.Eq/kg after the fifth frying process. Sunflower oil, from 1.0 m.Eq/kg in
control sample to 8.8 m.Eq/kg after the fourth frying process to 4.7
m.Eq/kg after the fifth frying process). The PV increased significantly
(P< 0.05) directly after second frying time. Similar increase in PV of
sesame oil were observed earlier by deep frying (Khattab et al., 1974).
46
Table (4): The effectiveness on the oil colour* during frying
Corn oil colour Sunflower oil colour Frying times
Yellow Red Yellow Red
0.0 25 0.9f 25 0.4f
1st 25 1.8e 25 1.2e
2nd 25 2.7d 25 1.9d
3rd 25 4.1c 25 2.8c
4th 25 4.9b 25 3.5b
5th 25 6.4a 25 5.1a
*Mean values having different superscript letter(s)
differ significantly (P< 0.05)
47
0
1
2
3
4
5
6
7
0 1 2 3 4 5
Frying time
Corn Sunflower
Fig. (3): The effectiveness on the oil colour* during frying
48
Table (5): The effectiveness on the oil peroxide value (m.Eq/kg)* during frying.
Frying times Corn oil Sunflower oil
0.0 1.0f 1.0f
1st 2.9e 2.2e
2nd 6.7d 4.0d
3rd 11.9b 6.9b
4th 9.6c 8.8a
5th 13.9a 4.7c
*Mean values having different superscript letter(s) differ significantly (P< 0.05)
49
0
2
4
6
8
10
12
14
0 1 2 3 4 5
Frying time
Corn Sunflower
Fig. (4): The effectiveness on the oil peroxide value (m.Eq/kg) during frying.
50
Table (6): The effectiveness on the oil free fatty acids value* during frying
Frying times Corn oil Sunflower oil
0.0 0.125f 0.110f
1st 0.130e 0.115e
2nd 0.135d 0.120d
3rd 0.180c 0.140c
4th 0.185b 0.145b
5th 0.200a 0.150a
*Mean values having different superscript letter(s)
differ significantly (P< 0.05)
51
00.020.040.060.080.1
0.120.140.160.180.2
0 1 2 3 4 5
Frying time
Corn Sunflower
Fig. (5): The effectiveness on the oil free fatty acids value* during frying
52
4.3.2 The effectiveness on the oil free fatty acids value during frying (FFA):
Table (7) shows changes in free fatty acids (FFA) of corn and
sunflower oils as a result of frequent use in frying of potato chips(corn
oil, from 0.125 in control sample to 0.200 after the fifth frying process,
sunflower oil, from 0.110 in control sample to 0.150 after the fifth frying
process). The FFA significantly decreased (P< 0.05) in both oils by
frequent frying. The increase of FFA in both oils was more or less
similar.
53
CHAPTER FIVE
CONCLUSIONS AND RECOMMENDATIONS
5.1 Conclusions:
Fat frying causes the hydrolysis, oxidation, and polymerization of
the oil. Hydrolysis increases the amount of free fatty acids, mono- and
diacylglycerols, and glycerols in oils. Oxidation occurs at a greater rate
than hydrolysis during deep-fat frying. Oxidation produces
hydroperoxides and then low molecular volatile compounds such as
aldehydes, ketones, carboxylic acids, and short-chain alkanes and
alkenes. Dimers and polymers are also formed in oil by radical and Diels-
Alder reactions during deep-fat. Replenishment of fresh oil, frying
conditions, quality of frying oil frying oil, food materials, fryer,
antioxidants, and oxygen concentration affect the quality and flavor of oil
during deep-fat frying. Intermittent frying with a lower turnover rate and
higher temperature accelerates the oxidation and polymerization of oil
during deep-fat frying. The frying quality of oil with high unsaturated
fatty acids and free fatty acids is not as good as oil with low unsaturated
fatty acids and free fatty acids. Addition of spinach and ginseng extract to
the dough increases the oxidative stability of oil. Frying in a fryer with a
small surface-to-volume ratio is desirable to slow down the oxidation of
frying oil. Tocopherols, BHA, BHT, PG, and TBHQ decrease oil
oxidation, but they become less effective at frying temperature. Lignan
compounds in sesame oil are more stable than tocopherols, BHA, BHT,
PG, and TBHQ at frying temperature and are effective antioxidants
during deep-fat frying. Desirable fried flavor compounds such as
54
hydroxynonenoic acid, 2, 4-decadienal, and nonenlactone are produced
during deep-fat frying at an optimal concentration of oxygen.
5.2 Recommendations:
- This study is conducted using one item of food which is potatoes,
while so many other items exist and they may give different results
if tested, especially in connection with physical characteristics.
Therefore, similar studies using other food items are
recommended.
- More research is needed in this field to cover more times of points
and food products.
- Fatty acids composition and iodine value are important to test in the
next researches.
55
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Appendix:
Figure 1: Physical and chemical changes of oil during deep-fat frying (Choe and Min, 2007).
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Figure 2: Free fatty acid formation in soybean and sesame oil mixture during consecutive frying of flour dough at 160 °C (Chung et al. 2004)
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Figure 3: The initiation, propagation, and termination of thermal oxidation of oil(Choe and Min,2007).
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Figure 4: Acyclic polymer formation from oleic acid during deep-fat frying(Choe and Min,2007).
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Figure 5: Polymer contents of cottonseed and corn oils heated at 190°C and 204°C (Takeoka et al. 1997)
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Figure 6: Formation of cyclic dimers and polymers from linoleic acids during deep-fat frying(Choe and Min, 2007).
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Figure 7: Cyclic compound formation from linoleic acid by Diels-Alder reaction during deep-fat frying(Choe and Min,2007).