UNIT 4 - Yola · period of time to saturated pressurised steam (about 0.1 - 0.6 MPa according to the product), followed by an abrupt pressure drop ... FBD. Pre & post drying treatment

UNIT 4

Learning outcomes

At the end of this chapter, student should be able :

1) Explain basic principle in drying agriculture produce and application of main group of dryers.

2) Explain operation of mechanical and solar dryer.3) Describe the effect of phytochemical loss during drying and seek

measures to minimize their losses during this process.

Processing of foodto avoid the spoilage of foods due to various reasons; to increase shelf life;to make food products available through out the year The spoilage could be due to physical damage, chemical damage, microbialattack Various food processing methods –Freezing, canning, preserving in syrup, food irradiation, salting, vacuum packaging,dehydrationDrying/Dehydration –very much cost‐effective; product takes much less storage space than cannedor frozen foods; Some dehydrated products have very good rehydration properties

Principles of dryingDrying involves removing water from the food product into thesurrounding air. For effective drying, air should be hot, dry and moving.These factors are inter-related and it is important that each factor iscorrect:

air must be dry, so it can absorb the moisture from the fruits andvegetablesheating the air around the product causes it to dry more quicklyif the air is not moving across the food, it cannot get rid of thewater vapour that it has collected. A fan or air blower is neededto keep the air circulating.

- In summary – when food is dried, hot dry air comes into contact with the food. The hot airabsorbs water from the food and is moved away from the food. New dry air takes its place andthe process continues until the food has lost all its water.

Measuring drying rate

• If a new type of dryer is to be used, or if a different type of food is tobe dried, it is necessary to carry out a few experiments to find therate of drying. The information can then be used to find the time thatthe food should spend in the dryer before the moisture content islow enough to prevent spoilage by micro-organisms.

• The rate of drying also has an important effect on the quality of the dried foods and (in artificial dryers) the fuel consumption. To find the rate of drying you will need a clock or watch and a set of scales.

How to measure drying rate

1) Weigh the food, 2) Place it in the dryer and leave for 5-10 minutes. 3) Remove the food, reweigh it and replace in the

dryer.4) Continue like this until the weight of the food does

not change any more (it has reached a constant weight and all the moisture has been removed).

• Drying rate is obtained by calculating the drying rate for each 10 minute period as follows:

Drying rate = initial weight - final weight time interval (eg 10 minutes)

• The moisture content of both the fresh food and the final dried food can be found by weighing the food, heating at 100°C in an oven for 24 hours and reweighing. The moisture content is found as follows:

Moisture content (%) = initial weight - final weight x 100 initial weight

Do you know what is this?

Moisture Content Analyzer

Factor that control drying rate?…particle size…rate that moisture can move from the interior of a piece of food to the surface.the shorter the distance that moisture has to travel, the faster the drying rate. For this reason, wherever possible, products should be cut into smallpieces prior to drying. Reducing the size also increase the surface areaof the food in relation to the volume of the pieces, which increases therate at which water can be evaporated from the food.

To ensure safe storage the final moisture content of the food should be less than ;20% for fruits and meat10% for vegetables10-15% for grains

The stability of a dried food during storage depends on its moisturecontent and the ease with which the food can pick up moisture fromthe air. Clearly the risk of moisture pick up is greater in regions of highhumidity!For foods that readily pick up moisture it is necessary to package themin a moisture proof material.

Why so many dryer types ?• Different sources of energy input( conduction, convection, radiation,etc)• Energy input continuous or intermittent (occurring at irregular intervals; not continuous or steady)• Batch, continuous or semi‐continuous operation• Quality is key parameter for many products• Need to reduce the cost• Need to consider drying system rather than dryer, i.e. Pre‐ and postdryingstages are important and often cost more than dryer• Environmental regulations demand new drying techniques

Oven dryer

Infrared dryer

Vacuum dryer

Solar dryer

1) Thermal (heat/gas) based

Hot air is used both to supply the heat for evaporation and to carry away the evaporated moisture from the product.In convective drying, heat transfer will occur through flow of heat. Mass transfer in the drying of solid depends on 2 mechanism :i) Movement of moisture internally within the solidii) Movement of water vapor from the material surface

Application : Oven dryer

Comparison gas dryer & electric dryer

2) Solar basedSolar dryers operate by raising the temperature of the air to between 10-30ºC above room temperature. This makes the air move through the dryer and also reduces its humidity.

Types of solar dryer:

1) Direct - expose the substance to be dehydrated to direct sunlight.2) Indirect - the black surface heats incoming air, rather than directly

heating the substance to be dried.* Refer group presentation

There are advantages to solar drying as follows:

• The combination of higher temperature, movement of the air and lower humidity increases the rate of drying.

• Food is enclosed in the dryer and therefore protected from dust, insects, birds and animals.

• The higher temperature deters insects and the faster drying rate reduces the risk of spoilage by micro-organisms.

• The dryers are water proof and the food does not therefore need to be moved when it rains.

Direct solar dryer mechanism

Indirect solar dryer

Solar dryer

3) Infrared based

Infrared heating or drying involve a heat transfer by radiation betweena hot element and a material at lower temperature that needs to beheated or dried. The peak wavelength of the radiation is dependant ofthe temperature of the heated element.The heat energy is directly transfered from the infrared source to theproduct to be heated without the need of an intermediate such as airor water.It is important to select the infrared emitters with the suitable spectraaccording to the product to have an optimized absorption of theradiation that will be subsequently converted into heat.*Refer group presentation

Infrared dryer

Microwave and Infrared Radiationsimilarities• Both are forms of electromagnetic radiation• Both are transmitted as waves• Waves are converted to heat

Differences between Microwave and IR

• Specified/specific frequency bands (915 MHz)• Deeper penetration in food• Induce molecular friction in water• Extent of heating depends on MC of food• Heating throughout the food

Less controlled and wider range of frequency• Shallower penetration• Radiation is absorbed and converted to heat• Heating depends on surface characteristics of food and color of food• Heating is limited to surface of food

Microwave

Infrared

4) Abrupt pressure drop & vacuum basedAbrupt pressure drop• Instant Controlled Pressure Drop process, is based on the thermo-

mechanical effects induced by subjecting the raw material for a shortperiod of time to saturated pressurised steam (about 0.1 - 0.6 MPaaccording to the product), followed by an abrupt pressure droptowards a vacuum (about 5 kPa). This abrupt pressure drop triggerssimultaneously auto vaporization of volatile compounds, swelling,possibly rupturing some cell walls and instantaneously cooling theproducts, which stops thermal degradation.

*Refer group presentation

4) Abrupt pressure drop & vacuum basedVacuum based• Vacuum drying is the mass transfer operation in which the moisture present in a

substance usually wet solid is removed by means of creating vacuum.

• vacuum drying is generally used for the drying of those substances which arehygroscopic and heat sensitive and is based on the principle of creating vacuum.With the help of vacuum ;

1) the pressure is reduced around the substance to be dried2) and boiling point of water inside that product decreases3) and rate of evaporation of water increases significantly4) thus increasing drying rate.

• Application : Vacuum dryer*Refer group presentation

Vacuum dryer

• Most cost effective drying method are solar drying. Solar dryer is leastexpensive, followed by convection oven, fluidized, etc.

• The drying method of choice is generally based on the characteristicof the product and relative ability to retain or reducing loss of plant’sphytochemical.

GROUP PRESENTATION

• Find a journal/article that study the effect of drying in reduction of phytochemical content.

- name the compound (at least 5 compound) that is heat sensitive

Dryers-system & designHigh temperature mechanized dryer (eg oven,cabinet,thermalhouse)i) Forced air flow systemii) Moist air exhaustiii) Hot circulation

Intermediate low temperature mechanized dryer (eg fluidized bed dryer)-Principle : quantity of fresh produce in a holding vessel is placed under pressurized conditions to cause the product particles to continuously ‘jump/float’ similar to free flow fluid behavior under gravity.

FBD

Pre & post drying treatment for phytochemical preservationa) Differences in pre cooling requirement for various herbRole of cool temperature before drying ;- Helps in stabilizing cell structure- Temperature for cooling; (0 - 10◦C) - Pre & post cooling handling for leafy, root, stem based cropsb) Gradual/stage drying- Role of tempering ( cooling in between drying process) in stabilizing

cellular structure & phytochemical evaporation.