Processing of Horticultural Crops

Factors affecting dehydration

Factors affecting dehydration
Important factors affecting the rate of drying are:

1. Initial moisture content of the raw material
2. Composition of raw material
3. Initial load of the food kept in drier
4. Size, shape and arrangement of stacking of the raw material
5. Temperature, relative humidity and velocity of air used for drying
6. Rate of heat transfer on the surface of the food
7. Pre-treatment of the raw material prior to drying (peeling, blanching, sulphuring etc.)

During drying process the control of air temperature and its circulation in the system is important. If temperature is too low and humidity is too high, the food will dry more slowly and microbial growth may occur. Conversely, if the temperature is too high in the beginning, a hard shell will develop on the surface of the food which will prevent the removal of moisture from the interior portion of the fruit and moisture will trap inside the food material. This condition is known as case hardening. Further, the temperature that is too high at the end of drying period causes the food to scorch. Temperature between 49^oC to 60^oC is recommended for drying of fruits and vegetables. Temperature up to 65^oC may be used at the beginning, but should be lowered as food begins to dry. While, during the last hour of drying period, the temperature should not exceed 55^oC.

Procedure for drying
Drying of fruit and vegetables generally involves three stages: pre-drying treatments, drying and post drying handling, packaging and storage. Common flow-sheet consisting of different unit operations for drying of fruits and vegetables is shown in Fig 10.2.

1. Pre-drying treatments: Fruit and vegetables are selected and sorted according to size, maturity and soundness. They are then washed in running water to remove dust, dirt, insects, mould spores, plant parts, soils, debris and other materials that might contaminate or affect colour, aroma, flavor or taste of final product. Depending upon the type and quantity of produce to be dried, any method of peeling can be selected like hand peeling, steam, hot water, lye peeling or abrasive peeling. Fruits like grapes, plums, apricot are dipped in boiling caustic soda (0.5% NaOH) for few seconds and immediately placed in cold water to remove the waxy layer (grape, plum), pubescence (apricot, peach). After peeling and washing, the fruit and vegetables are sliced or cut into desired size and thickness as it affects the rate of drying (grapes, plums, apricots are dried whole).

2. Blanching: Generally all vegetables and mushrooms either whole or slices after preliminary preparations are blanched in boiling water or under steam to pre-determined period followed by immediate cooling to inactivate enzymatic activity prior to drying.

3. Sulphuring /Sulphiting: Majority of fruits are treated with sulphur dioxide by placing them in an enclosed chamber in which sulphur (3 g/kg fruit) is burnt to allow the SO₂ fumes to be absorbed by the fruits. The process is called as sulphuring or sulphur fumigation. Sulphiting on the other hand refers to dipping of prepared fruit or vegetables in a solution of potassium meta-bi-sulphite to serve the same purpose as that of sulphuring. Sulphuring or sulphiting helps to preserve colour, retard browning, reduces destruction of carotene and ascorbic acid besides checking spoilage of the dried product.

2. DRYING METHODS
a) Sun Drying: It consists of spreading fruit/vegetable either on roof tops or floor for drying in open sun. This method is limited to certain fruits such as raisins, figs, apricots, dates, peaches and salted fish. After 10-12 days of drying the product is packaged in gunny bags/wooden boxes and sent to market for local or distant sale. The moisture content is generally not lowered below 15% which is too high for storage. The quality of product is inferior, characterized by brownish outer appearance and contaminated with insect dirt/dust particles. The quality can be improved by spreading the produce on black polythene sheet and covering it with thin muslin cloth to avoid entry of insects, dust or dirt particles on to the product.

b) Solar drier: Solar drier is an inclined rectangular box of 1.8 x 0.9 x 0.3 cubic meters. The internal dimensions made up of wood and lined internally with thermocol and tin sheets. Its top is covered with a glass sheet and inner sides are painted black. The air inlets and outlets (dampeners) are provided at the lower and upper ends to help in regulation of air flow and temperature. On an average 20-30^oC higher temperature can be attained inside the dehydrator as compared to ambient temperature. The loading capacity in solar drier of this size is 25-30 kg fresh fruit/vegetable spread on the trays each measuring 0.9 m× 0.45 m in size. Its fabrication cost is 5,000 (app.).

c) Polytunnel solar drier/ polyhouse solar drier: The basic principle is similar to solar drier. Polyhouse solar drier developed at Acharya N G Ranga Agriculture University, Baptla has a capacity to dry about 22-24 quintals of ripe chilies. It consists of 12×7.8×2.4 m (40’x26’x8’) size arch type polyhouse with 1600 sq ft as tray area. Thirty two trays of size 10’×5’×3 (L×B×H) each are used to load 22-24 quintals of ripe chillies. Whole frame structure is covered with a UV stabilized 150 Gsm cross laminated transparent polyethylene sheet with well crow type ventilators at bottom and top of all three sides except on one side (north side) and two chimney ventilators on the roof cover to facilitate removal of moist air. Two chimney ventilators are placed on the top of the roof at a spacing of 4 meters. The height and diameter of each chimney ventilator are 0.6 and 0.25 meters respectively. Generally, temperature of about 15-17⁰C higher than the ambient temperature can be obtained inside the drier.
Solar radiation is predominantly short wave radiation. The transmitted radiation through polyethylene sheet becomes long wave radiation after absorption and cannot escape from the polyethylene sheet thus leading to increase in temperature inside the drier. Covering of floor with black polyethylene sheet helps to retain better heat. Drying takes place due to convective movement of air caused by the difference in density of air inside the drier. The cold air enters through bottom ‘well crow’ ventilators and gets heated due to higher temperature inside the drier. The hot air moves through the bed of commodity being dried due to natural convection and transports moisture through the top well crow ventilators and chimney ventilators. The cost of drier is approximately Rs 1.20 lakh.

4. Dehydration equipment
A. Hot air Driers: In hot air driers the food is in contact with a moving stream of hot air. Heat is supplied to the product mainly by convection. Kiln drier, cabinet tray drier, tunnel drier, conveyor drier, bin drier, fluidized bed drier, pneumatic drier, rotary drier and spray drier use hot air for drying of fruits and vegetables.

1) Kiln drier: These are mainly used for hops, apple rings and slices and malt drying. It consists of two storeys with a furnace or burner located on the ground floor and wet material placed on the top floor. The heated air from the furnace rises by natural or forced convection and passes through the slotted floor of the second storey, on which the wet material is spread in an even layer of 10-20 cm. The humidified air is exhausted through a flue in the upper story. However, in kiln drier there is limited control over drying condition and drying time is longer. Besides, regular turning of product is necessary.

2) Cabinet (tray) drier: In tray driers the wet food is spread evenly/thinly on trays in which drying takes place. These consist of an insulated cabinet fitted with shallow mesh or perforated trays, each of which contains a thin (2-6 cm deep) layer of food. Hot air is circulated through the cabinet at 0.5- 5 m/s per square meter tray area. Ducts or baffles are used to direct hot air through each tray, to promote uniform air distribution. Heating is by conduction from heated trays or by radiation from heated surface. The heated air also removes the vapours. Tray driers are used for small scale production (1-20 t/day) or for pilot scale work. They have low capital and maintenance costs but have poor control and more variable product quality.

3) Conveyor drier (Belt drier): These are similar to tunnel drier, but the wet material is conveyed on moving belts rather than trucks. Continuous conveyer driers are up to 20 m long and 3 m wide. Food is dried on the belt in beds of 5-15 cm deep. The air flow is initially upward through the bed of food and then downwards in later stages to prevent dried food from blowing out of the bed. There can be 2 or 3 stage driers in which the food is mixed, repiled into deeper beds (15-25 cm in two stages and up to 250-900 cm in third stage). Major factors of conveyor driers are:

1. It improves uniformity of drying and saves floor space.
2. Food is dried up to 10-15 % moisture contents and finished in bin driers.
3. This equipment has good control over drying conditions and offers high production rates.
4. Can be used for large scale drying of foods (fruit and vegetable dried in 2.0-3.5 hours with a capacity up to 5.5 t/h).
5. Reduces labour costs, since it has independently controlled drying zones and is automatically loaded and unloaded.
6. Offers good replacement for tunnel driers.

Tunnel drier: It is the improvement of tray drier, in which trays move through a tunnel where heat is applied and vapours are removed. In most cases, air is used in tunnel drying and the material can move through the drier either parallel or counter current to the air flow. Typically a 24 m long tunnel contains 12-15 racks with total capacity of 5000 kg of food. The time taken for drying is 5-16 hrs.

Parallel or co-current air flow: In this arrangement wet product as well as hot air moves in the same direction.

Advantages

• High rates of evaporation at the wet end without over heating of the material.
• Rapid initial drying.
• Little shrinkage of food
• Low bulk density
• Less heat damage to food.
• No risk of spoilage, since moisture content is removed at once.
  

Limitations

• Low moisture content in the finished product is difficult to achieve as cool moist air passes over dry food at the outlet.Flow patterns in tunnel driers
  

b) Counter current type: As such, relatively low initial rates of drying at the wet end are achieved. This causes high shrinkage of cellular material.

Advantages

• More economical use of energy.
• Low final moisture content can be achieved as hot air passes over dry food at the outlet.

Limitations

• Relatively low initial rates of drying at the wet end.
• Causes high shrinkage of cellular material.
• Possible risk of heat damage to the product due to the heat at dry end.
• Long exposure of food to a warm moist atmosphere and may lead to shrinkage.
• Two or more tunnels can be used in series.

c) Central exhaust tunnel drier: In this both features of co-current and counter current flow are combined.

Advantages: Combined benefits of parallel and counter current driers, but less than cross flow driers.

Limitations: More complex and expensive than single direction air flow.

d) Cross flow tunnel drier: In this arrangement the hot air is introduced at different compartments of wet food, thus the rate of drying is uniform.

Advantages

• Flexible control of drying conditions due to air heating facility between stages.
• Uniform drying due to frequent changes in direction of hot air.
• High drying rates.

Limitations

• More complex and expensive to buy
• Operate and maintain.

5). Foam mat drying: In this the liquid foods (fruit juices) are formed into stable foam by the addition of a stabilizer and aeration with nitrogen or air. For drying of lemon juice, carboxy methyl cellulose (CMC) can be mixed with juice to convert into foam. The foam is spread on a perforated belt to a depth of 2-3 mm and dried rapidly in two stages by parallel and thin counter current air flows. Foam mat drying is about three times faster than drying a similar thickness of liquid owing to thin surface area. The thin foam mat of dried food is ground to a free flowing powder which has good rehydration properties. The rapid drying and low product temperature give rise to high quality product.

Limitation: Large surface area is required for high production rates and capital costs are high.

6). Bin drier (Deep bed drier): Bin driers are used for final drying of dried food material. They are provided with cylindrical or rectangular containers fitted with mesh (false bottom). Hot air passes up through a bed of food at relatively low speed 0.5 ms-1 per square meter of bin area. In bin drier, the food stuff is contained in a bin with perforated bottom through which warm air is blown vertically upwards, passing through the material and drying it. Bin driers are also used for equalization of moisture content within the bulk of dried food material.

Advantages

• These driers have a high capacity and low capital and running costs.
• Bin driers improve the operating capacity of initial driers by taking the food when it is in falling rate period, when moisture removed is most time consuming.
• The deep bed permits equalization of varied moisture content in different layers of food.

7). Fluidized bed drier: Heated air is forced up through a bed of solids under such conditions that the solids are suspended in to the air. The heated air acts both a fluidizing and drying medium. Some units have vibrating bases to assist movement of the product. The drier may be batch or continuous type. The drier is used successfully for drying of peas, beans, carrots, cocoa, coffee etc. In some cases, dust separators (cyclones) are included in the exhaust air line to remove fumes.
Advantages

• Fluidized bed driers are compact and have good control over drying conditions.
• They provide high thermal efficiencies and high drying rates.
• Since product is mixed by fluidization, it leads to uniform drying.
• Fluidized bed driers are limited to small particulate foods that are capable of being fluidized without excessive mechanical damage.

8). Pneumatic drier: It is the extension of fluidized bed drier where higher air velocities are used. In this, the solid food particles are conveyed rapidly in an air steam, the velocity and turbulence of the air maintain the particles in suspension. Heated air accomplishes the drying and often classifying device is included in the equipment. The dry matter passes out as product and the moist product is recirculated for further drying.
In pneumatic driers, powders or particulate foods are continuously dried in vertical or horizontal metal ducts. A cyclone separator is used to remove the dried product. The moist food (less than 40% moisture content) is placed into the ducting and suspended in hot air. In vertical driers, the air flow is adjusted to classify the particles; lighter and smaller particles, which dry more rapidly, are carried to a cyclone more rapidly than are heavier and wetter particles which remain suspended to receive additional drying required.

Rotary drier: In rotary drier, slightly inclined rotating cylinder is fitted internally with flights to cause the food to move through a stream of hot air as it moves through the drier. Air flow may be parallel or counter current.
The food stuff is contained in a horizontally inclined cylinder through which it travels. The heating is done either by air flow through the cylinder or by conduction of the heat from the cylinder walls. In some cases, the cylinder rotates and in others the cylinder is stationary and the paddle or screw rotates through the cylinder conveying the material. The drier is used for drying of sugar crystals and cocoa beans.

Advantages

• The agitation of the food and the large area of food exposed to the air produce high drying rates and uniformly dried product.
• The drier is suitable for the products that tend to mat or stick on the belt or tray driers.

Limitations

• It may cause damage to the product by impact or abrasion in the drier.

10). Trough drier (belt-trough drier): Small, uniform pieces of food like peas and other dried vegetable are dried in a mesh conveyor belt which hangs freely between rollers, to form the shape of trough. Hot air is blown through the bed of food, and the movement of conveyor mixes and turns it to bring new surfaces continually into contact with the drying air. The mixing action moves food away from the drying air and then allows time for moisture to move from the interior of the pieces to dry the surface. The moisture is then rapidly evaporated when the food again contacts the hot air. The drier operates in two stages to 50-60 % moisture and then to 15-20 % moisture content. Final finishing of dried product is carried out in bin driers.

Advantages

• These driers have high drying rates (55 minutes for dried vegetable compared to 5 hours in a tunnel drier).
• High energy efficiencies with good control over drying conditions.
• Minimum heat damage to the product.

Limitation: Not suitable for sticky foods.


11). Spray driers: A fine dispersion of pre-concentrated food is first “atomized” (sprayed) to form droplets (10-200 µm in diameter) which are sprayed into the drying chamber at a temperature of 150-300⁰C of heated air. The feed rate is controlled to produce an outlet air temperature of 90-100⁰C, which corresponds to a wet-bulb and product temperature of 40-50⁰C. Very short drying time and relatively low product temperature are the main features of spray driers. The main components of a spray drier include:

• Air heating and circulating system
• A spray forming system comprising of pressure nozzle, centrifugal atomizer and bowl nozzle
• A drying chamber
• Cyclone separator for product recovery (2 cyclones, scrubber with cloth filter)

For successful drying, complete and uniform atomization is necessary. Different types of atomizers are centrifugal atomizer, pressure nozzle atomizer, two fluid nozzle atomizer and ultrasonic atomizer.

Advantages:

• Due to very large surface area of the droplets, the drying is very rapid (1-10 s)
• The temperature of the product remains at the wet bulb temperature of the drying air.
• There is minimum heat damage to the food.

In spray drier, liquid or fine solid material in slurry form is sprayed in the form of fine dispersion into a current of heated air. Drying occurs very rapidly, thus this process is very useful for foods that suffer heat damage on long exposures. Spray driers are mostly used for milk, egg, coffee, cocoa, tea, potato, ground chicken, ice cream mix, butter, cream, yoghurt, cheese powder, coffee whitener, fruit juices, meat, encapsulated flavours, wheat and corn starch products.

B). Heated surface driers: Unlike hot air driers, the heat in heat surface driers is supplied to the food by conduction, thus resulting in higher thermal efficiency.
Advantages:

• It is not necessary to heat large volumes of air before drying commences.
• Drying can be carried out in absence of oxygen to protect foods that are sensitive to oxidation.
• Heat consumption is less than that of hot air driers. Heated surface drier utilize 2000 – 3000 kJ energy per kg of water evaporated while in hot air driers it is about 4000-10,000 kJ energy per kg of water evaporated.

Limitations: Since, foods have low thermal conductivities, which become further lower as the food dries. Therefore, the thin layer of food is required to conduct heat rapidly without causing heat damage.