Drying and Dehydration

Drying refers to the method of removing moisture content from food to a level at which food spoilage and the activities of food toxic micro-organisms are inhibited. Open sun drying is probably the oldest method of food preservation used for agricultural crops including food grains, oilseeds as well as fruits and vegetables. Most fruits and vegetables contain sufficient moisture for the activities of enzymes and drying is necessary to reduce the activity of micro-organisms and water. Therefore, the reduction of water activity of food is the main principle of preservation by drying. Two commonly used terms for dry foods are low-moisture foods and intermediate-moisture foods (IMF). During drying, fruits and vegetables are sliced ​​either whole into a single layer, or after primary treatment spread on trays that are placed inside a dehydrator or in the open sun to dry. In mechanical dehydrators, the initial temperature is usually kept at 43°C, which is then gradually raised to 60-66°C for vegetables and 66-71°C for fruits.

  • Dry foods or low-moisture foods typically contain no more than 25% moisture and have a water activity (aw) in the range of 0 to 0.6.
  • Intermediate Moisture food (IMF) on the other hand contains 15 – 50% moisture with a water activity (aw) of 0.6 to 0.85.
  • Open sun drying is probably the oldest method of food preservation used for agricultural crops including food grains, oilseeds as well as fruits and vegetables.

Advantages of drying

  1. Helps in food preservation by reducing water activities.
  2. Reduction in weight and volume of food (weight or volume of dried product is reduced by 4 to 10 times its initial fresh weight/volume).
  3. The reduction in bulk leads to a reduction in space requirement and hence reduces the cost of packaging, storage, and transportation.
  4. Drying is the cheapest and simplest method of preservation as compared to other methods.
  5. Dried foods diversify the diet and provide consumers with convenient foods to eat.
  6. Nutrient content per unit weight of dry produce is very high.

Difference between drying and dehydration

Drying and dehydration are related terms and are used interchangeably to describe the unit tasks involved in the removal of water by evaporation or sublimation. Drying generally refers to the method of removing moisture from food under natural conditions such as sunlight and wind, such as open sun drying, shade drying, etc. Whereas, dehydration refers to the process of removing moisture by artificial heat under controlled conditions (temperature, humidity, and airflow).

Drying mechanism

Drying or dehydration involves the removal of water from food by heating. When hot air is passed over wet food, the air expands through a boundary film of water vapor surrounding the food and is carried away by the air. A water vapor pressure gradient is established from the moist interior of the food to the dry air, which provides the driving force for the removal of water from the food. The boundary film acts as a barrier to both heat transfer and water vapor removal during drying. The moving air velocity determines the thickness of the boundary film. The water vapor leaves the surface of the food and increases the humidity of the surrounding air, thereby decreasing the water vapor pressure gradient, thereby reducing the drying rate. Therefore, the moving air must be fast to reduce the thickness of the boundary film and achieve a faster drying rate. Therefore, the parameters that are taken into account for drying moist horticultural products are moderately high dry bulb temperature, low relative humidity, and high air velocity

Drying Curve: For each product, there is a representative curve that describes the drying characteristics at specific temperature, velocity, and pressure conditions for that product. This curve is known as the drying curve for a specific product. Drying occurs in three distinct periods or stages namely the first stage or initial period, the second stage or steady rate period, and the third stage or falling rate period.

Advantages of Dehydration Over Open Sun Drying:

  1. Dehydration is more rapid, controlled, and efficient than sun drying in the open.
  2. Dehydration requires less space (floor area) than sun drying.
  3. Dehydration is more hygienic than drying in the open sun.
  4. Dehydration is not dependent on weather conditions whereas drying is not possible in cloudy weather or during rains.
  5. Uniform drying temperature keeps the colour of the dehydrated product uniform.

Factors affecting dehydration

The important factors affecting the drying rate are:

  1. The initial moisture content of raw material
  2. Composition of raw materials
  3. Initial load of food placed in the drier
  4. The size, shape, and arrangement of the raw material stack
  5. Temperature, relative humidity, and velocity of air used for drying
  6. Rate of heat transfer to the surface of the food
  7. Pre-treatment of raw materials before drying (peeling, blanching, sulfuring, etc.)

Things to keep in mind during the drying process

Control of air temperature and circulation in the system is important during the drying process. If the temperature is too low and the humidity is too high, the food will dry out more slowly and microbial growth can occur. Conversely, if the temperature is too high initially, a hard shell will develop on the surface of the food which will prevent the removal of moisture from the interior of the fruit and the moisture will be trapped inside the food material. This condition is known as case hardening. In addition, temperatures that are too high at the end of the drying period tend to scorch the food. It is recommended to keep the temperature between 49°C to 60°C for drying fruits and vegetables. Temperatures up to 65°C can be used initially but should be reduced as soon as the food begins to dry out. Whereas, during the last hour of the drying period, the temperature should not exceed 55°C.

Drying process

Drying fruits and vegetables generally includes three stages: pre-drying treatment, drying and post-drying handling, packaging, and storage.

  1. Pre-drying treatment

Fruits and vegetables are selected and sorted according to size, maturity, and soundness. They are then washed under running water to remove dust, dirt, insects, mold spores, plant parts, soil, debris, and other materials. Depending on the type and quantity of product to be dried, any peeling method may be selected such as hand peeling, steam, hot water, lye peeling, or abrasive peeling. Fruits such as grapes, plums, and apricots are immersed in boiling caustic soda (0.5% NaOH) for a few seconds and immediately placed in cold water to remove the waxy layer (grape, plum), pubescence (apricot, peach). After peeling and washing, fruits and vegetables are cut to the desired size and thickness as this affects the drying rate.

  1. Blanching

Generally, all vegetables are whole or pieces after preliminary preparation and are boiled in boiling water or by steam for a pre-determined period of time, followed by immediate cooling to inactivate enzymatic activity before drying.

  1. Sulphuring/Sulphiting

Most fruits are treated with sulphur dioxide by placing them in a closed chamber in which sulphur (3 g / kg of fruit) is burned so that the fruits can absorb SO2 fumes. The process is called sulphuring or sulphur fumigation. Sulphiting, on the other hand, refers to the immersion of prepared fruits or vegetables in a solution of potassium meta-by-sulphite to serve the same purpose as sulphiting. Sulphuring helps preserve color, prevents browning, and reduces the destruction of carotene and ascorbic acid, in addition to preventing spoilage of the dried product.

  1. Drying and Dehydration Methods

A) Drying Methods

a) Sun-drying: In this, fruits/vegetables are spread either on terraces or on the floor to dry in the 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 packed in sacks/wooden boxes and sent for sale in a local or distant markets. The moisture content is usually not less than 15% which is too high for storage. The product quality is substandard, as the product is brown in color from the outside and contaminated with insect dirt/dust particles. The quality can be improved by spreading the product on a black polythene sheet and covering it with a thin muslin cloth to protect it from the ingress of insects, dust, or dirt particles.

b) Solar Dryer: Solar dryer is an inclined rectangular box of size 1.8 x 0.9 x 0.3 m3. The interiors are made of wood and thermocol and tin sheets are attached as a layer inside. Its top is covered with a sheet of glass and the inner edges are painted black. Air inlets and outlets (dampeners) are provided at the lower and upper ends to help with the regulation of airflow and temperature. An average of 20-30°C higher temperatures can be achieved inside the dehydrator than the ambient temperature. The capacity of this size solar dryer is 25-30 kg of fresh fruits/vegetables, each tray having a size of 0.9 m × 0.45 m.

c) Polytunnel Solar Dryer / Polyhouse Solar Dryer: The basic principle is the same as that of the solar dryer. The polyhouse solar dryer developed at Acharya NG Ranga Agricultural University, Bapatla has the capacity to dry about 22-24 quintals of ripe chilies. It has an arch-type polyhouse of size 12×7.8×2.4 m (40’x26’x8’) with 1600 sq ft as tray area. Thirty-two trays of size 10’×5’×3 (L×W×H) are used to load 22-24 quintals of ripe chillies. The entire frame structure is covered with a UV stabilized 150 gsm cross-laminated transparent polyethylene sheet with crow-type ventilators on all three sides down and top well except one side (North side) and two chimneys on the roof cover. These are ventilators to facilitate the removal of moisture. Typically, a temperature of about 15-17°C above the outside environment temperature can be achieved inside the dryer.

Solar radiation is mainly short-wave radiation. The radiation transmitted through the polyethylene sheet becomes long-wave radiation after absorption and cannot exit from the polyethylene sheet leading to an increase in the temperature inside the dryer. Covering the floor with a black polyethylene sheet helps in better heat retention.

B) Dehydration Equipment

a) Hot air driers: In hot air driers, the food is exposed to a moving stream of hot air. Heat is supplied to the product mainly by convection. Such as Kiln dryer, cabinet tray dryer, tunnel dryer, conveyor dryer, bin dryer, fluidized dryer, pneumatic dryer, rotary dryer and spray dryer.

i. Kiln dryer: These are mainly used for drying apple rings, slices, and malt. It consists of two floors with a furnace or burner on the ground floor and wet material on the top floor. The heated air from the furnace is raised by natural or artificial convection and passes through the perforated floor of the second floor, over which the wet material is spread in a uniform layer of 10–20 cm. The humid air is drained out through a chimney on the upper floor. However, furnace dryers have limited control over drying conditions and have longer drying times. In addition, the regular turning of the product is necessary.

ii. Cabinet (Tray) Dryer: In the tray dryer, the wet food is spread evenly/on a thin tray in which drying takes place. These consist of an insulated cabinet equipped with shallow mesh or perforated trays, each containing a thin (2–6 cm thick) layer of food. The hot air is circulated through the cabinet at 0.5- 5 m/s per square meter of tray area. To promote uniform air distribution, ducts or baffles are used to direct hot air through each tray. Heating occurs by conduction from the hot tray or by radiation from the hot surface. The hot air also removes the vapor. Tray dryers are used for small-scale production (1-20 tons/day) or initial scale work. They have lower capital and maintenance costs but have poorer controls yielding more variable product quality.

iii. Conveyor dryers (belt dryers): These are similar to tunnel dryers in that the wet material is conveyed on a moving belt. Continuous conveyor dryers are up to 20 meters long and 3 meters wide. The food is dried in a bed 5-15 cm deep on the belt. The airflow is initially upward through the food bed and then downward to prevent dry food from exiting the bed in later stages. There may be 2 or 3 stages of drying in which the food is mixed and refilled in the deep bed. The major advantages of a conveyor dryer are:

  • It improves drying uniformity and saves floor space.
  • The food is dried to a moisture content of 10-15% and then completed in the bin dryer.
  • These have good control over drying conditions and provide high production rates.
  • Can be used for large-scale drying of foods (fruits and vegetables are dried in 2.0-3.5 hours with a capacity of up to 5.5 tons/hour).
  • Reduces labor cost, as load and unload automatically.
  • Provides a good replacement for a tunnel dryer.

iv. Tunnel dryer: This is an improvement of the tray dryer, in which the tray runs through a tunnel where heat is given off and vapor is removed. In which air is used in drying and the material moves through the dryer parallel to the flow of air or through a counter current. Usually, a 24-meter-long tunnel consists of 12-15 racks with a total capacity of 5000 kg. The drying time is 5-16 hours.

Flow pattern in a tunnel dryer

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


  • High rate of evaporation at the wet end without overheating the material.
  • Fast initial drying.
  • Slight shrinkage of food
  • low bulk density
  • Less heat damage to food.
  • There is no risk of spoilage, as the moisture content is removed in one go.


  • Achieving low moisture content in the finished product is difficult because cool moist air passes over the dried food at the outlet.

Counter current type: In these, a relatively low initial rate of drying is achieved at the wet end. This causes a high shrinkage of the cellular material.


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


  • High shrinkage of cellular material occurs.
  • The potential risk of heat damage to the product due to dry end heat.
  • Prolonged exposure to food in a warm moist environment can cause shrinkage.

Central Exhaust Tunnel Dryer: It combines both the characteristics of co-current and counter-current flow.


  • Combined advantages of parallel and counter-current dryers, but less than cross-flow dryers.


  • More complicated and expensive than single-direction airflow.

Cross Flow Tunnel Dryer: In this arrangement hot air is introduced into different compartments of wet food, thus drying rate is uniform.


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


  • More complicated and expensive to buy
  • Operation and Maintenance.

v. Foam mat Drying: In this, liquid foods (fruit juices) form a stable foam with the addition of a stabilizer and aeration with nitrogen or air. Carboxymethyl cellulose (CMC) can be added to the juice to turn lemon juice into a froth to dry it. The foam is spread over a perforated belt to a depth of 2-3 mm and is rapidly dried in two steps by parallel and thin counter-current airflow. Drying a foam mat is about three times faster than drying the same thickness of liquid due to the thinner surface area. The thin foam mat of dry food makes for a free-flowing lather that has good rehydration properties. Fast drying and low product temperatures lead to higher-quality products.

Limitation: Higher production rates require a larger surface area and higher capital costs.

vi) Bin Dryer (Deep Bed Dryer): Bin dryer is used for the final drying of dried food material. They are poured into cylindrical or rectangular containers with netting at the bottom. Hot air passes through the food bed at a relatively low speed of 0.5 ms–1 per square meter of bin area. Air passes vertically upwards through the material and dries it. Bin dryers are also used to equalize the moisture content in the bulk of dried foods.


    • These dryers have high capacity and low capital and operation costs.
    • Bin dryers improve the operational efficiency of the initial dryer, when the food is in the falling rate period, when moisture removal takes the most time.
    • The deep bottom helps to equalize the moisture content of different layers of the food.

vii). Fluidized dryer: Hot air is forced through a solid bed under conditions such that the solids are suspended in the air. Hot air serves as both a liquefaction and drying medium. Some units have vibration bases to aid in the movement of the product. Dryers are successfully used for drying peas, beans, carrots, cocoa, coffee, etc.


    • Fluidized bed dryers are compact and provide good control over drying conditions.
    • They offer high thermal efficiency and a high drying rate.
    • Since the product is mixed by liquefaction, it leads to uniform drying.
    • Fluidized bed dryers are limited to foods with small particles that are able to liquefy without excessive mechanical damage.

viii). Pneumatic Dryer: This is an extended form of fluidized bed dryer where high air velocities are used. In this, the solid food particles are rapidly carried in the air stream, and the velocity and turbulence of the air keep the particles in suspension. The hot air completes the drying; the dry matter is expelled as the product and the moist product is re-circulated for further drying.

In pneumatic dryers, powdered or particulate foods are dried continuously in vertical or horizontal metal ducts. A cyclone separator is used to remove the dried product. Moist food (less than 40% moisture content) is placed in ducts and suspended in hot air. In vertical dryers, the airflow is adjusted to classify the particles; Lighter and smaller particles, which dry more quickly, are carried to the cyclone more quickly than heavier and wetter particles, which remain suspended to meet the need for additional drying.

ix). Rotary dryer: In a rotary dryer, a slightly inclined rotating cylinder is fitted internally for the flight so that the food can move through the hot air stream as it moves through the dryer. Airflow can be parallel or counter-current.

Food is contained in a horizontally inclined cylinder through which it travels. Heating is done either by the flow of air through the cylinder or by heat conduction of 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 dryer is used for drying sugar crystals and cocoa beans.


    • A large area of ​​food exposed to food agitation and air produces a high drying rate and uniformly dried product.
    • The dryer is suitable for products that stick to the belt or mat on the tray dryer.


    • Impact or abrasion in this dryer may damage the product.

x). Trough dryer (belt-trough dryer): Small, uniform pieces of food such as peas and other dried vegetables are dried in a mesh conveyor belt that hangs freely between rollers, forming a trough. Hot air is sifted through the food bed, and the conveyor speeds up the mixing and turning of its new surfaces to continuously expose the drying air. The mixing action moves the food away from the dry air and then gives the time for moisture from the inside of the pieces to dry off the surface. When the food is exposed to hot air again, the moisture evaporates rapidly. The dryer operates in two stages 50-60% moisture and then up to 15-20% moisture content. The final finishing of the dried product is done in the bin dryer.


  • These dryers have a high drying rate (55 minutes for dry vegetables, compared to 5 hours in tunnel dryers).
  • High energy efficiency with fine control over drying conditions.
  • Minimal heat damage to the product.


  • Not suitable for sticky foods.

xi). Spray dryers: For a fine dispersion of pre-concentrated food, the first droplet (10-200 µm in diameter) is “atomized” which is sprayed in a drying chamber at 150-300°C hot air temperature. The feed rate is controlled to produce an outlet air temperature of 90-100°C, which corresponds to a wet bulb, and a product temperature of 40-50°C. Very short drying time and relatively low product temperature are the main characteristics of spray dryers.


  • Due to the large surface area of ​​the droplets, the drying speed is very fast (1-10 sec)
  • The temperature of the product is maintained at the temperature of the wet bulb of the dry air.
  • Minimal heat damage to food.

In a spray dryer, the liquid or fine solid material in the form of a slurry is sprayed into a stream of hot air as a fine dispersion. Drying is very rapid, thus making this process very useful for foods that have been damaged by prolonged heat exposure. Spray dryers are mostly used for milk, egg, coffee, cocoa, tea, potato, ground chicken, ice cream mix, butter, cream, yogurt, cheese powder, coffee whitener, fruit juices, meat, wheat, and corn starch products.

b) Heated surface dryers: Unlike hot air dryers, heat is supplied to food by heat conduction in heat surface dryers, resulting in its higher thermal efficiency.


  • It is not necessary to heat the air in large quantities before starting drying.
  • Drying can be done in the absence of oxygen to preserve food sensitive to oxidation.
  • Heat consumption is lower in comparison to hot air driers.

Limitations: Since, foods have low thermal conductivity, which further decreases as the food dries. Therefore, the thin layer of food needs to conduct heat rapidly without loss of heat.

Types of hot surface dryers

1. Vacuum Drum Dryer: Vacuum drum dryers work in a vacuum and in these the heat transfer occurs by conduction or radiation. The rollers are attached to a large cabinet in which a vacuum is created. A vacuum Drum Dryer is used for drying potato flakes, dried soup, and fruit juice.

2. Drum dryer (roller dryer): In a drum dryer, a slowly rotating hollow steel drum is heated internally to 120-1700C by pressurized steam. A thin layer of food is placed on the outer surface of the drum by spraying or spreading by feed rollers. Before the drum completes one rotation (within 20 seconds to 3 minutes) the dried food is removed by a ‘doctor’ blade that contacts the drum surface evenly along its length. Dryers can have single drums, or double drums (twin drums). A drum dryer is used to dry milk powder.


  • High drying rate with high energy efficiency and their suitability for slurry where the particles are too large for spraying.


  • Heat damage to sensitive foods and high capital costs.

3. Vacuum shelf dryer: A vacuum shelf dryer consists of hollow shelves in a vacuum chamber. Food is placed in thin layers on flat metal trays that are carefully made to ensure good contact with the shelves. A partial vacuum of 1-70 torr (1.3 kPa to 93.3 kPa) is created in the chamber and steam or hot water is passed through the shelves to dry the food. They are used to produce puff dried foods.


  • It gives fast drying and limited heat damage
  • Suitable for heat-sensitive foods.


  • Relatively high capital and operating costs and low production rates.

4. Explosion puff drying: This involves partially drying the food item to moderate the moisture content and then sealing it in a pressure chamber. The pressure and temperature in the chamber rise at once and are released immediately. The rapid reduction in pressure causes the food to expand and a finer porous structure is developed. This allows for rapid final drying and rapid rehydration. The sensory and nutritional properties are well retained in this dryer.

5. Freeze Drying: Freeze drying uses a vacuum to sublimation/remove water content from frozen food, the food product being solidly frozen during the process. In the freeze drying process, the food is first frozen at -18°C in the lower chamber of the freeze dryer and then the frozen material is placed in the upper chamber initially at 30°C for 24 hours then at 20°C under high vacuum (0.1 mm Hg). It is dried till it dries completely. The final product is highly hygroscopic and excellent in taste and flavour. Mango pulp, concentrate, passion fruit juice, and guava pulp are dehydrated by this method.

The freeze dryer uses the principle that a high vacuum (27-133 Pa pressure) allows frozen water to be removed from the food and collected without passing through the liquid phase. Since the material is frozen, there is no heat damage.

6. Osmotic dehydration: Fruits that are highly acidic and have a sensitive aroma can be dried using osmotic dehydration. In this method, the fruits are kept in a hypertonic solution of 70°Brix syrup for 4 hours after initial treatment. During this, water seeps into the syrup due to osmosis. About 50% of the moisture is removed from the fruit in the can process. The fruits are then removed from the sugar syrup, washed, and dried in a hot air dryer to the desired moisture content. During osmotic drying, the acid from the fruit is released into the sugar syrup while some sugar enters the fruit, thus the final product achieves the required sugar-acid balance. Apricots, grapes, apples, etc. can be dried by the osmotic method.

5. Dry products packaging and storage

After the foods have dried, cool them completely. The packaging is carried out in clean moisture-vapor-resistant containers. Glass jars, metal cans, or freezer containers are good storage containers if they have tight-fitting lids. Sulphurized or sulphite fruits should not touch the metal. Put the fruit in a plastic bag before placing it in a metal box. Dried food should be stored in a cool, dry, dark place. Most dried fruits can be stored at 60°F for 1 year, and 80°F for 6 months. Dried vegetables have about half the shelf life of fruits. Laminated Polypropylene (PP), Metallized Polyester (PET), and Low-Density Polyethylene (LDPE) are suitable for a shelf life of at least 6 months.

The different types of materials used in the packaging of dry products are:

  1. Rigid containers such as metal cans and plastic containers, are airtight and lightweight and prevent the penetration of moisture and oxygen. They are easy to handle during transport.
  2. Semi-rigid packs, such as lined cartoons and bag-in-boxes, preserve the freshness of the food product until it is opened. An ideal laminate is made of layers of paper/Low-Density Polyethene (LDPE)/Al-Foil/ which ensures the required shelf life.
  3. The flexible pouch can be easily handled and opened.