Drying and Dehydration of fruits and Vegetables – Concept And Methods, Osmotic drying.

Drying and Dehydration


Fruits and Vegetables drying is a method of food preservation in which food is dried (dehydrated or desiccated). Drying inhibits the growth of bacteria, yeasts, and mold through the removal of water.(Drying and Dehydration)


Fruit and Vegetables dehydration is the process of removing water from fruit by circulating hot air through it, which prohibits the growth of enzymes and bacteria.

Agri Exam Important 10000 Agriculture MCQ


  • The practice of drying of food stuffs, specially fruits and vegetables, for preserving them is very old.
  • The term drying‘ and dehydration‘ means the removal of water.
  • The former term is generally used for drying under the influence of non-conventional energy sources like sun and wind.
  • If fruits (or) vegetables are to be sun dried, they (or) their pieces should be evenly spread in single layer (on) trays or boards and exposed to the sun.
  • In sun drying there is no possibility of temperature and humidity control.
  • The hottest days in summer are, therefore, chosen so that the foods dry very fast, thus preventing them from getting spoiled due to souring.
  • Souring (or) turning acidic is usually due to growth of microorganisms which convert the carbohydrates in the food to acid.(Drying and Dehydration)
  • Quick removal of moisture prevents the growth of the microorganisms.
  • Dehydration means the process of removal of moisture by the application of artificial heat under controlled conditions of temperature, humidity and air flow.
  • In this process a single layer of fruits (or) vegetables, whole or cut into pieces (or) slices are spread on trays which are placed inside the dehydrator.
  • The initial temperature of the dehydrator is usually 43oC which is gradually increased to 60-66 oC in the case of vegetables and 50-71 oC for fruits.

Advantages of dried / dehydrated foods

1. Dried foods are in more concentrated form than foods preserved in other ways.

2. They are less costly to produce than canned or preserved food, because of lower labour costs and because of no sugar is required.(Drying and Dehydration)

3. Due to reduction in bulk of the product, it requires less storage space.

4. The weight of a product is reduced to 1/4th to 1/9th its original (or) fresh weight and thus the cost of its transport is reduced.

Methods of Drying/Dehydration:

1. Sun Drying

2. Drying by mechanical driers,

3. Osmotic dehydration,

4. Spray drying,

5. Foam mat drying,

6. Freeze drying


Sun drying of fruits and vegetables is practiced widely in tropical and subtropical regions where there is plenty of sun shine and practically little or no rain during the drying season. Sun drying in direct (or) diffused sunlight (shade drying), one of the earliest method of food preservation, is still used for the production of dried fruits, and also for drying nuts.(Drying and Dehydration)

It was originally limited to fruits high in sugar content, which when harvested, would dry naturally without hazard of loss from fermentation and molding.

Process for drying of fruits Fruits (mature and free from insects and disease→ Washing → Peeling / removal of outer skin → Preparation → Pretreatments → Spreading on flat wooden trays → Sulphuring → Drying → Sweating → Packaging in air tight tin containers (or) polythene bags → Storage (at ambient temperature).


Lye peeling

Dipping the fruits (grapes and dates) in 0.5% to 2.5% boiling caustic soda solution for 0.5 to 2.0 minutes depending on their nature and maturity.

Hot lye loosens the skin from the flesh by dissolving the pectin. The peel is then removed easily by hand. Any trace of alkali is removed by washing the fruit thoroughly in running cold water (or) dipping it for a few seconds in 0.5% citric acid solution.(Drying and Dehydration)


Sulphuring is done only for fruits and not vegetables. So2 fumes act as a disinfectant and prevent the oxidation and darkening of fruits on exposure and thus improves their colour.

This phenomenon is generally seen in sliced fruits which darken due to oxidation of the colouring matter. Sulphur fumes also act as a preservative, check the growth of molds etc. and prevent cut fruit pieces from fermenting while drying in the sun.

Vitamins in sulphured fruits are protected but not in unsulphured ones. The whole fruits, slices (or) pieces are exposed to the fumes of burning sulphur inside a closed chamber known as sulphur box for 30-60 min. or in small airtight rooms. Sulphur box is a closed airtight chamber of galvanized iron sheet.(Drying and Dehydration)

It is fitted in a wooden frame work having runways on both sides to hold the trays. For small scale sulphuring, a box of size of 90 x 60 x 90 cm which can hold 11 trays, each of 80 x 60 x 5 cm size is suitable. A box holding 10 trays will require burning of about 3 g of sulphur in one charge.


Sweating Keeping dried products in boxes or bins to equalize moisture content.

Sun drying of fruits

1. Banana

Dried ripe banana is known as banana fig‘. The fruit is peeled, sliced lengthwise, sulphured and dried in the sun. Unripe bananas are peeled after blanching in boiling water and cut into discs for drying. The dried slices are either cooked or fried. They can also be converted into banana flour which can be used as such or in combination with cereal flours.

2. Date

In the hard dried dates, sucrose sugar predominates, whereas in the soft dried dates, invert sugars predominates. Dates are picked in the dung stage, that is when the tip of the fruit has turned a translucent brown.(Drying and Dehydration)

They are spread on mats for 5 to 8 days for curing. This is rather expensive as several pickings have to be made as the date attain other proper stage of ripening.

Scientists have found that dates could be picked 3 to 4 days before the dung stage and then dipped for ½ to 2 min in 0.5-2.5 % caustic soda solution before placing them for drying in order to get a good dried product.

3. Fig

The fruits are allowed to ripen on the tree and gathered when they drop. They are then spread thinly on the drying yard for 3 to 4 days for drying.(Drying and Dehydration)

After drying they are sorted and packed. Figs are treated with salt and lime (1 kg of each per 1000 litrs of water) to remove the hair from the skin and also to soften the flesh. They are then dried without sulphuring, till there is exudation of juice on pressing the dried fig between the fingers.

4. Grapes

Large quantities of seedless grapes known as kishmish grapes are imported into India from Afghanistan. Ripe bunches of grapes are hung inside dark rooms known as kishmish khanas till the berries acquire a greenish or light amber tint.

These shade dried grapes are considered to be a far superior to the ordinary sundried (or) dehydrated grapes. The other important dried grape called Monucca (or) Raisin is prepared from the large seeded Haitha grapes which are lye dipped prior to the sun drying.(Drying and Dehydration)

For efficient drying, grapes should have a high sugar content of 20 to 24 degree brix. From this point of view, some of the varieties of grapes are not suitable for drying.

The higher sugar content grapes are dried without any sulphuring till there is no exudation of juice on pressing dried grape between the fingers.

The yield and quality of the final dried product depend on the brix of the fresh grape taken for drying. In California, the Sultanina (or) Thompson seedless varieties of grapes are dried.

The grapes are sometimes dipped for 3-6 seconds in caustic soda and sodium bicarbonate, covering the surface of a solution with a thin layer of olive oil. This treatment removes only the wax and the bloom on the grapes without cracking the skin.(Drying and Dehydration)

The dried product has a glossy appearance. Lye dipped grapes are sometimes treated with sulphur fumes, for 3-5 hrs for bleaching them, because certain markets prefer such glossy product.

In Australia, potassium carbonate solution with a layer of olive oil (or) sometimes grape seed oil itself, is used for dipping the grapes. The drying is carried out on wire net racks arranged inside a shed. In this way, the grapes are dried without direct exposure to the sun.

Drying takes 10-20 days depending upon the variety of grape. The dried product is generally of high quality.(Drying and Dehydration)

5. Jack fruit

Jack fruit bulbs of ripe fruit are sliced and the seeds removed. The slices are dried with (or) without sulphuring. The bulbs can also be made into a fine pulp, which can be dried in the form of sheets or slabs.

6. Mango

Unripe, green mangoes are peeled, sliced and dried in the sun. The dried product is used for the preparation of mango powder which is added as a relish in various food preparations. Ripe mangoes are taken and the juicy pulp squeezed by hand.

The pulp is spread on Bamboo mats and a small quantity of sugar sprinkled over it. Whet the first layer has dried, another layer of pulp is spread over it for drying. This process is repeated until the dried slab is 1.2 to 2.5 cm thick.(Drying and Dehydration)

The dried product has a light yellow amber colour and possess a delicious taste. Other fruits Pomegranate seeds are dried, and the dried product known as anardana is sued as a savoury and acidulant like tamarind in cooking.

Apple rings are threaded and dried by hanging them out to dry in the sun. The cereals, pulses and oilseeds are usually sundried in most of the areas after harvesting from the crop. Sundried vegetables results poor quality in physical and chemical characteristics during storage.(Drying and Dehydration)


a) Air convection driers

All air convection driers have some sort of insulated enclosures, a means of circulatingvair through the enclosure and a means of heating this air. They also will have various means of product support, special devices for dried product collection, some will have air driers to lower drying air humidity.

Movement of air generally will be controlled by fans, blowers and baffles. Air volume and velocity will affect drying rate, but its static pressure also is important since products being dried become very light and can be blown off trays or belts.(Drying and Dehydration)

The air may be heated by direct or indirect methods. In direct heating the air is in direct contact with flame (or) combustion gases. In indirect heating the air is in contact with a hot surface, such as being blown across pipes heated by steam, flame or electricity. The important point is that indirect heating leaves the air uncontaminated.

On the other hand in direct heating the fuel is seldom completely oxidized to CO2 and water. Incomplete combustion leaves gases and traces of soot and this is picked up by the air and can be transferred to the food product.(Drying and Dehydration)

Direct heating of air also contributes small amounts of moisture to the air since moisture is a product of combustion but this is usually insignificant except with very hygroscopic foods. These disadvantages are balanced by the generally lower cost of direct heating of air compared to indirect heating, and both methods are widely used in food dehydration.

b) Kiln drier

Kiln driers of early design were generally two storey constructions. A furnace or burner on the lower floor generated heat, and warm air would rise through a slotted floor to the upper story. Foods such as apple slices would be spread out on the slotted floor and turned over periodically. This kind of drier generally will not reduce moisture to below about ten per cent. It is still in use for apple slices.(Drying and Dehydration)

c) Cabinet, Tray and Pan driers

Food may be loaded on trays or pans in comparatively thin layers upto a few centimeters. Fresh air enters the cabinet is drawn by the fan through the heated coil and is then blown across the food trays to exhaust. In this case the air is being heated by the indirect method. Screens filter out any dust that may be in the air.

The air passes across and between the trays in this design. The air is exhausted to the atmosphere after one pass rather than being recirculated within the system. The moisture laden air, after evaporating water from the food, would have to be dried before being recirculated, or else it would soon become saturated and further drying of the food would stop. Cabinet, tray and pan driers are usually for small scale operations.(Drying and Dehydration)

They are comparatively inexpensive and easy to set in terms of drying conditions. They may run upto 25 trays high, and will operate with air temperatures of about 93oC dry bulb and air velocities of about 2.5 to 5.0 M/5 across the trays.

The commonly are used to dry fruit and vegetable pieces, and depending upon the food and the desired final moisture, drying time may be of the order or 10 of even 20 hr.(Drying and Dehydration)

d) Tunnel and continuous belt drier

For larger operations we elongate the cabinet, place the trays on carts. If drying time to the desired moisture is 10 hr then each wheeled cart of trays will take 10 hr to pass through the tunnel.

When a dry cart emerges it makes room to load another wet crat into the opposite end of the tunnel. Such an operation becomes semi continuous. A main construction feature by which tunnel driers differ has to do with the direction of air flow relative to tray movement.(Drying and Dehydration)

The wet food carts move from left to right. The drying air moves across the trays from right to left. This is the counter flow or countercurrent principle. Its significance is that the air, when it is hottest and driest, contacts the nearly dry product, whereas the initial drying of entering carts gets cooler, moisture air that has cooled and picked upto moisture going through the tunnel.

This means that the initial product temperature and moisture gradients will not be as great and the product is less likely to undergo case hardening or other surface shrinkage leaving wet centers. Further, lower final moistures can be reached since the driest product encounters the dried air.(Drying and Dehydration)

In contrast, there also are concurrent flow tunnels with the incoming hottest, driest and travelling in the same direction. In this case rapid initial drying air slow final drying can cause case hardening and internal splits and porosity as centers finally dry, which sometimes is desirable in special products.

Just as carts of trays can be moved through a heated tunnel, so a continuous belt may be driven through a tunnel or oven enclosure. Then we have a continuous belt or conveyor drier and a great number of designs are possible.(Drying and Dehydration)

e) Belt trough drier

A special kind of air convection belt drier is the belt trough drier in which the belt forms a trough. The belt is usually of metal mesh and heated air is blown up through the mesh. The belt moves continuously, keeping the food pieces in the trough in constant motion to continuously expose new surface.

This speeds drying and with air of about 135oC, vegetable pieces may be dried to 7-5% moisture in about 1 hr. But not all products may be dried this way since certain sizes and shapes do not readily tumble. Fragile apple wedges may break.(Drying and Dehydration)

Onion slices tend to separate and become entangled. Fruit pieces that exude sugar on drying tend to stick together clump with the tumbling motion. These are but a few additional factors that must be considered in selecting a drier for a particular food.

f) Air lift drier

Several types of pneumatic conveyor driers go a step beyond tumbling to expose more surface area of food particles. These generally are used to finish dry materials that have been partially dried by other methods, usually to about 25% moisture, or at least sufficiently low so that the material becomes granular rather than having a tendency to clump and mat.

g) Fluidized bed drier

Another type of pneumatic conveyor drier is the fluidized bed drier. In fluidized bed drying, heated air is blown up through the food particles with just enough force to suspend the particles in a gentle boiling motion.

Semi Dry particles such as potato granules enter at the left and gradually migrate to the right, where they are discharged dry. Heated air is introduced through a porous plate that supports the bed of granules. The moist air is exhausted at the top.(Drying and Dehydration)

The process is continuous and the length of time particles remain in the drier can be regulated by the depth of the bed and other means. This type of drying can be used to dehydrate grains, peas and other particulates.


The moisture is drawn out from all cell tissues. The water is then bound with the solute, making it unavailable to the microorganisms. In osmotic dehydration of fruits, the method involves the partial dehydration of fruits by osmosis in a concentrated sugar solution or syrup.

Fruits Suitable For Osmotic Dehydration:

Fruits such as apple, banana, cherry, citrus, grapes, guava, mango, papaya, pineapple, plum, etc. Osmotic Dehydration can remove 30-50% of the water from fresh ripe fruits e.g mangoes, pineapple, banana, sapota and papayas.(Drying and Dehydration)

The final drying of these osmotically dehydrated fruits by vacuum drying provides a product which has good quality, attributes with respect to appearance, taste, flavour and colour as compared to sun drying.

Advantages of Osmotic dehydration

  • Minimum loss of colour and flavour.
  • Browning is prevented.
  • Sweetening of the product.
  • Reduces the water removal load
  • Increases the solid density of the product
  • Textural quality will be better
  • Simple facility and equipments are required
  • The process is less expensive.

Limitations of Osmotic dehydration Limitations of Osmotic dehydration:

The reduction in acidity level reduces the characteristic taste of some products

Sugar uptake may not be desirable in certain product. Factors influencing Osmotic dehydration

  • Pre-treatments.
  • Osmotic agents
  • Concentration
  • Temperature.
  • Agitation /circulation
  • Duration of osmosis
  • Size and thickness
  • Variety and maturity of fruits used.

Products made through Osmotic dehydration

The product is suitable as a ready to eat snack item. Also the dehydrated product could be powdered if desired, and mixed with milk powder for making other products and confectionery items.(Drying and Dehydration)


The most important kind of air convection drier is the spray drier. Spray driers turn out a greater tonnage of dehydrated food products than all other kinds of driers combined, and there are various types of spray driers designed for specific food products. Spray driers are limited to foods that can be atomized, such as liquids and low viscosity pastes or purees.

Atomization into minute droplets results in drying in a matter of seconds with common inset air temperatures of about 200oC. Since evaporative cooling seldom permits particles to reach above about 80oC (180oC) and properly designed systems quickly remove the dried particles from heated zones, this method of dehydration can produce exceptimally high quality with many highly heat sensitive materials, including milk, eggs and coffee.(Drying and Dehydration)

In typical spray drying we introduce the liquid food as a fine spray or mist into a tower or chamber along with heated air. As the small droplets make intimate contact with the heated air they flash off their moisture, become small particles and drop to the bottom of the tower from where they are removed.

The heated air which has now become most is withdrawn from the tower by a blown or fan. The process is continuous in that liquid food continues to the pumped into the chamber and atomized, along with dry heated air to replace the moist air that is withdrawn, and the dried product is removed from the chamber as it descends. Milk and coffee powder is usually dried in the spray drier. Thermoplastic materials /substances viz., fruit juices are spray dried in a specially developed BIRD spray drier.(Drying and Dehydration)

a) Drum (or) Roller driers

In drum (or) roller drying, liquid foods, purees pastes and mashes are applied in a thin layer onto the surface of a revolving heated drum. The drum generally is heated from within by steam. Drier may have a single drum or a pair of drums.(Drying and Dehydration)

The food may be applied between the nip where two drums come together, and then the clearance between the drums determines the thickness of the applied food layer, or the food can be applied to other areas of the drum. Food is applied continuously and the thin layer loses moisture.(Drying and Dehydration)

At a point on the drum or drums a scraper blade is positioned to peel the thin dried layer of food from the drums. The speed of the drum is so regulated that the layer of food will be dry when it reaches the scraper blade, which also is referred to as a doctor blade.

The layer of food is dried in one revolution of the drum and is scrapped from the drum before that position of the drum returns to the point where more wet food is applied. Using steam under pressure in the drum, the temperature of the drum surface may be well above 100oC, and is often held at about 150oC. With a food layer thickness commonly less than 2 mm, drying can be completed in 1 min or less, depending on the food material. Other features of drum driers include hoods above drums to withdraw moisture vapor and conveyors in troughs to receive and move dried product.(Drying and Dehydration)

Typical products dried on drums include milk, potato mash, heat tolerant purees such as tomato paste, and animal feeds. But drum drying has some inherent limitations that restrict the kinds of foods to which it is applicable. To achieve rapid drying, drum surface temperature must be high, usually above 120oC.

This gives products a more cooked flavour and colour than when they are dried at a lower temperature. Drying temperature can, of course, be lowered by constructing the drums within a vacuum chamber but this increases equipment and operating costs over atmospheric drum or spray drying.(Drying and Dehydration)

A second limitation is the difficulty of providing zoned temperature control needed to vary to drying temperature profile. This is particularly important with thermoplastic food materials. Whereas dried milk and dried potato are easily scraped from the hot drum in brittle sheet form, this is not possible with many dried fruits, juices and other products which tend to be sticky and semi molten when hot. Such products tend to crimp, roll up, and otherwise accumulate and stick to the doctor blade in a taffy like mass.

This condition can be substantially improved by a cold zone to make the tacky material brittle just prior to the doctor blade. But zone controlled chilling is not as easy to accomplish on a drum of limited diameter and therefore limited arc, as it would be in perhaps 6 m of length of a horizontal drying belt 45 m long.(Drying and Dehydration)

One means of chilling is by directing a stream of cool air onto a segment of the product on the drum prior to the doctor blade. For relatively heat resistant food products, drum drying is one of the least expensive dehydration methods.

Drum dried foods generally have a somewhat more cooked character than the same materials spray dried; thus drum dried milk is not up to beverage quality but is satisfactory as an ingredient in less deligately flavoured manufactured foods. More gentle vacuum drum drying or zone-controlled drum drying increases dehydration costs.(Drying and Dehydration)

b) Vacuum driers

Vacuum dehydration methods are capable of producing the highest quality dried products, but costs of vacuum drying generally also are higher than other methods which do not employ vacuum.

In vacuum drying, the temperature of the food and the rate of water removal are controlled by regulating the degree of vacuum and the intensity of heat input. Heat transfer to the food is largely by conduction and radiation.

All vacuum drying systems have four essential elements. These include a vacuum chamber of heavy construction to withstand outside air pressures, that may exceed internal pressures by as much as 9800 kg/cm2; means to supply heat; a device for producing and maintaining the vacuum; and components to collect water vapour as it is evaporated from the food.(Drying and Dehydration)

The vacuum chamber generally will contain shelves or other supports to hold the food and these shelves may be heated electrically or by circulating a heated fluid through them. The heated shelves are called platens.

The platens convey heat to the food in contact with them by conduction, but where several platens are one above another they also radiate heat to the food on the platen below. In addition, special radiant heat sources such as infrared elements can be focussed onto the food to supplement the heat conducted from platen contact.

The device for producing and maintaining vacuum will be outside the vacuum chamber and may be a mechanical vacuum pump or a steam ejector.

A steam ejector is a kind of aspiration in which high velocity steam jetting past an opening draws air and water vapour from the vacuum chamber by the same principle that makes an insect spray gun draw fluid from the can.

The means of collecting water vapour may be a cold wall condenser. It may be inside to vacuum chamber or outside the chamber but most come ahead of the vacuum pump so as to prevent water vapour from entering and fouling the pump.(Drying and Dehydration)

When a steam ejector can condense water vapour as it is drawn along the air from the vacuum chamber and so a cold wall vapour condenser may not be needed except where a very high degree of efficiency is required.

Degree of vacuum

Atmospheric pressure at sea level is approximately 15 psi, or sufficient pressure to support a 30 inch column of mercury. This is equivalent to 760 mm of Hg or 1 in Hg is approximately 25 mm. At 1 atmp or 30 in., or 760 mm of Hg, pure water boils at 100oC.

At 10 in or 250 mm of Hg pure water boils at 72oC. At 2 in. or 50 mm of mercury pure water boils at 38oC. High vacuum dehydration operates at still lower pressures such as fractions of mm of Hg. Freeze drying generally will operate in the range of 23 mm down to about 0.1 mm of Hg.(Drying and Dehydration)

There are two kinds of vacuum drier.

1. Vacuum shelf driers

Batch type If liquids such as concentrated fruit juices are dried above about 55 mm Hg, the juice boils and splatters, but in the range of about 3 mm Hg, and below, the concentrated juice puffs as it loses water vapour. The dehydrated juice then retains the puffed spongy structure. Since temperature well below 40oC can be used, in addition to quick solubility there is minimum flavour change or other kinds of heat damage. (Drying and Dehydration) A vacuum shelf drier is also suitable for the dehydration of food pieces. In this case, the rigidity of the solid food prevents major puffing, although there also is a tendency to minimize shrinkage.

2. Continuous vacuum belt drier

Continuous type drier This drier is used commercially to dehydrate high quality citrus juice crystals, instant tea and other delicate liquid foods. The drier consists of a horizontal tank like chamber connected to a vacuum producing, moisture condensing system. The chamber is about 17 m long and 3.7 m in diameter.

Within the chamber are mounted two revolving hollow drums. Around the drum is connected a stainless steel belt which moves in a counter clock wise direction. This drum on the right is heated with steam confined within it. This drum heats the belt passing over it by conduction. As the belt moves, it is further heated by infrared radiant elements.

The drum to the left is cooled with cold H2O circulated within it and cools the belt passing over it. The liquid food in the form of a concentrate is pumped into a feed pan under the lower belt strand. An applicator roller dipping into the liquid continuously applies a thin coating of the food onto the lower surface of a moving belt.(Drying and Dehydration)

As the belt moves over the heating drum and past the radiant heaters, the food rapidly dries in the vacuum equivalent to about 2 mm Hg. When the food reaches the cooling drum, it is down to about 2% moisture. At the bottom of the cooling drum is a doctor blade which scrapes the cooled, embrittled product into the collection vessel.

The belt scrapped free of product receives additional liquid food as it passes the applicator roller and the process repeats in continuous fashion. Products dried with this equipment have a slightly puffed structure. If designed, a greater degree of puffing can be achieved.(Drying and Dehydration)

This has been done in the case of milk by pumping nitrogen gas under pressure into the milk prior to drying. Some of the gas goes into solution in the milk. Upon entering the vacuum chamber this gas comes out of solution violently and further puffs the milk as it is being dried.


The foam is deposited on a perforated tray or belt support as a uniform layer approximately 3 mm thick. Just before the perforated support enters the heated oven it is given a mild air blast from below.(Drying and Dehydration)

This forms small craters in the stiff foam which further expands foam surface and increases drying rate. At oven temperatures of about 82oC foam layers of many foods can be dried to about 2 to 3% moisture in approximately 12 min.


Freeze drying can be used to dehydrate sensitive high value liquid foods such as coffee and juices, but it is especially suited to dry solid foods of high value such as straw berries, whole shrimp, chicken dice, mushroom slices and sometimes food pieces as large as steaks and chops. These types of foods, in addition to having delicate flavours and colors, have textural and appearance attributes which cannot be well preserved by any current drying method except freeze drying.(Drying and Dehydration)

Any conventional drying method that employs heat would cause considerable shrinkage distortion and loss of natural strawberry structure (texture), upon reconstitution such as dried strawberry would not have the natural colour, flavour or turgor and would be more like a strawberry preserve or jam.

This can be largely prevented by drying from

the solidly frozen state, so that in addition to low temperature, the frozen food has no chance to shrink or distort while giving up its moisture. The principle behind freeze drying is that under certain conditions of low vapor pressure, water can evaporate from ice without the ice melting. When a material can exist as a solid, a liquid and a gas but goes directly from a solid to a gas without passing through the liquid phase.(Drying and Dehydration)

The material is said to sublime. Dry ice sublimes at atmospheric pressure and room temperature. Frozen water will sublime if the temperature is 0oC or below and the frozen H2O is placed in a vacuum chamber at a pressure of 4.7 mm (or) less. Under such conditions the H2O will remain frozen and water molecules will leave the ice block at a faster rate than water molecules from the surrounding atmosphere reenter the frozen block.

Within the vacuum chamber heat is applied to the frozen food to speed sublimation and if the vacuum is maintained sufficiently high usually within a range of about 0.1 to 2 mm g and the heat is controlled just short of melting the ice, moisture vapour will sublime at a near maximum rate. Sublimation takes place from the surface of the ice, and so as it continues the ice front recedes towards the center of the food piece; i,e. the food dries from the surface inward. Finally, the last of the ice sublimed and the food is below 5% moisture. Since the frozen food remains rigid during sublimation, escaping H2O molecules leave voids behind them, resulting in porous sponge like dried structure.

Thus freeze dried foods reconstitute rapidly but also must be protected from ready absorption of atmospheric moisture and O2 by proper packaging. A heating plate is positioned above and below the food to increase heat transfer rate but an open space is left with expanded metal so as not to seal off escape of sublimed H2O molecules.(Drying and Dehydration)

Nevertheless, as drying progresses and the ice front recedes, drying rate drops off for several reasons. Thus the porous dried layer ahead of the reducing ice layer acts as an effective insulator against further heat transfer and the porous layer slows down the rate of escape of H2O molecules subliming from the ice surface.

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