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Food Preservation by Drying

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Food Preservation by Drying

  • Food has been preserved by drying for centuries. If properly stored, some foods, such as grains, can be preserved without spoiling for long periods if they are sufficiently dry when harvested or slightly dried after harvest. 
  • Dryness preserves food by preventing enzymes and microbes from acting by removing moisture or binding it (by solutes, for example).
  • A food can be dried by removing its water, or it can be dried by reducing its available moisture, i.e., reducing its AW. 
  • Microorganisms cannot access moisture in flesh when dried fish is heavily salted, for instance. 
  • To reduce the amount of moisture available, sugar can be added, as in sweetened condensed milk.
  • Several methods exist for removing moisture from foods, such as sun-drying or artificial methods. 
  • In connection with the drying of foods, a number of inexact terms are used. When a food is sun-dried, moisture is removed from it without the use of artificial heat, relative humidity, or air velocity controls. 
  • Under controlled conditions of temperature, relative humidity, and air flow, a dehydrated or desiccated food has been dried artificially by heat. 
  • It is usually implied by the term condensed that moisture has been removed from a liquid food, and by the term evaporated that moisture has been removed from a liquid food.

Principle of Preservation by Drying/ Dehydration/ Concentration

In order to allow microorganisms to grow and enzyme activity to occur, sufficient moisture must be removed. Water activity is reduced when moisture is removed from the product, extending the product’s shelf life. A product can be stored for more than a year if the moisture content is reduced to 1 to 5 percent. After drying, the fresh material should retain its natural flavour, food value, and characteristic cooking quality. It is important that a dried product appears similar to its original form when reconstituted with water.

Methods Of Drying

There are two main ways to dry, namely:

  1. natural drying and
  2. mechanical dehydration or artificial drying.

There are three types of natural drying influenced by sunlight and wind: sun, solar, and shade drying. Temperature, air flow, and humidity are uncontrollable in natural drying, while they can be controlled in artificial drying.


Depending on the conditions employed in drying process, mechanical dehydration or artificial dehydration can be classified as atmospheric or subatmospheric dehydration. At atmospheric pressure, drying can be divided into batch and continuous types based on the mode of drying. Among mechanical drying methods, there are 

  1. heated air drying, 
  2. direct contact with a heated surface, such as drum drying, and 
  3. the application of energy from a microwave or dielectric source.

There are two types of commercial dehydrators, natural and forced draught. Natural dehydrators are generally larger in size and use circulation of air. The natural draught method dries food by bringing heated air to the surface. There are several types of driers, such as kilns, towers, and cabinets. Heat is moved across the food by currents of air, usually in tunnels. As an alternative, heated air can be used to move the food or a conveyor belt. Tunnel driers and belt driers are examples. Due to the high cost of forced draught dryers, temperature and humidity can be carefully controlled to produce a good dehydrated product.

1. Sun drying

  • As the name implies, sun drying involves drying food products under natural sunny conditions. During the drying process, no energy is consumed. 
  • A hot day with a minimum temperature of 35°C and low humidity is ideal for sun drying of foods. 
  • Natural drying cannot be used on poor quality produce to achieve good quality dried products. 
  • It is estimated that this method limits moisture content to approximately 15%. 
  • With sun drying, contamination and intermittent drying are common problems. Humidity must be low in order for it to be possible.
  • Knives, peelers, trays and other simple equipment are required. A plastic sheet can also be used. 

Process of Sun Drying

  • The process involves washing, peeling, and preparing fruit and vegetables. 
  • Generally, fruits are sulphured before drying, while vegetables are blanched before drying to prevent enzymatic browning. 
  • It is rare for fruits to be blanched. When held firmly in the hand, dry fruits do not display signs of moistness or stickiness.
  • Brittle vegetables are considered dry. The dehydrator should be removed at this point. 
  • In vegetables, residual moisture should not exceed 6-8 percent, and in fruits, 10-20 percent. 
  • It is usually necessary to soak dried fruits in water overnight and then cook them, while dried vegetables are usually soaked in water overnight and then cooked.

2. Solar drying

  • Structures are designed to collect and enhance solar radiation for solar drying. 
  • Drying is faster with solar dryers because they produce high air temperatures and low humidity. 
  • Drying with this drier is faster than with the sun, and it also requires less drying space. However, it cannot be used on cloudy days. 
  • Solar driers can be classified into three types: 
    1. Those that are directly heated by the sun, such as absorption driers or hot boxes,
    2. A solar absorber or heat exchanger heats the air used in indirect or convection driers by exposing the product to warm air.
    3. A combination of the first and second types of dryer.

3. Shade drying

  • When foods are exposed to direct sunlight for drying, they lose their colour. 
  • Herbs, green and red chillies, okra, beans, etc. are generally dried in shaded areas with good air circulation.

4. A home scale dehydrator or drier:

  • The box has dimensions of 90x90x60 cm and is made from galvanized steel. It consists of a perforated iron tray at the bottom. 
  • Approximately 2-3 feet above ground, the box is mounted on a wooden frame. 
  • There are two slits at the top that can be closed by shutters. The dryer can hold about seven trays. 
  • A gas stove or any other source of heating can be used to heat the material on trays. 
  • Vegetables and fruits are usually dehydrated at 43°C initially, then gradually increased to 60-66°C and 66-71°C as the temperature increases. 
  • For 25 kg fruit, 100-200 g of sulphur is necessary for a home scale drier. An average drying time of 1/2 hours to 2 hours is required

5. Oven drying

  • You can dry fruits, vegetables, fruit leathers, and meat in a conventional oven with a thermostatic setting of 60°C. A cabinet drier like this.

6. Kiln drier

  • A kiln evaporator is also known as a kiln. 
  • It has two floors. 
  • Food to be dried is spread on the top floor, while the furnace is located on the lower floor. 
  • A ventilator conveys heat. Large pieces of food are usually placed in it.

7. Tower drier

  • It is also called as stack type drier. 
  • This drier consists of a furnace room containing the furnace, heating pipes and cabinet in which fruits are kept in perforated trays. 
  • Heated air from the furnace rises through the trays. 
  • Heating is through steam coils placed between the trays. 
  • The trays are interchanged as drying progresses.

8. Tunnel belt drier

  • Above a furnace room are several parallel, sloped, and narrow chambers. 
  • An upper conveyor belt passes trays, which are removed at the lower conveyor belt. 
  • A fan circulates air for drying. Air is occasionally let in to control humidity. It is possible to heat directly or by radiation. 
  • Heat is passed over material indirectly, which is sufficient, but there is a risk of overheating. 
  • A furnace’s hot gases are carried over pipes by radiation. There is also the possibility of using steam pipes.

9.Belt-trough drier

  • This dryer has belts made of metal mesh that resemble troughs. 
  • In the trough, the food is dried by hot air passing through the mesh.

10. Fluidized bed drying

  • Food particles are placed on a porous plate, and hot air is blown up from the pores through the particles with just enough force to suspend them in a gentle boiling motion. 
  • Dried food is removed from one end of the dryer and fresh food is fed from the other.

11. Foam mat drying

  • This method is used to dry pulps and concentrates. 
  • A foaming agent is used to whip the fluid food material into a stable foam of low density. The foam is cast in thin layers onto trays or belts.
  •  It exposes a large surface area for rapid moisture escape, resulting in a rapid atmospheric drying process at a lower temperature. 
  • Foam is spread on trays and dried at low temperatures as a mat. Foam mat drying is one such method of dehydration.
  • It takes less time to dry this way, and the dried product can be reconstituted easily. 
  • Compared to drum drying, spray drying, and puff drying, this method is less expensive. 
  • This drying technology is used to produce orange, tomato, pineapple, and lemon juice powder, apple sauce, and baby foods. 
  • Glycerol monostearate (GMS), egg albumin, guar gum, groundnut protein isolate, and carboxymethylcellulose are used as foaming agents.

12. Spray drying

  • Purees, pastes, and liquids that can be atomized can be dried using spray driers. 
  • A hot air current is used to spray the material. Drying chambers collect dried products at the bottom. 
  • Using inlet air temperatures of about 200°C and properly designed systems, atomization into minute droplets results in drying within seconds. 
  • With this method of dehydration, milk and coffee, which are highly heat sensitive, can be dehydrated to an exceptionally high quality.

13. Drum or roller drying

  • This method is used to dry purees and liquid foods. 
  • The process is used to prepare mango flakes, orange flakes, baby foods, etc. 
  • Blending the pulp of fruit with edible starch and adjusting the acidity of the blend are the steps involved. 
  • Mixture is poured little by little into stainless steel drums that are heated. Slowly rotating drums. 
  • A continuous thin sheet or powder is formed when the product dries. A tin container is used to collect the pieces after they have been broken down into smaller pieces. 
  • Because the flakes are highly hygroscopic, the lid must be placed immediately. 
  • Drying high viscous liquids or pureed foods with drums is one of the most energy-efficient drying techniques.

14. Microwave drying

  • Microwaves are used to dry food products in this method.

15. Vacuum puffing and dehydration

  • Vacuum driers are required for drying at low temperatures. A sudden application of vacuum causes food to puff up. 
  • Generally used for preparing orange juice powder or preparing potato pieces with porous structures. 
  • While hot, potato pieces are vacuumed for a short period and then dried. 
  • As a result of the instant flashing of water vapour from inside, the texture becomes porous and dries quickly.

16. Drying by freezing

  • By using this method, food in pieces and liquids can be dried. The process of freeze drying is used to manufacture fruit juice concentrates. 
  • Under vacuum, the material is frozen on trays and then dried. As a result of vacuum drying, the material dries directly without passing through an intermediate liquid stage. 
  • In freeze drying, water evaporates from ice without melting under certain conditions of low vapour pressure.
  • Foods such as juices, coffee, strawberries, chicken dice, mushroom slices, and coffee are generally dried using freeze drying. 
  • Despite being highly hygroscopic, the dried product can easily be reconstituted. Fruit juice concentrates have excellent taste and flavour, as well as good reconstitution properties. Equipment costs make this method expensive.
  • Combining freeze drying with air drying can reduce drying costs. It is possible to air dry vegetables pieces down to about 50% moisture and then freeze dry them down to 2-3% moisture, for example.

17. Accelerated freeze drying (AFD):

  • It is used to dry food material without affecting its shape or texture. The product has a good reconstitution property, taste, and flavour. 
  • In a cabinet freeze dryer, the pieces of material are pressed between two perforated or wire mesh trays. 
  • By decreasing the clearance between the trays, the bulk of the pieces gradually reduces as the material dries. 
  • On rehydration, the dried material regains its shape. This technique is used to dry meat and other products.

Factors In The Control Of Drying

Fresh produce’s rate of drying is affected by several factors, including:

  • Composition of raw materials: Sugary foods and foods containing other solutes dry slowly.
  • Size, shape and arrangement of stacking of produce: The larger the surface area, the faster the drying.
  • Temperature, humidity, and air velocity: The greater the temperature differential between the product and the drying medium, the faster the product dries. The lower the humidity of the environment, the faster the drying will be.
  • Pressure (atmospheric or under vacuum): The lower the atmospheric pressure, the lower the temperature needed to evaporate water.
  • Heat transfer to surface (conductive, convective and radiative): Radiation transfers heat the fastest, followed by convection and conduction.

Process of Food Drying

A. Treatments Of Foods Before Drying

The pretreatments that are used to prepare food items to be dried have a significant impact on the microbial population as will be outlined. The pretreatments could include

  1. Sorting and selection based on size maturity, soundness, and size,
  2. Washing, especially of vegetables and fruits
  3. Peeling vegetables and fruits by hand machines or lye bath abrorsion
  4. division into halves and slices, shreds or cubes
  5. alkali dipping is primarily used to dip fruits like grapes, raisins and prunes (for drying in the sun) and makes use of high temperatures of 0.1 or 1.5 percent of lye, or sodium carbonate
  6. the blanching or scalding process of some fruits and vegetables (apricots peaches, apricots) and
  7. sulfurization of light-colored fruit and certain vegetables and certain fruits. Fruits are naturally sulfurized by exposure to the gas sulfur dioxide produced by the combustion of sulfur, so that a range of between 1,000 and 3000 ppm, based on the type of fruit, can be absorbable.

It is possible to sulfurize vegetables after blanching in the same method or by dips into the sulfite solution or by spraying it with solutions. Sulfuration helps preserve a light-colored color, preserve vitamin C, and possibly vitamin A. It also helps keep insects away; it can also kill many of the microorganisms in the.

B. Procedures After Drying

The drying process is different according to the dried food.

  1. Sweating: The term “sweating” refers to “Sweating” involves storage generally in containers or bins to equalize moisture levels or reading-dition of the moisture to a desired degree. It is usually used in conjunction along with dried fruit and certain walnuts (almonds, English walnuts).
  2. Packaging: The majority of foods are packaged immediately after drying in order to guard against moisture, contamination by microorganisms, as well as infestation by insects. However, some dried food items (e.g. fruit, nuts and nuts) could be kept for up to one year prior to packaging.
  3. Pasteurization: Pasteurization is restricted in the majority of cases to dried fruit and eliminates any pathogens that might be present, in addition to eliminating spoilage organisms. The fruit is usually processed in the packaging and the process, which varies according to the fruit, can range between 30 and 70 min at 70-100 percent relative humidity between 65.6 or 85 C.

Examples of some Dried Foods

Dried Fruits

  • The microorganisms present in the majority of fresh fruits range between a small number to several depending on the pretreatments used and, on the majority of dried fruits, they range from a few hundred for each kilogram of fruit to thousands. In whole fruits, they’re mostly located on the exterior of the fruits.
  • Spores of molds and bacteria tend to be most abundant.
  • If a portion of the fruit is causing the growth of mold and its sporulation prior to drying or following the fruit has dried, moldspores can be found in large quantities.

Dried Vegetables

  • The number of microbes found in dried vegetables range from small amounts to millions per grams.
  • The percentages on the vegetables just prior to drying can be high because of the growth and contamination that occurs after blanching. The proportion of the vegetable killed during the dehydrating process is typically less than that of the more acidic fruits.
  • If drying trays are not properly loaded, souring of veggies as potatoes and onions by lactic acid bacteria , accompanied by an increase in the number of bacteria could occur in the drying process.
  • The risk is more pronounced when onions aren’t blanched. Most bacteria can be present on dried vegetables.
  • Numerous researchers have identified various genera of bacteria they have found (summarized by Vaughn 1951) to include Escherichia, Enterobacter, Bacillus Clostridium, Micrococcus Pseudomonas and Streptococcus. Vaughn identified Lactobacillus as well as Leuconostoc species dominant in numerous varieties of dehydrated vegetables.

Dried Eggs

  • Dry eggs can contain anywhere from to a few hundred microorganisms most of them bacteria, in gram, up to 100 million depending on the eggs which have been broken and the method employed.
  • Because the eggs in fresh eggs that are of high quality are usually not contaminated by microorganisms or comprise just a handful of dried eggs must be free of microorganisms.
  • However, the addition of eggs that have been poorly cleaned or eggs that were allowed to sweat eggs that are dirty and cracked and eggs already infected by microorganisms can result in the addition of numerous organisms. Also, contamination and growth could occur when breaking eggs and other handling, before drying.
  • The drying process can decrease the number of animals by ten to a hundredfold , yet still allow huge numbers to live.
  • Many kinds of organisms have been identified in eggs that have been dried, including micrococci, streptococci and coliforms such as sporeformers, molds, and microorganisms.
  • If the egg white has been pretreated with fermentation and the count on the dried product could be very high. Egg yolk is a more effective cultivator than white and will be more crowded at the breaking point and will help to promote growth prior to drying.

Dried Milk

  • The amount of microorganisms found in milk can range between a few hundred per gram to millions, contingent on the type of milk being dried and the process of drying.
  • Drum drying or rollers kill more organisms than the spraying process. The most prevalent kinds of microbes found that are found in milk dry are thermoduric streptococciand micrococci, and sporeformers.

Microbiology Of Dried Foods

1. Before Reception at the Processing Plant

  • The microbiology of food prior to their arrival at the processing plant will likely to be similar regardless of whether the food are drying or frozen, chilled or canned. processed.
  • The fruits and vegetables are surrounded by water and soil organisms that grow on them after harvesting and also their own soil flora. Also, rotten areas contain microorganisms responsible for the degradation.
  • The growth of certain organisms can occur before the food products get to the processing plant when the conditions are favorable. Therefore, the piled veggies can be heated and help support the surface growth of slime-forming and flavor-harming or bacteria that cause rot.
  • Meats and poultry can be contaminations by soil and intestinal contents handles, equipment, and handlers.
  • Fish can be contaminated by the water they drink and their own intestinal contents and slime and handling equipment and handlers, and the growth process can begin prior to the time the fish enter the plant for processing.
  • Eggs can be ruined by the hens, nests, and by the handler. If they are properly and quickly handled, they could support some Microbial growth.
  • Milk is susceptible to contamination at the point of release by the cow until its acceptance at the processing plant . It could aid in the growth of psychrotrophic bacteria.

2. In the Plant before Drying

Microorganisms growing that have started on food items before they’ve reached the drying facility may remain in the plant until the time of drying. Additionally, the equipment and workers can cause contamination to food.

As you will see, certain pretreatments decrease the number of organisms and others can raise them, but food products could be affected by one treatment.

  1. The grading, selection, and sorting of foods: The selection, grading and sorting of food items including eggs, fruits, vegetables and milk, will determine the type and quantity of microorganisms that are present.
  2. The elimination of spoiled fruits and vegetables: Removal of fruits and vegetables that are rotten or parts that are spoiled will decrease the number of microorganisms within the product that need to be dried. The refusal of dirty, cracked eggs or those that are rotten is a similar goal and so does the refusal of milk that doesn’t meet the bacteriological standards of quality.
  3. Washing fruits and vegetables: Washing vegetables and fruits removes soil and other adhering substances and helps to eliminate microorganisms. There is also the possibility of adding organisms in the event that the water is not of good quality and the water’s surface can encourage the growth of microorganisms when the opportunity arises for it. The washing of eggs can be to be more harmful than beneficial, unless they are done quickly, as the moisture aids in the penetration of bacteria into the shell.
  4. Peeling fruits or vegetables: Peeling vegetables or fruits in particular using steam or lye, will decrease the number of microorganisms because the majority of microorganisms typically reside in the outer layer of the food. 
  5. Slicing or cutting: Cutting or slicing not result in an increase in numbers of organisms, but it can cause harm if the equipment isn’t properly cleaned and clean.
  6. Dipping in alkali: Dipping Dosing in alkali, applied to certain fruits prior to drying in the sun, could decrease the number of microbial colonies.
  7. Blanching or scalding: scalding or blanching vegetables can reduce the number of bacterial colonies up to 99 percent in some cases. 
  8. Sulfuring of fruits and vegetables: Sulfuration of vegetables and fruits after blanching, quantities of bacteria could increase due to contamination of equipment and the potential to grow. Sulfurization of fruits and vegetables can also result in a significant decrease in the number of microorganisms. It also helps stop the growth of dry products.

3. During the Drying Process

  • The drying process results in a decrease in the total number of microorganisms. However the effectiveness of the drying process varies according to the type and quantity of microorganisms present at the time and the drying process used.
  • Generally, all yeasts and the majority of bacteria are destroyed. However, the spores of molds and bacteria typically remain as do the vegetative cells of a handful of varieties of heat-resistant bacteria.
  • In the future, we’ll be able to see Drying in poor conditions can even allow the expansion of microorganisms.
  • More microorganisms are killed through freezing than are killed by dehydration during the freeze drying process.
  • Any process that requires drastic and abrupt variations in temperature, such as the increase in drying due to temperature or a decrease when freezing, is more likely to result in metabolic damage to certain organisms, which makes them more nutrient-deficient.

4. After Drying

  • When the conditions for drying and storage are appropriate the food will not be affected by development of microorganisms within the dried food.
  • In the course of storage, there is a gradual reduction in the amount of organisms present that is more rapid at the beginning and then slower. Microorganisms that are resilient to drying will fare most effectively, and the proportions of these organisms will rise.
  • Particularly resistant to storage in humid conditions is the spores of bacteria , molds, a few of the micrococci, as well as microbacteria.
  • There is a possibility for contamination of dried food in the packaging process or other handling after drying.
  • The special treatments that are given to certain dry food items can affect the microbial count. The sweating of dried fruits to balance moisture could allow some Microbial growth.
  • The pasteurization process for dry fruits can lower the amount of microorganisms. Certain products are repackaged to sell at retail, e.g., figs in the area of the middle east and can be affected by contamination later on.
  • The microbial content as well as the temperature of the water that is used to rehydrate dried products can impact the durability of the product that has been rehydrated.
  • The bacteria in the chicken meat that is freeze dried are further reduced through rehydration using drinking water that is 50 C and they are eliminated when the temperature of the water is between 85 and 100 C.
  • The growth of bacteria within the meat that has been rehydrated occurs at temperatures that are conducive to growth, however the shelf life is excellent (keeping duration) when the temperature is 4. C. Staphylococcus aureus has been observed to be able to withstand freezing and rehydration up to 60 C Therefore, rehydrating in a temperature of 100 C is suggested.

Advantages of Drying

  • The principal benefit of eating dry food is the fact that it’s convenient for the person who owns it and is simple to measure, simple to serve, and simple to keep in the fridge.
  • They are able to store for an unlimited time in the right storage conditions.
  • Storage, transportation, and handling costs are cut.
  • Make sure you meet the latest marketing need.
  • Simple and affordable way to store excess food to be used at a later date.
  • It’s portable – Take it wherever you travel.
  • Aids in reducing loss after harvest.


  • Ahmed, Dr-Naseer & ٭‬, Jagmohan & Singh, Harmeet & Chauhan, Prerna & Gupta, Anisa & Anjum, Harleen & Kour,. (2013). Different Drying Methods: Their Applications and Recent Advances. 4. 34-42. 
  • https://www.slideshare.net/Nugurusaichandan/preservation-by-drying
  • https://www.partselect.com/JustForFun/Preserving-Food-by-Drying.aspx
  • http://ecoursesonline.iasri.res.in/mod/page/view.php?id=111449


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