Drying Method of Food Preservation – Types, Methods, Examples

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.

Purpose of drying

The purpose of drying in food processing is multi-fold and serves several important functions. Here are the key purposes of drying:

1. Extending Shelf-Life: The primary objective of drying is to extend the shelf-life of foods. By removing moisture from food, the water activity (aw) is reduced, creating an unfavorable environment for microbial growth. Lower water activity inhibits the growth of bacteria, yeasts, molds, and other microorganisms, thus helping to prevent spoilage and foodborne illnesses.
2. Inhibition of Microbial Growth: Drying effectively reduces the moisture content in foods, which inhibits the growth of microorganisms. Without sufficient moisture, microorganisms cannot multiply and cause food spoilage. This preservation effect helps to maintain the quality and safety of the dried foods for longer periods.
3. Enzyme Activity Reduction: Enzymes in food play a significant role in promoting chemical reactions that can cause undesirable changes, such as browning, texture deterioration, and nutrient degradation. Drying lowers the water content, which slows down enzymatic activity and helps preserve the natural characteristics, flavor, and nutritional value of the food.
4. Reduction in Weight and Bulk: Drying removes a significant amount of water from foods, leading to a reduction in weight and bulk. This reduction in moisture content makes the dried foods lighter and more compact, resulting in reduced transportation and storage costs. It also makes the dried foods more convenient and portable for consumers.
5. Preservation of Nutrients: Drying can help preserve the nutritional value of foods by minimizing nutrient degradation. While some vitamins and minerals may be slightly affected during the drying process, the preservation of nutrients is generally better compared to other preservation methods that involve higher temperatures or longer exposure to air.

Principle of Preservation by Drying/ Dehydration/ Concentration

The principle of drying in food processing is based on the state of water in the food and its effect on the drying process. Here are the key principles of drying:

1. Bound Water: Bound water refers to water that is not easily removable from the food. It is unfreezeable, immobile, non-squeezable, and non-solvent water. Bound water requires more energy to remove compared to free water during the drying process.
2. Free Water: Free water is the water present in the food that is easily removable. It is the water available for enzymatic activity and microbial growth, which can lead to food spoilage. The removal of free water is crucial for extending the shelf-life of foods.
3. Moisture Content: Moisture content refers to the total water content in the food, which can exist in the free or bound state. The moisture content affects the drying process and determines the amount of water that needs to be removed from the food to achieve the desired level of dryness.
4. Water Activity (aw): Water activity is a measure of the availability of free water in a food product. It indicates the level of moisture that supports enzymatic activity and microbial growth. Lowering the water activity inhibits microbial growth and helps prevent food spoilage.

The effectiveness of the drying process is influenced by the moisture content and water activity of the food. Foods with high moisture content and water activity have a shorter shelf-life and are prone to microbial growth. On the other hand, dried foods with lower moisture content and water activity have an extended shelf-life, lasting from days to years, depending on the specific food product.

Stages of drying

The process of drying can be divided into two stages: the constant rate period and the falling rate period. Here’s an explanation of each stage:

1. Constant Rate Period: In the constant rate period, as heat is applied to the food in the dryer, moisture moves from the interior of the food to the surface at the same rate as it evaporates from the surface. The surface of the food remains wet during this period until it reaches a critical moisture content. This period is characterized by a relatively high drying rate, as moisture is easily removed from the food.
2. Falling Rate Period: Once the moisture content of the food falls below the critical moisture content level, the drying rate gradually decreases. The moisture in the food reaches equilibrium with the drying air, and the process enters the falling rate period. During this stage, the rate of water movement from the interior to the surface becomes slower compared to water evaporation from the surface. Removing moisture from the food becomes more challenging, and the drying process slows down.

The movement of water from the interior of the food to the surface during drying can occur through various mechanisms:

• Movement of Liquid by Capillary Forces: Water can be transported through the capillary network present in the food, driven by capillary forces. Capillary action helps in moving liquid water towards the surface for evaporation.
• Diffusion of Liquids: Diffusion plays a role in the movement of moisture within the food matrix. It involves the movement of water molecules from regions of higher concentration to regions of lower concentration. This process aids in the transport of moisture towards the surface.
• Water Vapor Diffusion: As moisture evaporates from the surface, water vapor diffuses through the air surrounding the food. This diffusion process helps in maintaining a concentration gradient, facilitating the movement of moisture from the interior of the food to the surface.

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.

Types of dryer

1. Solid surface dryer

A solid surface dryer is a type of drying equipment that utilizes a solid surface for heat transfer to remove moisture from the product. Here are some specific types of solid surface dryers:

1. Drum Dryer: In a drum dryer, the product in the form of a slurry is deposited onto a rotating drum in a thin film. Heat is transferred to the product through the drum wall, typically using steam. As the drum rotates, the moisture in the product evaporates, resulting in the formation of a fine powder. Drum dryers are commonly used in the food industry for the production of powdered products like milk powder or fruit powders.
2. Vacuum Shelf Dryer: The vacuum shelf dryer consists of a cabinet with hollow shelves. The heating medium, such as hot water or steam, is circulated within the shelves. The product is placed on pans or trays, which are positioned on the shelves. The unit is closed and vacuumed, creating a low-pressure environment that enhances the drying process. Vacuum shelf dryers are utilized for drying sensitive or heat-sensitive food products, such as citrus powder or tomato powder. However, they are relatively expensive due to the complex setup.
3. Continuous Vacuum Dryer: In a continuous vacuum dryer, the product is placed on a stainless steel belt, and heat is transferred to the product film through the belt. The drying process occurs in an enclosed setup under a vacuum, promoting efficient moisture removal. Continuous vacuum dryers are suitable for continuous production processes and are commonly used in industries where a continuous drying operation is required.

Solid surface dryers offer advantages such as efficient heat transfer, controlled drying conditions, and the ability to handle heat-sensitive products. The specific type of solid surface dryer chosen depends on the characteristics of the product being dried, the desired output, and the available resources and infrastructure.

2. Adiabatic dryers

Adiabatic dryers are a type of drying equipment where heat is transferred to the dryer through hot gas, which in turn transfers heat to the water in the food and carries away the resulting water vapor. Here are some specific types of adiabatic dryers:

1. Cabinet Dryer: Cabinet dryers consist of a chamber where food product trays are placed. Hot air generated by a heater is circulated in the chamber using a fan, drying the material. Cabinet dryers are less expensive and have an easy setup, making them suitable for small-scale dehydration of vegetables and fruits.
2. Tunnel Dryer: Tunnel dryers are long, tunnel-like structures typically ranging from 10 to 20 meters in length. Trays of food are placed inside the tunnel, and hot air is blown across them. The trays are inserted from one end and taken out from the other. Tunnel dryers are commonly used for dehydrating fruits and vegetables.
3. Kiln Dryer: Kiln dryers have a two-story structure. The upper story has narrow slats where pieces of food products are placed, while the lower story houses a furnace that produces hot gas. The hot gas is passed through the product, facilitating drying. Kiln dryers are suitable for drying products like apple slices, hops, and potatoes.
4. Spray Dryer: Spray dryers are used to dry solutions, pastes, or slurries. The products are dispersed as small droplets and dried with drying air. This can be done in a horizontal long chamber or a vertical long chamber where a conveyor carries the dried products in powder form.
5. Air-Lift Dryer: In air-lift dryers, food products like potato granules are mixed with hot air and carried up a narrow column. As the product gets dried, it becomes suspended in the air due to the incorporated air velocity. At the top of the drying column, the air velocity is lowered, allowing the product to settle into a collector.

Adiabatic dryers offer efficient drying by utilizing hot gas for heat transfer. The specific type of adiabatic dryer chosen depends on factors such as the nature of the food product, scale of production, and desired drying outcomes. These dryers are commonly used in the food industry for various drying applications.

3. Foam-mat dryer

• The foam-mat dryer is a type of drying equipment specifically designed for drying liquid foods. In this drying method, the liquid food is first prefoamed using an edible foaming agent. The foaming process helps create a stable foam with increased surface area. The stable foam is then spread on a mat or conveyor belt and subjected to hot air for drying.
• The foam-mat dryer is particularly suitable for drying heat-sensitive, viscous, sticky, and liquid food products. It provides an effective solution for drying foods that would otherwise be challenging to dry using conventional methods. The foam layer created by the prefoaming process allows for faster and more efficient drying.
• During the drying process, hot air is directed onto the foam layer, promoting the evaporation of moisture. The increased surface area of the foam enables faster moisture removal compared to drying the liquid food in its original liquid form. The hot air circulates around the foam, extracting the moisture and reducing the moisture content of the foam layer.
• The foam-mat dryer offers several advantages for drying liquid foods. It allows for quick drying times, with the foam layer of many liquids and semi-solid foods reaching a moisture content of about 2-3% in approximately 12 minutes. This rapid drying helps preserve the quality and characteristics of the food product.
• Additionally, the foam-mat drying method is suitable for achieving uniform drying across the entire foam layer, ensuring consistent moisture removal and preventing uneven drying or clumping of the product.
• Overall, the foam-mat dryer is a specialized drying technique that provides an efficient and effective solution for drying heat-sensitive, viscous, sticky, and liquid food products. By utilizing the foam layer, it allows for faster drying times and helps maintain the quality of the dried food.

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.

Uses of the drying process for food preservation

The drying process is widely used for food preservation across various types of dryers and food products. Here are some examples of different dryers and the foods that can be preserved using them:

1. Drum Dryer: Drum dryers are commonly used for drying milk, vegetable juices, and bananas. The heat applied to the rotating drum helps remove moisture from these liquid or semi-liquid food products, resulting in the production of powdered or dried forms.
2. Vacuum Shelf Dryer: The vacuum shelf dryer is utilized for the preparation of citrus powder. The low-pressure environment created by the vacuum enhances the drying process, allowing for the production of powdered citrus products.
3. Continuous Belt Dryer: Continuous belt dryers are suitable for drying vegetables. The food products are placed on a moving belt, and hot air is circulated to remove moisture, resulting in the preservation of dehydrated vegetables.
4. Foam-Mat Dryer: Foam-mat dryers are used for drying juices. The prefoaming process creates a stable foam layer that is then dried using hot air, leading to the production of dried juice products.
5. Spray Dryer: Spray dryers are commonly used for drying whole eggs, egg yolk, and milk. The liquid products are dispersed as small droplets, which are then dried with hot air, resulting in powdered forms.
6. Cabinet Dryer: Cabinet dryers are suitable for drying apples and vegetables. The food products are placed in trays inside a chamber, and hot air is circulated to remove moisture, preserving the dried fruits and vegetables.
7. Tunnel Dryer: Tunnel dryers are often utilized for drying a variety of fruits and vegetables. The food products are placed on trays or racks inside a tunnel-like structure, and hot air is blown across them, facilitating the drying process.
8. Conveyor Dryer: Conveyor dryers are suitable for drying cereals, fruits, nuts, and biscuits. The food products are placed on a moving conveyor belt, and hot air is applied to remove moisture, preserving the dried goods.
9. Solar Dryer: Solar dryers harness solar energy to dry fruits and vegetables. The food products are exposed to sunlight, allowing the heat to remove moisture and preserve the dried fruits and vegetables.

The drying process is used for value addition across different types of food products. Low-value products such as cereals, legumes, and root crops can be preserved and transformed into value-added products. Intermediate value products like vegetables, fruits, meat, and fish can be dried to extend their shelf life. High-value products such as spices, herbs, medicinal plants, nuts, and bioactive materials can be dried for enhanced preservation and concentrated flavors or bioactive compounds.

Overall, the drying process is a versatile preservation method that can be tailored to various food products, offering extended shelf life, reduced weight and bulk, and value addition to the final dried products.

Pretreatments before drying

Pretreatments play a crucial role in preparing food products before the drying process. Here are some common pretreatments that are often applied before drying:

1. Blanching: Blanching involves briefly immersing the food product in boiling water or steam and then rapidly cooling it. This process helps inactivating naturally occurring enzymes that can lead to enzymatic browning and off-flavor development during drying. Blanching also helps in preserving the color, texture, and nutritional quality of the food.
2. Sulfur Dioxide Treatment: Sulfur dioxide (SO2) treatment is used to preserve the texture, flavor, vitamin content (such as ascorbic acid and carotene), and color of the food. It also acts as an antimicrobial agent, inhibiting the growth of microorganisms. SO2 treatment is particularly effective in reducing enzymatic browning in fruits and vegetables.
3. Salting or Curing: Salting or curing is a pretreatment commonly used for meat, fish, and cheese products before drying. Sodium chloride (salt) and potassium nitrite are applied to the food, acting as osmotic dehydrators. They reduce the moisture content, aid the drying process, and act as preservatives to prevent microbial growth.
4. Dipping in Sulfite Solution: Immersing foods in a sulfite solution with a concentration of less than 5% for a short period, typically less than 5 minutes, helps in dehydrating the food and improving its drying characteristics and overall quality. The sulfite solution aids in removing excess moisture from the food, enhancing the drying process.
5. Cooking: Cooking food products before the drying process serves multiple purposes. It helps in destroying microorganisms present in the food, reducing the risk of microbial growth during drying and storage. Additionally, cooking can decrease the water-holding capacity of certain products, such as meat, facilitating the removal of moisture during drying.

These pretreatments are employed to enhance the quality, safety, and efficiency of the drying process. By properly preparing the food products beforehand, it ensures that the drying process can proceed effectively, resulting in dried foods with improved shelf life, texture, color, flavor, and nutritional value.

Influence of drying on the nutritive value of food

The drying process can have an influence on the nutritive value of food. While drying helps in preserving food by reducing water activity, it can also lead to certain losses and changes in nutrient content. Here are some influences of drying on the nutritive value of food:

1. Vitamins: Drying can result in the loss of water-soluble vitamins, especially during pretreatment and exposure to heat. Oxidative damage can lead to the degradation of ascorbic acid and carotene. Heat-sensitive vitamins like thiamin and vitamin D can also be lost during the drying process. Spray drying has been found to lower carotene loss, and freezing fruits before drying can reduce vitamin C loss. Enriched milk with vitamin D before drying can help retain its vitamin content.
2. Proteins: High temperatures during drying can cause denaturation and decrease the nutritional value of proteins. However, low-temperature drying methods can help maintain the digestibility and nutritional quality of proteins.
3. Fats: The higher temperatures used in dehydration can lead to the oxidation of fats, resulting in a decrease in their quality. To mitigate this, low-temperature drying methods and the use of antioxidants can help preserve the nutritional integrity of fats.
4. Carbohydrates: Enzymatic or non-enzymatic browning can occur in fruits during drying, leading to discoloration. The use of sulfur dioxide as a treatment can help prevent browning and preserve the quality of carbohydrates.

FAQ

References

• 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