Removal of water in food is process in which we remove the water from the fresh food with the help of different methods such as sun drying, dehydration, etc. This method helps to increase shelf life of food products.
Water in food is essential for chemical and biochemical reactions and microbial activities in foods. Without available moisture, the spoilage agents would be completely inactive. It is present in practically all foods in varying quantities.
Fresh fruits, vegetables, meat and fish that contain high levels, spoil readily. Dry beans and cereal grains have relatively low moisture and can be stored for long periods without spoilage. This knowledge has been utilized for centuries to preserve foods by sun drying.
1) Function of water in food
a. Serve as a solvent
b. Participate in chemical and biochemical reactions as principal reactant in processes involving hydrolysis.
C. Be product of chemical reactions involving condensation,
d. Serve as a modifier of the catalytic activity of other substances in food, e.g., metallic catalysts associated with lipid peroxidation are inactivated by water.
2) Types or forms of water in foods
Water exists in foods in 5 different forms such as
a. Free water (e.g., in tomato juice).
b. Droplets of emulsified water (e.g., in butter).
c. Water tied in colloidal gels (e.g. in jellies).
d. Thin layer of adsorbed water (e.g., in powdered milk).
e. Chemically bound water of hydration (e.g., in sugar).
A The chemically bound water in the food is extremely most difficult to remove during drying. while free and adsorbed water is easy to manipulate.
In removal of moisture in the food, the free and absorbed moisture is removed by suitable means. In this manner the water becomes unavailable for the chemical and biochemical reactions, as well as the activities of enzymes and microorganisms.
3) Advantages of drying of foods
Dried foods, preserved by the removal of moisture or water in food, are less expensive than most foods preserved by other methods. These are concentrated sources of nutrients and are high in total solids.
The dehydrated foods, because of reduced weight, are less costly to transport.
They also require less storage space. While sun drying is restricted to countries with plenty of sunshine, dehydration can be practiced in any part of the world under any climatic conditions.
4) Methods of removal of moisture or water In the Food
A) Sun-drying used for removal of water or moisture in food
Sun drying refers to removal of water from foods by exposing them to the direct energy from the sun’s rays. Many food like cereals and pulses become naturally dried in the field by solar energy and are protected from the autolysis and microbial attack.
Their moisture content usually varies from 8-16% and water activity is normally 0.75 percentage below. In addition to these naturally preserved foods, other perishable commodities are sun-dried in order to prolong their shelf life.
In tropical and sub-tropical regions with abundance of sunshine, several fruits and vegetables are sun-dried. Examples are sun drying of grapes in Afghanistan, Greece and United States to produce raisins and the drying of dates in Saudi Arabia, Iraq, Tunisia, Egypt and Algeria.
Guava tomato, pears and peaches are also sun-dried in several parts of ne world. In Pakistan fruits including figs, plums, grapes, dates and apricots are sun dried.
a) Equipment used in sun drying method
Equipment’s are required for the removal water in food with the help of sun drying varies according to the product to be dried, local conditions and the scale of production. However, any one engaged in sun drying of fruits and vegetables would need wooden boxes of appropriate size for collection of raw material from the field.
In the preparation knifes, cutting tables, cutting knives, peeling cutters, aprons, etc. are required. So the prepared raw material needs to be placed in suitable sized trays that are usually mainly constructed of wood: Blanchers or sulphuring equipment may be needed.
After drying, the raw material is brought to the sweating chamber. Packaging – equipment is required to pack the dried product in suitable sized moisture-proof containers.
b) Procedure of sun drying method
The fresh raw material for sun drying required the preparatory treatments similar to those required for preservation by canning, freezing or other techniques. The fruit may be washed, peeled, cut and then blanched or sulphured.
It is left in the open sun in trays and turned occasionally until dried to the desired degree. The dried fruits are then sorted and stacked in boxes or bins for moisture equilibration-a process know as sweating? It may now be stored in insect- and rodent-proof storage rooms.
c) Precautions using of sun drying method
Since the raw material for sun drying is directly exposed to the atmosphere, there is danger of contamination from natural sources. Microorganisms floating in the atmosphere may settle on the product. Birds and animals should not be allowed in and around the vicinity of the drying yard.
Cleanliness in all operations is absolutely essential. Great care should be taken during the drying process to prevent contamination. All equipment should be properly sanitized. Waste fruit should not be dumped near the drying area and where this is not possible.
It should be treated in a pit with lime chloride to prevent its decay and multiplication of microorganisms and flying insects, especially flies. Dried products are subject to attack by insects and rodents.
These should be protected by providing insect-proof room for storage Occasionally this room is fumigated with suitable gas to kill insect pests. Since dried foods usually deteriorate at room temperature and lose both colour and flavour.
It is preferable to store them at lower temperatures, especially if stored in bulk bins. Most processors prefer to keep their products in cold stores at a temperature of 0-4°C. This also prevents damage by insects and rodents, apart from protecting product quality.
B) Dehydration used for removal of moisture in food
Dehydration is an operation in which water content of food is substantially lowered under controlled conditions of temperature. Humidity and airflow. Under these conditions, high quality products are obtained that retain their natural characteristics upon rehydration or reconstitution.
Foods that have been successfully preserved by dehydration include fruits, vegetables, milk, eggs, fish, cake mixes, soup mixes and others. The raw material determines the handling techniques and type of equipment needed for dehydration.
A) Drying atmosphere
The help of drying atmosphere removal of moisture from natural foodstuffs, heat energy is. supplied to the food. Water, which evaporates, is removed from the vicinity of the raw material.
Foods may be dried in hot air, superheated steam, vacuum, heated inert gas, or by direct application of energy from a radiant microwave or dielectric source.
However, air is the most common medium for removal of moisture from foods for several reasons: dehydrators using air are less to construct.
By using air as a drying medium, overheating, discolouration and scorching of the product is greatly avoided. Air also helps to drying of foods, thereby avoiding loss of juice by dripping.
Primarily, there are three big roles of air in dehydration of food. It conveys heat energy to food, vaporizes moisture from the commodity and transfers the liberated moisture to the atmosphere outside.
A larger volume of air is required to vaporize moisture from the food than to transport it out of the drying atmosphere.
The volume of air required for drying of food is calculated from the initial temperature of air entering to the drier, volume of moisture required to be evaporated, time in which the operation has to be completed and temperature of air leaving the dehydrator.
Other factors connected with this calculation are the heat losses through leakage and heat required to raise the temperature of trays and walls of the dehydrator.
B) Factors affecting removal of water in food
The rate of moisture evaporation of dryer from the free surface of a food material depends upon the nature of food material, the particle size, the bed depth (in case of pieces placed on a surface), humidity temperature and the velocity of air.
All food raw materials do not react in the same manner to changes in atmospheric conditions. Since moisture travels out of the food from inside, larger particles take longer time to dry than the smaller ones.
Thus, while it may take a few hours for okra pieces to dry in a tunnel drier, several hours may be required when whole okra is dried under the same conditions.
When tissue foods are dried in cabinet or tunnel driers, the depth of raw material on the tray surface will also determine length of the process. Atomization of fluid foods into small particles, as seen in spray drying of milk and instant beverages, cuts the drying time to a matter of seconds.
The relative humidity and velocity of air are other important factors that must be carefully controlled in dehydration processes. Air, high in relative humidity will not accommodate the extra moisture from the food.
At low temperature, air is easily saturated with moisture, hence will not be efficient for the purpose. The rate of moisture evaporation from the free surface of food material is directly proportional to the velocity of air, provided other factors remain constant.
It has been found that at an air velocity of about 70 meters per minute, drying would be twice as rapid as in still air and a 140 meters per minute, it would be three times as fast.
C) Types of Dehydrators
Depending upon the mechanism of transferring heat from the heat exchanger to the food, all dehydrators will be sorted into one in every of the following categories: –
a. Natural Draft Driers
The natural draft driers, in general, incorporates a furnace room or alternative heating arrangement (steam pipes, electrical heaters), surmounted” by a drying chamber.
The air is heated by contact with the diverging pipes and enters the drying chamber by natural convection currents. These driers are cheap to build and have low fuel efficiency. Most natural driers are currently equipped with fans that have significantly accumulated their efficiency.
The oven drier is one in every of the oldest kind and is used within the drying of hops (flowers of hop plant employed in production to impart bitter taste), chocolate tree and apples. it’s unsuitable for soft fruits.
The drier consists of an higher structure that homes the drying floor, and a ground floor that accommodates the heat. At the highest of the higher floor is an exit for escape of the spent air.
In the tower drier, the drying chamber will accommodate many drying trays stacked one over another within the variety of a tower. The heat is just like the oven drier. the cupboard drier is comparable to tower drier except that steam coils placed below the trays furnish heat.
During this kind, temperature of the air will be handily regulated and drying created a lot of speedy. the modern sorts of cupboard driers incorporates compartments that hold many trays over or through that hot air is blown by special fans (Fig. 6.1).
The OR tunnel drier consists of a series of parallel, sloping slim chambers on top of the room. the recent air enters at the lower finish of every tunnel through a gap or throat, whereas trays of food material (normally fruits) enter the drier at the higher or cool finish and progress towards the lower or heat finish.
b. Forced draft dehydrators
The forced draft dehydrators supply a lot of economical energy utilization and management over the product. in essence, all forced draft dehydrators incorporates a heating chamber, which can be heated by any typical means that, the drying chamber.
wherever food could also be placed and a system to flow into air through the foodstuff. commonly these driers have arrangement for re-circulation of air that will increase fuel potency and prevents casehardening.
The tunnel drier consists of a chamber within the form of a tunnel, that is longer than it’s wide. depending upon the direction of the of air over the merchandise, a tunnel drier could also be concurrent once air and stuff move within the same direction or counter-current if these move in other way to every alternative.
In some driers, air could enter from either side of the tunnel and is released from the center, Such a drier is termed centre-exhaust tunnel drier and has the advantage of producing a more robust quality product than the opposite driers. Another version, known as cross flow drier, the air flows across the direction of food.
So, in tunnel driers, food is generally placed in trays that are stacked on trolleys or trucks and these move the length of the tunnel. Heating of air is finished either by steam coils, electrically heated grids or hot air chamber. The air provide is maintained by victimisation disc, multivenue, propeller, axial or paddle wheel sorts of fans.
3) Evaporation and concentration used for removal of water in food
Evaporating some humidity, there by concentration the soluble solids, can lower the water exertion of a food material. Tomato paste is a common sample where evaporation is a part of the preservation approach. Originally tomato juice contains about 6 solids that are concentrated by evaporation to about 32.
Evaporation helps to reduce the weight of food accoutrements, thereby lowering packaging and transportation costs. Typically water exertion of most evaporated foods is still high enough to encourage growth of corruption microorganisms.
last are given heat treatment milder than would be the case for their unevaporated counterparts or may be concentrated by the addition of answerable ingredients to further drop their water exertion.
4) Freeze drying
In response to demand by customers for preserved foods, that show minimal quality variations from the fresh, food technologists have provided varied innovative solutions.
freezing is one process technique that will minimal harm to product quality. Foods that are freeze-dried are lightweight, porous in structure and once reconstituted, exhibit most smart characteristics of the contemporary goods. To boot, freeze dried product retain form and size of the initial staple.
Freeze-drying involves freeze of food material followed by removal of moisture by sublimation, i.e., evaporation of moisture from the solid state to vapour state while not first becoming liquid. The sublimation method is administered at low vacuum (usually at zero.1-2 mm Hg).
The product is usually finished in a normal drier since some moisture could stay within the food. Some disadvantages of freezing include the doable harm to staple cell structure once freeze is poorly dead. Moreover, the product is brittle and, therefore, at risk of mechanical harm.
Also, freezing method is extremely costly as compared with alternative standard drying techniques. This technique is appropriate for low, fruit juices, whole shrimps, chicken dices, etc.
5) Dehydro freezing
This is a less usually used technique of food preservation. essentially dehydro-freezing involves moisture reduction by any appropriate dehydration technique followed by freeze remainder of the wet gift within the food.
The freeze ensures more reduction in water activity. In apply, the method consists of removal of concerning five hundredth wet from the foodstuff then subjecting this part dehydrated material to traditional freeze operation.
During this manner, the majority of the food is reduced, thereby lowering storage, transportation and handling prices.The product is superior in quality to a strictly dehydrated one, because it higher retains the flavour and texture.
6) Intermediate moisture food technology
The obtainable wet in foods is certain, creating it unobtainable for spoilage agents. Such a way is utilized within the production of intermediate wet foods that have moisture content sometimes variable from 20-50%.
The water activity of intermediate wet foods typically ranges from 0.70 to 0.85, that is low enough to stop the expansion of the many spoilage bacterium and yeasts.
However, this water activity is comparatively high for many spoilage moulds and osmophoric yeasts. The intermediate moisture foods are honey, jams, jellies, confectionery product, sweet milk, and similar product.
The principle underlying the technology of intermediate moisture foods is that water activity of the fabric is lowered by part removing the moisture. this can be followed by the addition of such chemicals as common salt, glycerol, sorbitol, sucrose glucose-or others that bind a part of the remaining wet.
In these product, microbes growth is more simple by using antimicrobial agents, particularly antimycotics like antifreeze and/or carboxylic acid or its metallic element salt.
The technology of manufacturing intermediate wet foods may well be notably helpful and practicable for developing countries that lack the traditional vasoconstrictive storage facilities.
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