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Chapter: 11th Home Science : Chapter 4 : Food Preservation Methods

Preservation by Dehydration

Dehydration is the extraction of moisture from food products like fruits, vegetables, herbs and meat.

Preservation by Dehydration

Dehydration is the extraction of moisture from food products like fruits, vegetables, herbs and meat. 


It inhibits the growth of microorganisms and imparts a long stor-age life. This is a modern development of smoking and drying. Some changes that occur during the process of dehydra-tion are:


·        Chemical changes


·        Browning and flavour changes


·        Denaturation of proteins


·        Concentration on the surface of the food (case hardening)


Dehydration can be done by dry-ing and salting. Evaporation is quickened with the addition of moderate heat which is sometimes provided by natural sunlight. The ultraviolet rays from the sun serve to kill microbes. Modern methods of dehy-dration use circulating air that is heated just enough to promote dehydration with-out cooking the food. Food preservation by drying is one of the oldest methods used by human beings. Drying is one of the methods used for dehydration.



Drying is the method nature resorts to pre-serve foods. Natural drying was adopted by early man to dry fruits, fish and meat by exposing them to the sun.

Sun drying is used in many parts of the world for preserving certain foods, such as fruits and nuts. However, this method can be used only if the climatic conditions are hot with low humidity. In many cases foods are pretreated before drying to make the structure more porous and to facilitate transfer of moisture, thereby speeding the drying rate. Food porosity increases the chance of quick sol-ubility on reconstitution, but is at a disad-vantage due to increased bulk and shorter storage stability. Vegetables like beans, peas, potatoes, cauliflower, ladies finger, garlic, onion and all leafy vegetables can be sundried.


Changes during drying


·        Shrinkage occurs on the surface first and then proceeds to the inner lay-ers. With quick high temperature drying of food, the surface becomes dry and rigid long before the center dries out.

·        Dried food pieces may also contain cracks and pores of various diameters. The shrinking and pore clogging by the solutes is known as core hardening. It can be minimized by gradual drying with low surface temperature.

·        Foods that lack good structure and are high in sugar content, give an impres-sion of retaining moisture even after the drying process. Fruits like grapes and figs have high sugar content and lack good structure, hence appearing moist even after dehydration.

·        Complete prevention of these changes is impossible. They can be minimized by using appropriate technology.


Methods of Drying

A number of drying methods are available; some are suitable for liquids, others for solid foods or mixtures containing food pieces. The common drier types used for liquid and solid foods may be categorized as the air-convection drier, drum or roller drier and vacuum drier.


Types of Driers


Air-Convection drier – In the air-con-vection drier, hot air supplies the heat for evaporation. Though there are dif-ferent types of air-convection driers, they all have an insulated enclosure, a means of circulating air through the enclosure and a means of heating this air.

If liquid, the food may be sprayed or poured into pans or on belts. Food in the form of a fine spray or mist is introduced into a tower or chamber along with heated air. The small drop-lets come into contact with the hot air, blast off their moisture, become small particles and drop to the bottom where they are removed. This method can produce a high quality product even with heat sensitive products like milk, eggs and coffee.


Drum or Roller drier – Liquid foods, purees and mashes are dried by this method. The food to be dried is applied, as a continuous thin layer, on to the surface of a revolving drum or between a pair of drums moving in opposite directions heated by steam. The dried layer of food is scraped by a scraper blade positioned at a point on the drum. Foods that are sticky cannot be scraped when it is hot. Such a sticky food becomes brittle when cooled, which facilitates scraping. For heat resistant food products, drum drying is one of the inexpensive dehydration methods.


Vacuum driers – This method is quiet expensive but gives good quality foods. It consists of a vacuum chamber that can withstand air pressure and contains shelves to hold food. The shelves are heated. The food gets heated by con-duction and radiated heat. Liquid foods dehydrated by vacuum drying have a puffed structure and are easily dis-solved in water. There is minimum fla-vour change and heat damage because low temperature is used in this method.


Dried foods are very convenient as they are light weight, take up little storage space and can be stored for long periods as emergency foods.


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11th Home Science : Chapter 4 : Food Preservation Methods : Preservation by Dehydration |

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