FAQ

Preservation by Dehydration

• The word dehydration usually complies the use of controlled conditions of heating, with the forced circulation of air or artificial drying as compared with the use of sun drying. Drying as a means of preservation can be observed in cereal grains, legumes and nuts, which dry on the plants. Freeze drying is a method of drying involving freezing and then the sublimates of the ice under vaccum
• Dried foods are preserved because the available moisture level is so low that microorganisms cannot grow and enzyme activity is controlled.

Agricultural biotechnology is a collection of scientific techniques, including genetic engineering, that are used to develop, improve, or modify plants, animals, and microorganisms.

The Methods of F&V drying are Sun drying, Drying by mechanical driers, osmotic dehydration, Spray drying Foam mat drying, Freeze drying.

Sun drying It is an old and traditional method of drying. It is limited to climates with a hot sun and a dry atmosphere and to certain fruits such as raisins, prunes figs, apricots, pears and peaches. It is a slow process. Vegetables like french beans, curd chilli are preserved by this method.

Drying by mechanical driers:Most methods of artificial drying involve the passage of heated air with controlled relative humidity over the food to be dried or the passage of the food through such air. The simplest drier is the evaporator, where the natural draft from the rising of heated air brings about the drying of the food. The optimal temperature for drying are between 520 C and 600 C. lower temperature for a longer time yield better quality and better retention of vitamin content. It takes 6-15 hours for vegetables and 6-24 hours for fruits. Vegetables are dried until they are brittle but fruits feel leathery when dried. Liquid foods, such as milk, juices and soups may be evaporated by the use of comparatively low temperatures and in a vacuum pan or similar devise, drum dried by passage over a heated drum with or without vacuum, or spray-dried by spraying the liquid into a current or dry heated air.

Fruit bars, raisins, dehydrated products from fruits like fig, pomegranate and vegetables like onion, carrot, cauliflower, okra, mushrooms etc.

Osmotic dehydration

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

Osmotic dehydration process has gained lot of interest during the last two decades and has been applied to many fruits such as apple, apricot, banana, cherry, citrus, grapes, guava, mango, papaya, pineapple, plum, raspberry etc. Osmotic Dehydration can remove 30-50% of the water from fresh ripe fruits e.g mangoes, pineapple, banana, sapota and papayas. The final drying of these osmsotically dehydrated fruits by vacuum drying provides a product which has good quality, attributes with respect to appearance, taste, flavour and colour as compared to sun drying.

Advantages of Osmotic dehydration: Main advantages of osmotic dehydration are as follows:

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

Limitations of Osmotic dehydration

• The basic raw material is fruits at proper maturity. Hence activity is highly seasonal
• The reduction in acidity level reduces the characteristic taste of some products.
• Sugar uptake  may not be  desirable in certain product.
• The process combined with air or vacuum drying in much cheaper than freeze-  drying.

Factors influencing Osmotic dehydration:

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

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

Spray drying Thin fruit juices like pine apple, citrus are dried to a powder in spray driers in which the liquid is atomized and sprayed into a hot air stream for almost instant drying.

Freeze drying : Freeze drying is a method of drying involving freezing and then the sublimates of the ice under vacuum. Removal of water from a product while it is frozen by sublimation is called freeze drying. After the preliminary preparation of food is carried out as for other methods of drying and then the prepared food is frozen, placed in a vacuum chamber and a small amount of heat is applied. Fresh flavours and textures are better preserved by freeze-drying than by sun-drying or other procedures of artificial drying without vacuum. This method is costly. Freeze-dried fruits and vegetables generally have better texture and rehydration quality. In smoking drying is the main preservative factor, especially drying at the surface of the food.

Dehydration of grapes (Raisin making): The process of raisin making involves selection of raw material, preparation of bunches, lye treatment, dipping oil treatment, sulphur fumigation, drying in sun or shade or drier, separation of raisins, washing in luke warm water, curing, packing, storage for marketing. Work done at this institute indicated that apart from Thompson seedless, variety Arkavati is also suitable for raisin making. Grapes at 24 0 Brix produces best quality raisins.