Dehydration or Drying of Vegetables and Fruits
The preservation of fruits and vegetables is done since ancient times. Dehydration is extensively used in food preservation. Sun drying is among the oldest method for increasing the shelf life of many fruits and vegetables. Although the primary objective of dehydration is to preserve the food, it also reduces the weight and volume of the product and lowers the packaging and transportation costs (Murthy and Joshi 2007).
Process Description
Fruits and vegetables can be dried or dehydrated through various processes. The
the difference in these processes depends upon the type of fruit and the characteristics
of the final product. Different vegetables and fruits are dehydrated or dried by the
following processing steps: pre-drying treatments, like sorting/grading of fruits and
vegetables, peeling, blanching; drying or dehydration, through sun drying or using
artificial driers; and post dehydration treatments, like sweating, packaging and
inspection (Murthy and Joshi 2007).
Pre-Drying Treatments
Raw material to be dehydrated is firstly pretreated to prepare for the dehydration process. The raw material pretreatment includes selecting and sorting the vegetables and fruits, washing, peeling, and cutting into the desired and required size and form & blanching (for most vegetables and few fruits). Commodities are selected and sorted according to the size and maturity, followed by washing to remove the insect matter, dust, dirt, spores of molds, and plant parts that may cause contamination or affect the color flavor and aroma. Peeling and the removal of the undesirable parts are done after washing. Different peeling methods are used in food processing, like hand peeling, abrasive peeling, lye peeling, steam pressure peeling, high-pressure washing, and flame peeling.
Fruits usually peeled before dehydration include pineapple, apple, pear, and banana. While vegetables undergoing the peeling process include potatoes, garlic, beets, onions, and carrots. To facilitate the dehydration process in commodities like grapes and prunes, these are dipped in an alkali solution to remove natural wax coating. Then size is reduced to increase the rate of dehydration through cutting into halves, slicing, and in cubes form. However, some fruits do not require a size reduction process, such as maize (corn) and cherries. Some
vegetables and fruits are blanched in hot water (95 to 100oC) or exposed to steam
before dehydration or drying.
Color preservation, usually known as sulfuring, is the final step in pre-dehydration
process applied. SO2 is used due to its preservative and antioxidant activity in most fruits and vegetables, including apples, cabbage, and potatoes. This
technique is also very effective in retarding the browning of fruits and vegetables
(Murthy and Joshi 2007).
Drying or Dehydration
Various methods of dehydration or drying are used in food industries, and the choice of the appropriate method depends on the required product quality,
economic factors, and characteristics of the raw material. Three types of dehydration processes are used very often: sun and solar drying; continuous
processes (fluidized-bed, tunnel, spray, drum, continuous belt, foam-mat, and
microwave-heated driers), sub-atmospheric dehydration (vacuum belt, freeze driers
And vacuum drum), and atmospheric dehydration (cabinet, tower, and kiln driers).
Sun drying is mostly used for drying fruits, while some vegetables are also dried by using the heat energy of solar light. This technique is used only in climates that have
dry atmospheres and abundance of hot sunlight and in fruits, like pears, dates,
grapes, prunes, apricots, and figs. It is done by spreading the commodity on the racks, roofs, grounds, or trays until it is dried to the desired final product characteristics. Food is dried by the heat of the bright sun and the natural movement of air in the surrounding. The temperature should be 100 oF for effective drying. The advantages of this method are its simplicity and small capital investment, while the complete dependence on sunlight and moisture level (not less than 15-20%) are its disadvantages. Black-painted trays, collectors, and solar trays are used to reduce the drying time.
In atmospheric forced-air drier produce is passed through heated air with controlled relative humidity until the dried product or by applying heated air to the product are the most widely used for vegetable and fruit dehydration. Vegetables are mainly dehydrated using continuous processes such as fluidized-bed, tunnel, continuous belt, microwave, spray, and drum dryers. Tunnel driers are widely used as they are the most flexible and efficient. Batch processes include tower, cabinet, and kiln driers.
Sub atmospheric dehydration is also known as vacuum dehydration as it occurs at
low air pressures. Some of its types include vacuum drum, freeze drier, and vacuum belt drier. The vacuum drying ensures the dehydration of food at less than
the boiling point by using the ambient conditions., Due to the high operational and
installation expenses, this technique is used for dehydration of those fruits and
vegetables that may be modified chemically during exposure to air at high
temperatures and may deteriorate because of oxidation.
Vacuum driers are categorized into two types. In the first category, the moisture of the vegetables and fruits is removed by the sublimation process, in which ice is converted directly into water vapor. While in the second category, the moisture from the fruit surface is evaporated from liquid products. Some of its types include the vacuum belt, shelf, and drum driers. Some advantages of freeze-drying include the maximum
retention of nutritional value, high flavor retention, and preventing the damage of
product texture, color, and structure. Disadvantages include high processing costs,
high capital investment, and special packing of the product to avoid moisture gain
and oxidation (Murthy and Joshi 2007).
Post-Dehydration Treatments
Different kinds of treatments are used for vegetables and fruits, depending upon their intended use. Some of these treatments are inspection, screening, sweating,
instantiation treatments, and packaging. Inspection of fruits and vegetables removes discolored pieces, foreign materials like carpel, skin, and stem particles.
Screening is done to remove “fines” (dehydrated pieces of unwanted size).
In instantiation treatments, rehydration of products (having low moisture content) is improved. Sweating is the process of holding the dried product in boxes or bins to balance the moisture content. Packaging has a great role in enhancing the shelf life of dehydrated products. The primary objective of packaging is to protect the final product from dust, dirt, air, foreign odor, micro-flora, light, moisture, insects.
It also provides stability and strength to maintain the product's desired characteristics during handling, transportation, storage, and marketing.
And rodents. Package material (cartons, bins, plastic bags, drums, and cans) are selected according to the product's end-use (Murthy and Joshi, 2007).
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