Drying preserves foods by removing enough moisture from food to prevent decay and spoilage. The water content of properly dried food varies from 5 to 25% depending on the food. Successful drying depends on:

• Enough heat to draw out moisture, without cooking the food;
• Dry air to absorb the released moisture; and
• Adequate air circulation to carry off the moisture.

When drying foods, the key is to remove moisture as quickly as possible at a temperature that does not seriously affect the flavor, texture, and color of the food. If the temperature is too low in the beginning, microorganisms may grow before the food is adequately dried. If the temperature is too high, affects the flavor, texture, and color of the food.

Principles of drying and dehydration: The microorganisms require plenty of free water for their survival. Drying or dehydration removes biologically active water, thus the growth of microorganisms is stopped. This also results in a reduced rate of enzyme activity and chemical reactions. The food value, natural flavour, and characteristic cooking quality of fresh material are retained after drying. Fruits show no sign of moisture or stickiness and vegetables become brittle on drying. The residual moisture should not be more than 6-8% in vegetables and 10- 20% in fruits. Dried fruits can be used as such or after soaking, while dried vegetables are usually soaked in water over night before cooking.

Process of drying:

During Preparation

1. Select the best fruit and vegetables- As with canning and freezing, dehydrated foods are only as good as fresh fruit or vegetables. When selecting fruits and vegetables for dehydration, choose ones that are ripe, unbruised, and of peak-eating quality.
2. Prepare fruits or vegetables to be dehydrated – Apples, for example, maybe sliced, cut into rings, or pureed for fruit leather.
3. Keep pieces uniform in size and thickness for even drying-Slices cut 1/8 to 1/4-inch in thickness will dry more quickly than thicker pieces.
4. Some foods should be washed before drying– Foods such as herbs, berries, and seedless grapes need only be washed before dehydrating.
5. To prevent browning- Try steaming, sulfuring, or coating light-coloured fruits and vegetables with acids such as lemon juice or ascorbic acid before drying. Steaming or blanching is recommended for vegetables to inactivate enzymes that cause vegetables to mature or toughen during drying.

• Blanching: Most vegetables and some fruits are blanched before drying to inhibit enzyme activity and to help preserve the colour. The material is cut into appropriate sized pieces and plunged into boiling water for up to 5 minutes. They should be blanched in small batches to ensure that each piece is properly heated through. If too many pieces are put into the water at one time, the water temperature will drop and prolong the blanching time. After blanching for the required time, vegetables are rapidly cooled by plunging into cold (or iced) water.
• Sulphuring: is an optional stage of processing. The main benefit of sulphuring is to preserve the fruit’s colour. Some consumers object to chemical preservatives and prefer naturally dried fruits. Sulphur dioxide gas (SO2) is applied to the fruit pieces by placing them in a cabinet or tent in which sulphur is burned. The gas is absorbed by the fruit. For most fruits, 5-6g sulphur per kg food is adequate. The gas given off is toxic and corrosive. Therefore, sulphuring should be carried out in a well-ventilated place, using appropriate equipment.

During Drying

1. Select the right drying method and equipment- Foods can be dried in a conventional oven, a commercial dehydrator, or in the sun. Drying times vary with the method and foods chosen. Be sure to read the instructions with your dehydrator.
2. Maintain 130^OF to 140^OF with circulating air: Remove enough moisture as quickly as possible to prevent spoilage. A drying temperature of 130^O F to 140^OF allows moisture to be removed quickly without adversely affecting food’s texture, color, flavour and nutritive value. If the initial temperature is lower, or air circulation is insufficient, foods may undergo undesirable microbiological changes before drying adequately. If the temperature is higher, or humidity too low, nutrients can be lost or moisture may be removed too quickly from the product’s outer surface. This causes the outer surface to harden and prevents moisture in the inner tissues from escaping. When testing for sufficient dryness, cool foods before testing.
3. Know whether food is dry or not: Some foods are more pliable when cool than warm. Foods should be pliable and leathery, or hard and brittle when sufficiently dried. Some vegetables actually shatter if hit with a hammer. At this stage, they should contain about 10 percent moisture. Because they are so dry, vegetables do not need conditioning (Sweating) like fruits.

After Drying

• Sweating (Conditioning):- Keeping dried products in boxes or bins to equalizer moisture content.
• After conditioning, package and store the fruit.

Flowsheet for drying/dehydration of fruits and vegetables