Principles

Dehydration involves simultaneous heat transfer and moisture diffusion (mass transfer). The conversion of liquid (or solid in the case of freeze drying) to vapor demands the supply of latent heat to the product; this can be achieved by a variety of methods: conduction by contact with a heated metal plate, convection from heated air, radiation from an infrared source, or microwave energy. The process may be accelerated by the application of vacuum.

Dehydration Terms

Dried is the term applied to all dried products, regardless of the method of drying. Dehydration refers to the use of mechanical equipment and artificial methods under carefully controlled conditions of temperature, humidity, and airflow. Although the term dehydrated does not refer to any specific moisture content in the finished product, it is usually considered implying virtually complete water removal to a range of 1 to 5% moisture. Products with such low water content can be stored in a moisture-proof package at room temperature for periods well in excess of two years with no detectable change in quality.

Evaporation refers to the use of the sun or forced-air dryers to evaporate moisture from fruit to a fairly stable product. The moisture level of evaporated fruit is approximately 15 to 25%. In general, sun drying will not lower the moisture content of fruit below 15%. The shelf life of such fruit products does not exceed one year, unless they are held in cold storage. For the extended shelf life of most evaporated fruits, sulfur treatment (except for prunes, unbleached raisins, and dates) or the use of a chemical mold inhibitor is necessary.

Vacuum drying is a method of drying in a vacuum chamber under reduced atmospheric pressure to remove water from the fruit at less than the boiling point of water under ambient conditions.

Freeze drying is a method of drying in which the fruit is frozen and then dried by sublimation in a vacuum chamber under high vacuum (an absolute pressure of less than

Table 2. Production Volume and Value of Dried Fruits in California Five Year Averages, 1992-1996

Commodity Drying ratio: fresh to dry fruit" Average annual production (tons) Average farm value (dollars)

Table 2. Production Volume and Value of Dried Fruits in California Five Year Averages, 1992-1996

Commodity Drying ratio: fresh to dry fruit" Average annual production (tons) Average farm value (dollars)

Apples

7-10 to 1

2,720

3,168,800

Apricots

5.5-8.5 to 1

2,200

4,840,00

Dates

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