Drying or dehydration of foodstuffs is one of the oldest method of food preservation. It is particularly successful in hot, dry climates. Drying reduces moisture necessary for bacterial growth that eventually causes deterioration. Successful dehydration depends upon a slow steady heat supply to assure that food is dried from the inside to the outside. Drying is an inexact art. Size of pieces, relative moisture, and the method selected all affect the time required to dehydrate a food adequately.
Molds, yeast and bacteria need water to grow. When foods are sufficiently dehydrated (dried), microorganisms cannot grow and foods will not spoil. Dried fruits and fruit leathers may be used as snack foods; dried vegetables may be added to soups, stews or casseroles. Campers and hikers value dried foods for their light weight, keeping qualities and ease of preparation.
The nutritive value of food is affected by the dehydration process. Vitamins A and C are destroyed by heat and air. Using a sulfite treatment prevents the loss of some vitamins, but causes the destruction of thiamin. Blanching vegetables before drying (to destroy enzymes) results in some loss of Vitamin C, B-complex vitamins and some minerals because these are all water soluble. On the other hand, blanching does reduce loss of vitamins A, C and thiamin during dehydration and storage.
Foods may be sun dried or by using a solar dehydrator, in a gas or electric oven, or with a portable electric dehydrator. Dehydrators with thermostats provide better control over weather conditions and food quality than simple sun drying. Drying time varies widely because of the method selected and the size and amount of moisture in food pieces. Sun drying requires the most time; an electric dehydrator requires the least. Vegetables take from 4 -12 hours to dry; fruits take 6 - 20 hours. Meats require about 12 hours. Making raisins from grapes may require days/weeks when dried outside.
Solar drying is a modification of sun drying in which the sun's rays are collected inside a specially designed unit with adequate ventilation for removal of moist air. The temperature in the unit is usually 20 to 30 degrees higher than in open sunlight, which results in a shorter drying time. While solar drying has many advantages over sun drying, lack of control over the weather is the main problem with both methods.
It is important to control air temperature and circulation during the drying process. If the temperature is too low or the humidity too high (resulting in poor circulation of moist air) the food will dry more slowly than it should and microbial growth can occur. Careful temperature control is required at the beginning and end of the drying period. If the temperature is too high at first a hard shell may develop on the outside, trapping moisture on the inside. This is known as "case hardening." Temperatures that are too high at the end of the drying period may cause food to scorch. Temperatures between 120 degrees F - 140 degrees F are recommended for drying fruits and vegetables. Temperatures up to 150 degrees F may be used at the beginning, but should be lowered as food begins to dry. For at least the last hour of the drying period, the temperature should not exceed 130 degrees F.