The process of food preservation by drying involves removing the water from a food item so harmful bacteria and mold will not grow and cause the food to become inedible. There are several advantages to using drying for food preservation, including dramatically extending the shelf life of an item, concentrating some flavors and preserving many of the innate nutrients in the food. Disadvantages also exist, including modifying the texture of food, the loss of some vitamins, and the potential cost of drying the food. In comparison to other methods, food preservation by drying maintains more of the basic elements of the original state of certain types of food, but also can have disastrous results if the wrong foods are dried or if the process is performed incorrectly.
Air drying, which involves using moving air to remove the moisture from food, is a simple and common way to preserve some foods. The idea of air drying has been used for thousands of years to help keep food edible for a longer time. It works best with items such as fruit or thin pieces of unsalted meat.
A variation used in food preservation by drying is hot air drying. This process uses heated air to speed evaporation of the water in food, although the heat needs to be carefully regulated so as not to cook the food. The advantages to this method are that the speed at which the food dries will help to retain the flavors and vitamins in foods such as vegetables and fruits, while also actively killing some forms of bacteria. The disadvantages are that some foods do not respond well to the extended period of low heat and may dry out to the point of being completely inedible. The level of heat also can be a problem because, if it is allowed to get too high, the food will bake instead of drying out.
Freeze drying quickly converts the liquid in foods into ice and then extracts the ice. This is a very effective method of drying food and can extend the shelf life of an item by years. It preserves many nutrients but also changes the texture of most foods. Food preservation by drying in this way also is very expensive and requires specialized equipment.
All forms of food preservation by drying involve removing moisture, which concentrates the remaining elements of the food. This concentration of things such as sugars can be welcome in fruits and some vegetables, but can be hazardous when the concentrated ingredient is salt, as can occur when drying meats. One thing to be aware of is that certain beneficial nutrients, such as vitamins C and A, will dissolve in the air, reducing the vitamin content in dried food items such as citrus fruits.