Food Preservation by Drying or Dehydration

Water is needed by micro-organisms to survive and reproduce. Therefore, by removing the water in foods below a critical value (this is a value that is specific for different types of foods) spoilage is reduced. In the past, this was done by drying foods (e.g. fruits) in the sun; today many types of equipment are used, and the food is dried by the use of air at regulated temperatures and humidity.

Advantages of drying

• If kept dry, food keeps indefinitely.
• Food preserved by this method occupies less space than food preserved by other methods. Some dried foods occupy only 10 percent of the space that would be required when fresh.
• Dried foods are easily transported and stored.
• The cost of drying and the expenses incurred in storing are not high as those of other methods of preservation.
• There is no waste after purchase, therefore portion control and costing are simplified.

FOOD	EXPLANATION
Vegetables	Vegetables: peas, onions, beetroot, beans, carrots, lentils, cabbage, mixed vegetables and potatoes. Many vegetables are dried; those most used are the pulse vegetables (beans, peas and lentils), which are used for soups, vegetable purees and many vegetarian dishes. Usually potatoes are cooked, mashed and then dried. The other dried vegetables are used as a vegetable (cabbage, onions). Pulse vegetables may be soaked in water before use, then washed before being cooked. Vegetables that are dehydrated (having a lower content of water as more moisture has been extracted) are soaked in water. Dehydrated potatoes are in powder form and are reconstituted with water, milk, or milk and water. They often have manufactured vitamin C added as dehydration results in a loss if this vitamin.
Milk	Milk is dried either by the roller or spray process. With the roller method the milk is poured onto heated rollers, which cause the water to evaporate; the resulting powder is then scraped off. This method is not widely used now as it damages the milk proteins and resulting in a less soluble dried product, which is more difficult to reconstitute. With the spray process the milk is sent through a fine jet as a spray into hot air, the water evaporates and the powder drops down. The temperature is controlled so that the protein in the milk is not cooked. Milk powder may be used in place of fresh milk, mainly for economic purposes (especially skimmed milk powder), and is used for making custard and white sauce.
Eggs	Eggs are dried in the same way as milk, and although they have a food value similar to fresh eggs, dried eggs do not have the same aerating quality. When reconstituted the eggs should be used be used at once; if they are left in this state in the warm atmosphere of a kitchen, bacteria can multiply and food poisoning may result; although pasteurized before drying, the mixture may be contaminated in the kitchen and it is a very suitable food for the growth of bacteria. Dried eggs are mainly used in the bakery trade.
Fruit	Fruits: apples, pears, plums (prunes), apricots, figs, and grapes. Sultanas, currants and raisins are grapes that have been dried in the sun
Herbs	Fresh herbs are tied into bundles and allowed to dry out in a dry place.