| Centuries ago, man discovered that
removing moisture from food helps to preserve it, and that the easiest way to do
this is to expose the food to sun and wind. All foods contain water--cabbage and other leaf vegetables contain as much as 93% water, potatoes and other root vegetables 80% , lean meat 75% and fish anything from 80% to 60% depending on how fatty it is. If this water is removed, the activity of the bacteria(细菌) which cause food to go bad is checked. Nowadays most foods are dried mechanically. The conventional method of such dehydration(脱水) is to put food in chambers through which hoi air is blown at temperatures of about 110°C at entry to about 43°C at exit. This is the usual method for drying such things as vegetables, minced meat, and fish. Liquids such as milk, coffee, tea, soups and eggs may be dried by pouring them over a heated horizontal steel cylinder or by spraying them into a chamber through which a current of hot air passes. In the first case, the dried material is scraped off the roller as a thin film which is then broken up into small, though still relatively coarse flakes. In the second process it falls to the bottom of the chamber as a fine powder. Dried foods 'take up less room and weigh less than the same food packed in cans or frozen, and they do not need to be stored in special conditions. For these reasons they are invaluable to climbers, explorers and soldiers in battle, who have little storage space. They are also popular with housewives because it takes so little time to cook them. |