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  3. Centuries ago, man discovered that removing moisture from food helps to preserve it, and that the easiest way to do this is to e

Centuries ago, man discovered that removing moisture from food helps to preserve it, and that the easiest way to do this is to e

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问题     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. In this way the North American Indians produce pemmican (dried meat ground into powder and made into cakes), the Scandinavians  make stock fish and the Arabs dried dates (枣) and "apricot leather" (杏干).
    Fruit is sun-dried in Asia Minor, Greece, Spain and other Mediterranean countries, and also in California, South Africa and Australia. The methods used vary, but in general, the fruit is spread out on trays in drying yards in the hot sun. In order to prevent darkening, pears, peaches and apricots are exposed to the fumes of burning sulphur (硫磺) before drying. Plums (李子), for making prunes (梅干), and certain varieties of grapes for making raisins (葡萄干) and small dried grape cakes, are dipped in a salt solution in order to crack the skins of the fruit slightly and remove their wax coating, thus increasing the rate of drying.
    Nowadays most foods are dried mechanically. The conventional method of such dehydration (脱水) is to put food in chambers through which hot air is blown at temperatures of about 110℃ at entry to about 43℃ at exit. This is the usual method for drying such things as vegetables, cut meat, and fish.
    Liquids such as milk, coffee, tea, soups and eggs may be dried by pouring them over a heated horizontal steel roller or by spraying them into a chamber through which a current of hot air passes. In the first case, the dried material is removed off the roller as a thin film which is then broken up into small, though still relatively coarse thin pieces. In the second process it falls to the bottom of the chamber as fine powder. Where recognizable pieces of meat and vegetables are added, as in soup, the ingredients are dried separately and then mixed.
How long has tile open-air method of drying food been known by man?____________
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答案For hundreds of years/centuries.
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