单选题 All living cells on earth require moisture for their metabolism. Cereal grains when brought in from the field, although they may appear to be dry, may contain 20 per cent of moisture or more. If they are stored in a bin, there is sufficient moisture in them to support several varieties of insects. These insects will, therefore, live and breed and, as they grow and eat the grain, it provides them with biological energy for their life processes. This energy will, just as in man, become manifest as heat. Since the bulk of the grain acts as an insulator, the temperature surrounding the colony of insects will rise so that, not only is part of the grain spoiled by the direct attack of the insects but more may be damaged by the heat. Sometimes, the temperature may even rise to the point where the stored grain catches fire. For safe storage, grain must be dried until its moisture content is 13 per cent or less.
The direct drying of other foods has also been used. Fish has been dried in many parts of the world besides Africa. Slices of dried meat are prepared by numerous races. Biltong, a form of dried meat, was a customary food for travelers. The drying of meat or fish, either in the sun or over a fire, quite apart from the degree to which it exposes the food to infection by bacteria and infestation by insects, tends also to harm its quality. Proteins are complex molecular structures which are readily disrupted. This is the reason why dried meat becomes tough and can, with some scientific justification, be likened to leather.
The technical process of drying foods indirectly by pickling them in the strong salt solutions commonly called "brine" does less harm to the protein than straightforward drying, particularly if this is carried out at high temperatures. It is for this reason that many of the typical drying processes are not taken to completion. That is to say, the outer parts may be dried leaving a moist inner section. Under these circumstances, preservation is only partial. The dried food keeps longer than it would have undried but it cannot be kept indefinitely. For this reason, traditional processes are to be found in many parts of the world in which a combination of partial drying and pickling in brine is used. Quite often the drying involves exposure to smoke. Foods treated in this way are, besides fish of various sorts, bacon, hams and numerous types of sausages.

单选题 According to the passage, insects also spoil stored cereals by ______.

单选题 Direct drying affects the quality of meat or fish because ______.

单选题 It can be inferred from the passage that salting preserves food by ______.

单选题 The word "pickling" (para. 3) means ______.

单选题 What does the passage mainly discuss?