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 thus, 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.
Traditional arts of food preservation took advantage of this principle in a number of ways. The plant seeds, wheat, rye, rice, barley millet, maize, are themselves structures evolved by nature to provide stored food. The starch of their endosperm is used for the nourishment of the embryo during the time it over-winters (if it is a plant of the Temperate Zone) and until its new leaves have grown and their chlorophyll can trap energy from the sunlight to nourish the new-grown plant. The separation by threshing and winnowing is, therefore, to some degree part of a technique of food preservation.
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, by likened to leather.
The technical process of drying foods indirectly by pickling them in the p 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.
According to the passage, insects spoil stored cereals by （）
A．consuming all the grain themselves
B．generating heat and raising the surrounding temperature
C．increasing the moisture content in the grain
D．attacking each other for more grain