Drying the simplest and oldest method of food preservation, is used in almost every part of the world, and for foods of all kinds. Drying a food reduces its water content to a level so low that the micro-organisms and enzymes which cause spoilage cannot function.
When carried out by traditional methods, drying is a gradual process. Food, however, begins to decay immediately, so drying is a race against spoilage. Various factors influence the rate at which food dries. The larger its surface area in relation to its volume, the more quickly it loses liquid. In practice this means that it has to be in small pieces or cut into flat sheets, as when fish to be dried is split and opened out, or figs are squashed. The air has to be dry. It is helpful if there is a wind. Heat speeds drying, since hot air can hold, and thus carry away, more moisture than cold air; but it also speeds up decomposition. A problem which affects all dried foods containing fat is rancidity caused by the oxygen in the air. This is most severe in oily fish, whose oil is highly unsaturated and goes rancid easily. These cannot be successfully dried.