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.
Drying is often combined with salting, which arrests spoilage at once. The smoking of food also helps to dry it, as well as depositing a layer on the surface which is antiseptic and excludes oxygen to avoid rancidity.
Chemical agents may be used to arrest spoilage while drying takes place. The chief of these (apart from salt) is sulphur dioxide, which creates acid conditions and is effective against both enzymes and micro-organisms. It is widely used on dried fruit. Pepper and spices, which are mildly antiseptic, aid the preservation of some dried foods.
Meat can be dried without salt if it is cut into thin strips or sheets. This may well have been how Attila the Hun and various barbarians reputedly dried meat under their saddles as they rode across the steppes (a proto-ready-meal). However, many dried meats include salt, often added mainly for flavour. The ‘dry-salting’ of large pieces of meat, such as some kinds of bacon or ham, is not really a drying procedure, since it relies almost entirely on the action of the salt. However, the best hams, such as Parma or jamón serrano, include very lengthy drying times after their initial cure, normally from six to twelve months.
Genuinely dried meats include the Latin American charqui (or tassajo), made with beef, and its mutton equivalent chalona. The name charqui has been Anglicized to jerky (or jerked beef). Such foods were, and to some extent still are, used by travellers, cowboys, and other people who had to carry their food with them. See also biltong. Typically, these products consist of thin strips, air dried, usually salted, sometimes lightly smoked, often peppered or spiced.
Dried meats are often made in mountainous areas where windy conditions (and at great altitudes, low air pressure) favour drying: for example, conditions are good for drying pork in Nepal.
Thicker pieces of dried meat in which drying plays a significant part in the preservation process include some beef products such as the northern Italian bresaola and the Swiss Bündnerfleisch, two products made in adjoining border regions. On Orkney and Shetland wind-dried meat is known as vivda. The E. European and Turkish pastrami or pasturma is another example. See also pemmican.
Plain dried fish, prepared without salt, has been superseded by salted, dried fish, or fish preserved in other ways, in many parts of the world where it was formerly usual. In N. Europe, where the climate allows simple air-drying, it continues in use alongside combined salting and drying. Only white fish, whose flesh is not oily, are suitable for either process and, for drying without salt, they have to be fairly small.
In ancient Egypt, classical Greece, and the Near East salt was readily obtainable, and fish was salted to dry it from early times, as well as being preserved in brine. In medieval Europe stockfish was a major food, and was the subject of considerable trade. The name, originally a German word, was a general one for any dried white fish, most often cod, but also pollack, whiting, hake, and others. These might be dried with or without salt, although the modern usage of the term ‘stockfish’ is for an unsalted kind only. Salt cod is the prime example of the salted kind.
Eggs are difficult to dry, and it was not until the early 20th century that a workable process was developed, by German engineers in China. From the 1930s onwards other countries, including the USA and Britain, began to dry eggs, for baking and other processed foods. During the Second World War dried egg was used in Britain and elsewhere, for different and obvious reasons, but domestic use of the product has since almost ceased, although its quality has improved. Eggs may be dried whole, or separated into yolks and whites.
Dried milk is another modern product, foreshadowed by the Indian condensed milk khoya (see milk reduction). This is evaporated and coagulated by slow heating, often carried to the point at which the product is quite dry and crumbly. It is not intended to keep, but is used as an ingredient in confectionery. Dried milk is now made by either drum or spray drying.
Sun-drying of most fruits is easy in warm, dry climates if there is no objection to the fruit becoming brown or black, as is considered normal in raisins, prunes, and figs. Dried figs were a main article of the diet of ordinary people in classical Greece and Rome. In Arabia and N. Africa dried or partly dried dates are still a staple food.
Sun-dried tomatoes, formerly best known in the Mediterranean region (see Patience Gray, 1986, for an account of their preparation in Apulia and of how they can be stuffed), became fashionable as a speciality food in N. America and W. Europe in the closing decades of the 20th century.
In medieval Europe dried fruits were much in demand. Currants, prunes, figs, and dates were all imported from Mediterranean countries on the same ships that brought spices. Rich people used them liberally in pies, tarts, and pottages. The medieval mistrust of raw fresh fruit did not extend to exotic dried kinds. Apples and pears were also dried. Rather than being sliced into rings as in more recent practice, they were peeled and cored but left whole. Threaded on strings and hung across an airy room they became brown, leathery, and sweet through concentration of their sugar as they shrank.
Vegetables are in general less suitable for drying by simple processes than are fruits. They are watery and lack sufficient protective sugar or acid to resist decay. If air-drying were attempted many vegetables would simply go bad, or leaves would wither and brown. (There are exceptions. The leaves of melokhia, which is made into a soup loved by Egyptians, are sometimes air dried, and it is possible still to buy rings of dried aubergine in Turkey. Dried chillies are, of course, universal.)
Attempts were made long ago to dry some vegetables for use as military rations to ward off scurvy, but it was not understood until the 20th century that the procedure would make them almost useless for that purpose. The first process was patented in 1780, but was unsuccessful, as were other early attempts, e.g. to produce by drying an instant mashed potato. Indeed, some would say that full success has still not been achieved. Modern dried vegetables (other than freeze dried, for which see freezing) are prepared in an air draught whose temperature is a compromise between the conflicting needs of fast drying and flavour preservation. The vacuum method, which satisfies both requirements, is sometimes used for better-quality products.
Among dried vegetable products, much the most important are grains. Generally, whole grains harvested in favourable conditions need only a little air-drying to make them keep, although in wet climates, such as that of Britain, fuel-burning grain dryers often have to be used.
Pulses—peas, beans, lentils, chickpeas, and the like—are typically staple foods of dry areas, such as India and Mexico, where they can be spread out in the sun and left to dry naturally inside their protective seed coats.
Hard spices such as cinnamon and nutmeg are little altered by the small amount of drying necessary to preserve them. Drying may bring about a desirable change in flavour, as in pepper, where the pungency increases.
Among herbs, woody types such as rosemary and thyme stand up to drying well; slightly more fleshy plants such as marjoram and sage reasonably well; and fleshy moist ones like mint not very well. Worst is probably basil which, however carefully dried, loses the ‘top notes’ of its fragrance and has little virtue left.
Fungi have been dried with great success since early times. In a dry climate they can be air dried without any special preparation other than cutting the larger ones into slices. All kinds shrink considerably, concentrating the flavour. The best known of the dried mushrooms on sale in Europe is the cep; the Italian funghi porcini are of this kind. The British mushroom which was most dried in former times was the champignon or fairy ring mushroom. The Chinese dry many fungi, especially the wood ear.
Various beverages are prepared from dried ingredients. Tea leaves and coffee beans are obvious examples. Instant coffee and tea are made by brewing the drink and drying the liquid in a spray dryer (see below) or, in the case of high-quality instant coffee, by freeze-drying. Other dried plant substances used to make infused drinks are chicory (dried root), cocoa (dried powdered seeds), guarana (dried powdered seeds, made into smoked cakes), cola ‘nut’ (dried powdered seeds), and maté (dried leaves).
For an interesting form of dried soup, see portable soup.
Of modern mechanized drying techniques, the most commonly used is continuous tunnel drying. The food is loaded onto wire mesh trays which are drawn slowly through a long tunnel through which heated air is blown. By the time the food emerges from the far end of the tunnel it is fully dried.
A refinement of the tunnel dryer is the fluidized bed dryer, used for small items such as peas. The food travels along perforated plates through which warm air is blown from below. The air lifts and transports the food while it is drying. The further it goes the hotter the air gets. In the case of peas, moisture content falls from 80% to 50%. The food is then transferred to a stationary drying bin where it is finished off slowly with warm air. Finally, after a total of, say, sixteen hours, the moisture content is down to 5%.
Stationary bin or cabinet dryers are used throughout for less robust foods. In a closed cabinet a partial vacuum can be applied to the food, which allows the use of lower temperatures and so avoids damage to delicate fruits and vegetables.
Liquids such as milk, fruit or vegetable juices, and pastes, such as those for breakfast cereals, may be dried in drum dryers. Two heated stainless steel drums revolve slowly, almost touching. The liquid is poured into the gap between the drums and trickles down slowly. It dries on the lower surfaces of the drums and is scraped off by a blade. Usually drying is assisted by a partial vacuum.
Another drying method for liquids or semi-liquids, including milk, eggs, and instant coffee, is spray drying. A large, funnel-shaped chamber, typically 3 m (10′) high, has a whirling nozzle at the top through which the liquid is misted into the chamber. Hot air is also blown in at the top. The mist dries to a powder and collects at the bottom. The product is less damaged by heat than in drum drying. The fine powder has to be slightly moistened and formed into granules to make a product which will dissolve without bedding down into a lumpy mass.
Alan Davidson was a distinguished author and publisher, and one of the world's best-known writers on fish and fish cookery. In 1975 he retired early from the diplomatic service—after serving in, among other places, Washington, Egypt, Tunisia, and Laos, where he was British Ambassador—to pursue a fruitful second career as a food historian and food writer extraordinaire. Among his popular books are Seafood of South-East Asia, North Atlantic Seafood, and Mediterranean Seafood. In 2003, shortly before his death, he was awarded the Erasmus Prize for his contribution to European culture.
Gray, Patience (1986), Honey from a Weed, London: Prospect Books.
Riddervold, Astri with Ropeid, Andreas (ed) (1988), Food Conservation, London: Prospect Books.
