This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
The concentration of food is based upon the fact that many foods contain a large percentage of free water, which can be driven off by evaporation, thereby reducing the weight, and usually the bulk of the food as well. Slight further condensation may be accomplished by pressure through machinery which is capable of applying a force of several tons to the square inch.
If the process of drying is not carried beyond a partial evaporation, the food is called "condensed." It may, however, be continued until the food is wholly dry, in which case the substance may be either "desiccated " - i. e., torn into shreds - or pulverised. Condensed and powdered foods may be added to other foods in order to thicken them or add to their nutritive power.
The nutritive principles of foods may be extracted by glycerin or acid solutions and other materials, and the resulting extract is condensed by evaporation to the consistence of a paste or powder. Beyond this it is not possible to concentrate foods by chemical process, for a definite bulk of food must be daily consumed from which to derive the energy for the body and the substance needed for repair.
Drying is conducted either in the heat of the sun or by artificial means. The class of dried foods embraces the various forms of dried and desiccated meat and fish, dried milk, dried vegetables and fruits, such as peas, beans, lentils, corn, okra, apples, peaches, cocoa-nuts, grapes (raisins), figs, etc. The addition of sugar, flour, or salt by abstracting moisture, aids the drying and helps to prevent decomposition. (See Food Preservation by Drying, p. 278).
Milk may be dried in vacuo and preserved by itself as a powder, or mingled with other materials, such as malt and various starchy foods. Condensed milk has been already described (p. 95).
Eggs may also be successfully dried; they keep well, especially when mixed with farinaceous materials (p. 108).
Froissart relates how the King of France in the invasion of England in 1386 had the yolks of eggs packed and stored in barrels to furnish rations for the troops. Egg albumin dries in the form of thin scales which may be indefinitely preserved.
The preservation of meat and fish by drying is probably the oldest, as it is the most primitive, method in use. Meat drying is practised extensively among savage tribes in almost all parts of the world, but especially where purity of the atmosphere combined with intense heat and dryness of climate will cause the water to evaporate from the meat so rapidly that germs do not have time to decompose it. For this purpose only lean meat can be used, as the fat does not part with its water with sufficient readiness. Dried meat loses much in weight, becomes hard and tough, and in many cases tasteless. It is therefore usually indigestible, and requires prolonged cooking and proper seasoning. When prepared in this way, the drying process may be applied very thoroughly, and the food is more easily cooked and seasoned. Dried meat may be pre-digested, evaporated, powdered, and made into a paste for broths, or used to re-enforce various food preparations for invalids. Powdered meat is sometimes mixed with sugar and salt, or pulverised dried cooked vegetables, bread, etc.
Meat extracts have been described in detail on pp. 113 to 120.
Pemmican is a preparation of dried powdered meat, which has been described on p. 200.
Dried vegetables keep even better than dried meats. Tea and coffee are good examples of dried vegetable substances. They may be extracted and then concentrated by evaporation. Potatoes are concentrated by drying to less than one third of their original weight, and they may be thus preserved in slices or in granular form. Many other vegetables are prepared by drying, and, in addition, some are compressed, as, for example, cabbage, cauliflower, carrots, etc. The compression still further excludes air and moisture. As a rule, dried vegetables are only serviceable for relieving monotony of diet when fresh vegetables cannot be obtained. Desiccated vegetables have been used with some success in the United States navy, but they have less antiscorbutic property than fresh foods.
Bread may be preserved for a long time by drying, but it usually becomes tasteless, and is useful in this condition only in emergencies, or to make variety in the rations of sea biscuits, hard-tack, etc., which are furnished to soldiers and sailors in active service.
Major Woodruff, Assistant Surgeon, U. S. A., writes of this subject: "The Germans have been the first to take advantage of drying and compressing processes in the manufacture of a dried, compressed bread. The great difficulty in the use of bread for field service consists in the inability to supply it so that it will keep a long time and be digestible. Hard-tack is ruinous to many soldiers, as already pointed out. If baker's bread is compressed, it sinks into a heavy dough. Only strong stomachs can digest it, and it is far worse than the wet, soggy, hot breakfast bread with which we cultivate dyspepsia. If the bread is merely dried, it is too bulky for transportation. By a new process, which probably consists in drying the bread and at the same time compressing it by improved machinery, the Germans have secured a variety of field bread which is spoken of in very high terms. Small bits of it thrown into soup swell up like a dried sponge when thrown into hot water. The soldiers are said to be very fond of it, and as far as known it is entirely successful....
" The French Department of Intendance has been experimenting with dried bread, which is said to be superior for campaigning purposes both to biscuit and ordinary bread. From the results of the experiment, which are given in the Revue du Service de l'lntendance Militaire, it appears that this dried bread will absorb from five to six times its own weight of water, milk, tea, coffee, or bouillon. Biscuit absorbs hardly its own weight of liquid, although when thoroughly dried it contains only about 10 per cent of water, whereas the bread contains from 12 to 14 per cent. It can be made in cubes of convenient form".