All foods possessing an organized structure, as animal flesh and amylaceous sub stances, require to be cooked before being eaten, the only exceptions being the oyster and some ripe fruits. The processes of salting, pickling, and smoking harden the ani mal textures, and, as we shall presently see (at all events in the case of salting), induce chemical changes which render the meat less nutritious.
The ordinary operations of cookery are boiling, roasting, broiling, baking, and frying.
In the case of vegetables, boiling affects the solution of gummy aticF,saecharine mat ters, the rupture and partial solution of starch grains, the coagulation of albuminous liquids, and the more or less complete expulsion of volatile oil. In the boiling of flesh, there takes place a more or less perfect separation of the soluble from the insoluble con stituents, according to the duration of the boiling, the amount of water employed, and its temperature at the commencement of the operation. If we wish the boiled meat to contain the largest amount of nourishing matter, and disregard the soup or broth that is simultaneously formed, we introduce it into the boiler when the water is in a state of brisk ebullition. We keep up this boiling for a few minutes, in order to coagulate the albumen near the surface, and thus to convert it into a crust or shell, which equally prevents the entrance of water into the interior, and the escape of thejuice and soluble constituents of the flesh into the water. If cold water is then added, so as to reduce the temperature to about 160r, and this temperature is kept up for the necessary time— for which, in reference to the weight of the meat, see the article BoiLING—all the con ditions are, according to Liebig, united which give to the flesh the quality best adapted to its use as food.
If, on the other hand, we wish to obtain good soup from meat, we should place it in cold water, and bring this very gradually to the boiling-point. The interchange between the juices of the flesh and the external water, which was prevented by the former pro cess, here takes place without hindrance. " The soluble and sapid constituents of the flesh are dissolved in the water, and the water penetrates into the interior of the mass, which it extracts more or less completely. The flesh loses, while the soup gains, in sapid matters; and by the separation of albumen, which is commonly removed by skim ming, as it rises to the surface of the water, when coagulated, the meat loses its tender ness, and becomes tough and hard; and if eaten without the soup, it not only loses much of its nutritive properties, but also of its digestibility."—Liebig's Researches on the Chemistry of Food, p. 128.
Roasting is applied much more to meat than to vegetables. Both in roasting and broiling meat, the first application of heat should be considerable and rapid, so as to form an outer coating of coagulated albumen (just as in boiling), which retains the nutritive matters within the cooked meat. In 'roasted meat, nothing is removed but some of the superficial fat and the gravy, which is itself an article of food. The effect of roasting on such vegetables as apples and potatoes is to render them more nutritive than the: would be in the raw state, by splitting their starch grains, and rendering them more soluble.
Baking (q.v.) acts in the same manner as roasting, but meat thus cooked is less wholesome, in consequence of its being more impregnated with empyreumatic oil.
Frying is the most objectionable of all kinds of In tins operation, beat is usually applied by the interinedium of boiling fat or oil. Various products of the decomposition of the fat are set free, which are very obnoxious to the stomachs of invalids.
Liebig has shown that salted meat is, in so far as nutrition is concerned, in much the same state as meat from which good soup has been made. After flesh has been rubbed and sprinkled with dry salt, a brine is formed amounting in bulk to one third of the fluid contained in the raw flesh. This brine is found to contain a large quantity of albumen, soluble phosphates. lactic acid, potash, creatine, and creatinine—substances which are essential to the constitution of the flesh, which therefore loses in nutritive value in proportion to their abstraction.
The preservation of food requires some notice. Three methods—viz., preservation by cold, preservation by the exclusion of air, and preservation by salting—are noticed in•the article ANTISEPTICS. The first is only of comparatively limited application: the second, known as Appert's method, has been successfully used in the English navy for many years; the chief objection to it is its expense: the third method injures, as we have already seen, the character of the meat, and renders it both deficient, in nutritive materials, and actually injurious if it forms a principal and continuous article Of diet. To these methods we must add preservation by smoking, preservation with sugar. and with vinegar, and preservation by drying. It is well known that meat suspended in 1 smoke loses its tendency to putrefy, the substance from which the smoke derives its autiseptic,property being creasote, or some allied body. Smoked meat acquires a pecu liar taste, a dark color, and a somewhat hard consistence; but it retains all its nutritive constituents, and is thus preferable to salted meat. Sugar and vinegar are chiefly employed in the preservation of vegetable products. The most important mode of pre serving articles of food, whether animal or vegetable, is by direct drying. Meat is cut up into small slices about a quarter of an inch thick, and vegetables into smaller pieces; they are steamed at a high temperature, so as to coagulate the albumen ; and they are then completely desiccated by exposure to a current of .very hot dry air. .At the con. elusion of the process, the slices of meat are quite hard, and present a shriveled appear, ante. Dr. Marcet (On the Composition of hood, 1856, p. 174) speaks in high terms of this method, which he had himself seen in operation in Paris. " Food thus preserved," het says, " whether it be animal or vegetable, has the advantage (1) of remaining in a fresh condition, though freely exposed to the atmosphere for a great number of years, and (2) of being rechiced to one fifth of its original bulk from its having lost all its water." He adds, that the preserved vegetables resume their bulk when boiled in water, and that they so completely retain their aroma, that it is often difficult to distinguish between soups made with them, and others prepared with fresh vegetables.