The chemical composition of meat is modified by cooking, the results varying according to the method of cooking employed. Cooking abstracts a large amount of water, about 20 per cent, also part of the extractives, salts, and fat. A large amount of the extractives are removed by boiling; hence boiled beef is deficient in flavour. The cooking of animal food develops in it new flavours and a greatly improved appearance - two factors of importance in promoting the digestibility of meat, the attractiveness of well-cooked food producing an increased secretion of gastric juice. The functions of digestion and assimilation are best carried on at a blood heat, therefore another object of cooking is to raise the food to a suitable temperature. The application of heat exercises a mechanical and also a chemical action on all foodstuffs. The mechanical action is more important when the question of cooking vegetable substances is considered. The prolonged heat acts by softening hard tissues and preparing tough tissues for mastication. Cooking of food is also of value in destroying parasites and pathogenic bacteria. With regard to the influence of cooking on the digestibility of meat foods, it is probable that most forms of cooking tend to lessen the digestibility of meat, the reverse holding good for vegetable foods.
The effect of heat on the proteins is to coagulate them - the connective tissues being transformed into gelatine, and the protein of the meat at the same time developing odorous substances (extractives) which impart flavour and stimulate the appetite.
The fats of food do not appear to be much affected by heat. Fat which has been heated and allowed to cool again is often found to become more granular than before. This is probably due to the driving off of water, which tends to make the fat more brittle and therefore more digestible. This change is well seen in fried bacon.
The effect of different degrees of heat on albumin is the keynote of all the various culinary methods, which are variations of one another, depending on whether the meat juices are to be retained or extracted.
(a) When the meat juices are to be retained there is first a preliminary case-hardening of the albumin by intense heat; after which, cooking proceeds at a lower temperature, rising in the interior of the meat just to the coagulating point of albumin. This object may be carried out by any of the following methods: -
The heat being radiant heat and hot gases.
In this the heat is hot water.
This process is carried on by hot oil or hot oil and conduction.
By following these methods we retain all the nutritive sapid qualities within the portion, leaving none of them, or very little, in the medium used.
(b) When the juices are to be extracted from the meat, the result is a slew or a soup. Stewing first extracts the juice and then cooks the meat in it, and is thus conducted slowly and over a long time. Soupmaking aims at extracting as much as possible, and therefore begins in the cold, never rising above 1600 F. The processes will now be referred to in detail.