Cooking sterilizes the food and renders it more palatable. It also alters its composition in various ways, according to the method employed.
The effect of cooking upon the composition of meat has been the subject of numerous researches. In a recent paper by Grindley and Emmet describing experiments under conditions which were carefully controlled, the following conclusions were drawn. Cooking has but little effect upon the total nutritive value of meat, whether it be roast or boiled. The method of cooking, however, influences the quantity of the nitrogenous and nonnitrogenous extractives, to which the flavour is chiefly due, for by boiling about two-thirds of these substances, on the average, together with a part of the salts of the meat, some fat and a very little protein, are transferred to the broth. The longer a piece of meat is boiled the richer is the broth, but even after many hours the amount of nutritive matter extracted is small. The larger the piece of meat the smaller is the relative loss. There is of course no economic disadvantage in boiling if the broth be served for soup. In raw meat about one-eighth of the protein is in a soluble form; boiling converts most of this into insoluble protein. Whether the meat be put first into hot or cold water makes very little difference to its final composition.
In roasting, broiling, or cooking by any form of dry heat there is a slight loss of extractives and salts, but much less than with boiling, for it was found that dry-cooked meats contain twice as much of those soluble organic and inorganic flavouring substances as boiled meats. There is also a smaller loss of protein and fat in cooking meat by dry heat, but from the nutritive standpoint the difference is so slight that it is not of importance.
The cooking of vegetables breaks up starch grains and bursts the walls of the cells, enabling the digestive juices to penetrate the mass. The usual method, boiling, involves a great loss of nutritive material. Snyder, Frisby and Bryant have shown that with potatoes the loss is greatest when they are peeled and soaked in water before cooking, the shortage of total nitrogenous matter amounting, under these circumstances, to 50 per cent and of protein 25 per cent. The loss of mineral matter is 38 per cent. The loss of total nitrogenous matter and protein is about half the above if the peeled potatoes are put at once into boiling water : if they are boiled in the skins it is negligible. In boiling carrots about 40 per cent of the total nitrogen and 26 per cent of the sugar is lost, equal to about a pound of sugar in a bushel of carrots. At least a quarter of the nutritive material is therefore extracted. There is less waste if the carrot be in large pieces, and if the boiling be rapid and very little water be used. Cabbage does not contain much nutritive matter to begin with and it loses about a third of it in boiling. This loss is avoided if the cabbage be cooked with meat as a stew, the fluid part of which is also consumed. It is better to steam vegetables than to boil them. With cooking by dry heat these losses are obviated. Potatoes baked in their skins are therefore an economical food.
On the whole cooking renders meats slightly less digestible and vegetables more digestible.