This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
When meat is cooked most of its connective tissue is turned into gelatin, and the whole mass becomes softer and more readily broken up by the teeth. In boiling meat it is a good plan to put it first into boiling water which coagulates its surface layer of albumen, and this then keeps in flavoring and other matters which would otherwise pass out into the water. After the first few minutes the cooking should be continued at a lower temperature; meat boiled too fast is hard, tough, and stringy. In roasting or baking meat, the same plan is advisable. Put it close to the fire or in a hot oven for a short time, and then complete the cooking more slowly at a lower temperature.
The cooking of vegetable foods is of considerable importance. Starch is the chief nutrient matter in most of them, and raw starch is much less easily digested than cooked. When starch is roasted it is turned into a substance known as soluble starch, which is easily dissolved by the digestive liquids, so there is a scientific foundation for the common belief that the crust of a loaf is more digestible than the crumb, and toast than fresh bread.
The necessary quantity of daily food depends upon that of the material used by the body and passed out from it in each twenty-four hours; this varies both in kind and amount with the work done and the organs most used. In children a certain excess is required to furnish material for growth.
What happens when meat is cooked ?
Why does a good cook first put meat that she is to boil into very hot water ? Why should the boiling be completed at a lower temperature? How should meat be baked?
Why is it important to cook most vegetable foods ? Why is toast more easily digested than fresh bread?
What determines the necessary amount of daily food? How does it vary? Why do children require more in proportion to their size than adults?
It is impossible to state accurately beforehand just what amount of food any individual will require, but a general idea may be arrived at by taking the average daily losses, by excretion, of a man, as determined by many experiments made on different persons. Such experiments show that a man of average size and doing ordinary work needs rather more than 9 1/2 ounces (274 grams) of carbon to replace his loss of that element; and about 7/10 of an ounce of nitrogen (20 grams). Some hydrogen is also required, as the body daily loses more water than we take in our food; and this extra amount implies a loss of hydrogen, combined with oxygen in the body to form water.