=The principles of cooking.=——In science the word “principle” ordinarily means a formulation of some _general_ or _constant mode of behavior_——a generalization based on many observations of fact. In cookery the word is used in the same sense; for example, one may say that an important principle to bear in mind when cooking with any fat is that the fats may be melted without decomposition, but when too strongly heated they begin to decompose with the production of acrid and irritating products. Sometimes, however, we speak of “principles of cookery” in a broader and somewhat less exact sense to indicate the _general purposes_ of cooking operations, as when we say that the most important principle of vegetable cookery is to soften the fiber without destroying the flavor or dissolving away the ash constituents of the vegetable. That is, the change either chemical or physical that takes place in a certain foodstuff by the application of heat or cold or by the use of a fermentation process may be referred to as the underlying, working principle. We shall study in detail these changes as we experiment with and prepare each food material, but a general statement of the effect of heat on various foodstuffs will be helpful here. _Protein._——There are several forms of protein, with differences that we can understand only after a thorough study of chemistry. The most important proteins in meat, fish, eggs, milk, old beans and peas coagulate, or become slightly harder or firmer at a temperature below the boiling point of water. We shall perform an experiment to show this while studying the egg. There is no marked chemical change; that is, the protein is not changed to another substance. _Fats._——Solid fats are liquefied by heat, and freed from the tissue that contains them in animal fats like suet. When a fat begins to smoke with heat, a chemical change is taking place. If intense heat is continued, all the hydrogen and oxygen are driven off and pure carbon remains. When the fat is “brown,” giving the flavor we like, a part of the oxygen and hydrogen have been driven off. The “boiling” of fat in a kettle is ordinarily due to the boiling of the water contained in the fat. _Starch._——Starch occurs in the form of granules. See Fig. 39. In boiling water, the granule expands and finally bursts, and frees the content, the pure starch, and the whole mass thickens. Boiled with an acid the starch is changed to dextrin, a substance resembling a gum, and the mixture becomes thin; and this process continued changes the dextrin to dextrose. With intense “dry” heat, as in toasting, the granule expands and opens, and the contents change to dextrin. Continued heat reduces the starch to pure carbon. The brown color and pleasant flavor in toast are a stage on the road to carbon.