In science the word "prin-ciple" 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.