The Chemistry of Cookery

small piece of this ‘mucous membrane’ will coagulate the casein of three thousand times its own quantity of milk, or the coagulation may be effected by placing a small piece of the stomach (usually salted and dried for the purpose) in the milk, and warming it for a few hours. Many theoretical attempts have been made to explain this action of the rennet. Simon and Liebig suppose that it acts primarily as a ferment, converting the sugar of milk into lactic acid, and that this lactic acid coagulates the casein. This theory has been controverted by Selmi and others, but the balance of evidence is decidedly in its favour. The coagulation which occurs in the living stomach when milk is taken as food appears to be due to the lactic acid of the gastric juice. Casein, when thoroughly coagulated by rennet, then purified and dried, is a hard and yellowish hornlike substance. It softens and swells in water, but does not dissolve therein, nor in alcohol nor weak acids. Strong mineral acids decompose it. Alkalies dissolve it readily, and if concentrated, decompose it on the application of heat. When moderately heated, it softens and may be drawn into threads, and becomes elastic; at a higher temperature it fuses, swells up, carbonises, and develops nearly the same products of distillation as the other protein compounds. Note the differences between this and the soluble casein above described, viz. that obtained by simply removing the fat from the milk, then evaporating away the water, but using no rennet. I have good and sufficient reasons for thus specifying the properties of this constituent of food. I regard it as the most important of all that I have to describe in connection with my subject—the science of cookery. It contains (as I shall presently show) more nutritious material than any other food that is ordinarily obtainable, and its cookery is singularly neglected, is practically an unknown art, especially in this country. We commonly eat it raw, although in its raw state it is peculiarly indigestible, and in the only cooked form familiarly known among us