83. Fat-Rich Foods

Certain of the raw materials used in the human dietary are practically all fat or oil, such as lard, butter, and the salad oils. Meats such as pork, beef, and mutton are rich in fats, the proportion depending greatly on the condition of the animal from which the meat comes.

84. Nature And Kinds Of Fats

The vegetable and animal fats and oils may, for convenience' sake, be discussed in two divisions, the neutral fats or glycerides and the fatty acids. The neutral fats are combinations of the fatty acids with glycerin. When, for instance, lard is treated at a high temperature with the alkalies, potash, and soda, glycerin is set free, and an alkali takes its place in a union with the fatty acids. This is the chemical change which occurs in soap-making. There are several of these neutral fats, the ones most prominent and important in agriculture being those abundant in butter and in the body fats of animals; viz. butyrin, caproin, caprylin, caprin, laurin, myristin, olein, palmatin, and'stearin, the last three being the most abundant and important in human foods. Butyrin is a combination of butyric acid and glycerin, stearin of stearic acid and glycerin, and so on. Because these are combinations of three molecules of a fatty acid radical with one of glycerin, they are sometimes named tri-stearin, tri-palmatin, and tri-olein, and so on. Some single fats (glycerides) are compounds of two or three fatty acid radicle3 united with glycerin in the same molecule. As glycerin is an alcohol, and as combinations of an alcohol and acids are ethers, the neutral fats are really ethers (esters), although they differ greatly from our conceptions of an ether, which is gained from ethyl ether or the ether of drug stores.

85. Physical Properties

These individual fats possess greatly unlike physical properties. They are all soluble in benzine, chloroform, and ether, and insoluble in water. At the ordinary temperature of a room, some are liquid and some are solid, olein belonging to the former class, and palmatin and stearin to the latter. It is a matter of common observation that butter, lard, and tallow differ in hardness at a given temperature, and by the use of a thermometer it may easily be discovered that their melting points are not the same. As these animal fats are in all cases chiefly mixtures of olein, palmatin, and stearin, stearin and palmatin being a solid at ordinary temperatures, and olein a liquid at anything above the freezing point, it is evident that the relative proportions of these compounds will affect the ease of melting and the hardness of the mixtures of which they are a part. Stearin melts at 71.7° C. and palmatin at 62° C. Tallow, having more stearin than lard and butter, and less olein, is consequently much more solid on a hot day.

The composition and physical properties of the fat from a beef animal seem to vary according to the age of the animal and the locality of the body from which the fat is taken. Fat from an old animal melts at a lower temperature than that from a young animal, and the same is true of fat taken from the outside of the body as compared with that taken from the inside. Fat from herbivora is in general harder than that from carnivora.