Fats are composed of carbon. hydrogen, and oxygen. Their chemical nature was thoroughly explained by the French chemist Chevreul as far back as 1811. Chevreul showed that fats are combinations of ordinary glycerin and certain so-called fatty acids. Glycerin is a tri-atomic alcohol: that is to say, like the hydroxide of a tri•valent metal (such as ferric hydroxide), it acts like a weak base and is capable of combining with three molecule equivalents of a monobasie acid (such as ordinary acetic acid). The combination of an alcohol and an acid is called in chemistry an ester, or ethereal salt. The esters of glycerin and stearie. palmitic, and °Mere acids are called, respectively, tri stearin, tri-palmitin, and tri-olein. The vari 0119 fats are mixtures mainly of these esters or 'glycerides.' containing them in varying rela tive quantities. Fats. like tallow, containing a large proportion of tri-stearin and tri-pal mitin are comparatively hard: on the contrary. lard and similar fats, which are soft and pasty. are found to contain a high percentage of tri olein. The color, state, consistency. etc.. of fats vary with the source from which they arc de rived. The fat of carnivorous animals has a peculiar disagreeable odor. and is not so hard as that of herbivore. Human fat contains, be sides tri-palmitin, t•i-olein, and some tri-stearin, also a yellow substance resembling bile by its odor and bitter taste. The animal secretions all contain a certain amount of fat; ear-wax, for instance. has been shown to contain sonic tri stearin and some tri-olein. Butter contains about 87 per cent. of fat, including a considerable amount of tri-butyrin (the ester of glycerin and butyric acid) ; oleomargarine, prepared from the best beef tallow, differs but slightly in composi tion from butter.
When exposed to the action of steam heated to a temperature of 400' F. (about 200° ('.), all fats and fatty oils are decomposed into their chemical constituents. The same action takes place in the presence of moisture, though much more slowly, at ordinary temperatures (the ran chilly of fat is flue to the separation of free acids, and may. therefore, be removed by dissol• ing-, out the acids with water]. A similar decom position takes place in the animal organism; steapsin. one of the pancreatic ferments, splits up and emulsifies fat in the process of digestion. Even 1111)11e readily than with hot steam, and at a much lower temperature, is the decomposition of fat effiieted with caustic alkalies. When fats arc treated with sodium or potassium hydroxide, the metal takes up the acid of the fat, forming the salts known in common life as soap, while the glycerin of the fat is set free. The process is extensively employed in the manufacture of soap, and is therefore generally spoken of by chemists as the saponification of fats. See Es'rERs.
In the animal organism, or when heated with free access of oxygen, fats are burned (oxidized) completely, yielding. like other compounds of carbon, water and carbonic acid. But when they arc burned ineompletely, as sometimes in care less cooking, a number of combustible gases- are produced, including the vapors of acrolein, to which the irritating odor of superheated fat is due. See OILS.