Almost as widely distributed in nature as the carbohydrates, and constituting a much more concentrated form of fuel to supply energy in nutrition, are the fats. Fats are glyceryl esters of fatty acids, and since glycerol is a triatomic alcohol and the fatty acids are monatomic, a normal glyceride is a triglyceride and on hydrolysis yields three molecules of fatty acid and one molecule of glycerol. Thus, for example:

C3H6(C18H35O2)3 + 3 H2O → C3H5(OH)3 + 3 C18H36O2.

Stearin Glycerol Stearic acid (glyceryl tristearate).

When the splitting of the fat is brought about by means of an alkali instead of water, the corresponding products are glycerol and three molecules of the alkali salt of the fatty acid. Since alkali salts of the fatty acids are commonly known as soaps, this reaction is usually called saponification of the fat.

The fats are therefore a definite group of chemical compounds, and the term applies equally to the solid and the liquid members of this group. As a matter of convenience, however, the liquid fats are often called "fatty oils." The fatty oils are also sometimes called "fixed oils," since a spot made by dropping a fatty oil on paper cannot be removed by drying (as can a volatile oil), nor by washing with water (as can glycerin).

Another property which helps to characterize the fats is that glycerol, or the glyceryl radicle of a fat, when heated to a high temperature (3000 C. or over), decomposes with production of acrolein, an aldehyde of characteristic odor and very irritating to the mucous membranes. Doubtless also fatty acid radicles may sometimes contribute to the production of irritating fumes when fat is overheated.

The fats, including fatty oils, are lighter than water, their specific gravities ranging between 0.90 and 0.97. They are poor conductors of heat and therefore tend to conserve the heat of the body, while they show on oxidation a much higher fuel value than any of the other foodstuffs.

All of the fats are practically insoluble in water, and all except those of the castor oil group are sparingly soluble in cold alcohol, but dissolve readily in petroleum ether and mix in all proportions with light petroleum oils. Light petroleum distillate ("petroleum ether") is often used as a solvent for fat. All of the fats are readily soluble in ether, carbon bisulphide, chloroform, carbon tetrachloride, and benzene. Since neither carbohydrates, proteins, nor ash constituents are soluble in ether (or the other "fat solvents"), it follows that the fat of a food may be readily separated from the other chief components by drying the food and extracting the dry material with pure ether. After the fat has been completely dissolved away from the other foodstuffs, it can be recovered from the solvent by evaporating the latter at a relatively low temperature. This is the method commonly used to estimate the percentages of fat in foods and to obtain small portions of fat for examination. It must be noted, however, that the fat thus obtained is not always pure in the sense of consisting entirely of substances meeting the definition of fat as given above. Obviously, such an extract will contain, along with the fat, any other ether-soluble substances which were present in the food, and may contain substances which, while not appreciably soluble in ether alone, are dissolved by the mixture of ether and fat. It is therefore somewhat more accurate to speak of the material extracted by ether as "ether extract" rather than as "fat," and it will be found so designated in some statements of analytical results. In most human foods - at least those which are important as sources of fat - the constituents of the ether extract other than true fat are for the most part fat-like substances and we shall therefore be sufficiently accurate in most cases if we designate the material extracted by ether by the simple term "fat," remembering, however, that we may thus include along with the glycerides (and any free fatty acids which may be present) small amounts of fat-like substances or lipoids, and of fat-soluble or other ether-soluble matter.

The food fats of commerce have been separated from the materials in which they naturally occurred not by solvents as above described but by mechanical means such as churning (butter) or pressing (olive or cottonseed oil) but even in this case the naturally occurring fat-soluble substances will still remain dissolved in the separated fat. Recent investigations indicate that these fat-like and fat-soluble substances, although occurring only in small quantities, may have very important functions in nutrition. We shall have occasion to study them in that connection later.

The actual glycerides of any common natural fat, with the exception of butter, would if obtained absolutely pure be colorless, tasteless, and odorless. The colors, tastes, and odors of fats are therefore ordinarily due to substances present in small amount which might be removed by refining processes. All of the quantitative differences among the fats are to be accounted for by the kinds and the amounts of the fatty acids which enter into the composition of the glycerides.