I have already spoken of the formation of dextrin from starch. Dextrin has no particular dietetic qualities that do not exist in starch. It is, in fact, starch arrested at an intermediate point of digestion.
Pectins are a group of gummy substances found in fruits, especially green fruits which are in the process of being formed into sugar. These pectins form the basis of fruit jellies. Green grapes, as every housewife knows, will make better jelly than ripe grapes. This is because the pectins in ripe grapes have been transformed into sugar. The pectins in fruit are in most cases wholesome enough, though it would seem the better part of wisdom to eat all fruits in the ripened state, after Nature has completed her work.
Inulin is a compound closely related to starch, and upon digestion with acids, yields levulose just as starch yields glucose. It is of no particular interest to the food chemist, as it exists in but very small quantities in starch, and has no distinct dietetic value.
[Sidenote: Composition and formation of fats and oils]
The fats and oils in food products, whether of plant or animal origin, contain the elements carbon, hydrogen, and oxygen. These fats are formed by uniting the fatty acids with glycerin, which belongs to the alcohol group. The particular fat that is formed takes its name from the acid which enters into its composition; thus stearic acid unites with glycerin to form the fat stearin.
The following table gives the names of a few of the more common fatty acids and their corresponding fats:
Stearic acid Stearin Palmitic acid. Palmitin Oleic acid Olein Butyric acid Butyrin
[Sidenote: Distinction between tallow and lard]
A fat from any source will usually contain several of these chemical compounds. The ordinary animal fats, such as tallow and lard, are formed chiefly of the two fats stearin and olein. The different proportions of these fats will determine the melting point or hardness of the mixed product. Olein is a liquid at ordinary temperature, while stearin is solid. The reason that tallow is a firmer fat than lard or butter is because it contains a larger per cent of stearin.
Olive-oil, cottonseed-oil, and other vegetable oils contain large per cents of olein, which accounts for their being liquid at ordinary temperature.