Cellulose, from the standpoint of human nutrition, is not a food product, being insoluble by the digestive juices, but it is very important in the digestion and the alimentation of other foods. Its chief purpose is to excite stomach and intestinal peristalsis. All plant products in their natural form contain some cellulose, though the percentage is very small in such grains as rice and barley. The bran of wheat or of corn is chiefly cellulose. Wood is almost pure cellulose. Cellulose can be digested by strong acids into simple carbohydrates, in the same way that starch may be. Sugar can be manufactured from wood or rags, but the process is yet too expensive to be applied commercially. Some of us may live to see the time when the chief food of mankind will be manufactured from scrap lumber and waste paper. Bacteria have the power of digesting cellulose. The bacterial action or fermentation in the human intestines may cause a small amount of cellulose to be digested, but the quantity is of no consequence from a nutritive point of view. 4 GUMS The gums include a group of rather complex carbohydrates which are intermediate between starches and sugars. From plants are derived many varieties of gums which have various commercial uses in the market, such as gum arabic. 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. [Sidenote: Pectins in fruits] 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. 5 INULIN 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. FATS AND OILS [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: