The polysaccharides are all colloids insoluble in alcohol. Some "dissolve" in water in the sense that they form colloidal dispersions which will pass through filter paper; some swell and become gelatinous; some are unchanged. The members of greatest importance in nutrition are starch and glycogen, the typical reserve carbohydrates of plants and animals respectively.

Pentosans, (C5H8O4)x, occur in the greatest variety of plants and in various parts of the plant organism. As a rule, however, they are abundant only in the fibrous tissues and gummy exudations and not in the starchy and succulent parts which are more commonly used for human food. Moreover experiments have not yet succeeded in demonstrating in man or other mammals any enzyme capable of digesting the pentosans (Swartz). It is therefore believed that, notwithstanding their wide distribution in plants, the pentosans can play only a very small, if appreciable, part in the nutrition of man.

Starch, (C6H10O5)x, is the form in which most plants store the greatest part of their carbohydrates, and is of great importance as a constituent of many food materials and as the source of dextrin, maltose, commercial glucose, and many fermentation products. Starch is found stored in the seeds, roots, tubers, bulbs, and sometimes in the stems and leaves of plants. It constitutes one half to three fourths of the solid matter of the ordinary cereal grains and at least three fourths of the solids of mature potatoes.

Unripe apples and bananas contain much starch which is to a large extent changed into sugars as these fruits ripen, while, on the other hand, young tender corn (maize) kernels and peas contain sugar which is transformed into starch as these seeds mature.

Unchanged starch occurs in distinct granules, and those formed in different plants vary in size and structure,* so that in most cases the source of a starch which has not been altered by heat, reagents, or ferments can be determined by microscopical examination. Starch granules are scarcely affected by cold water; on warming they absorb water and swell. Finally the starch passes into a condition of colloidal dispersion or semi-solution, "starch paste" Starch which has been heated in water (either admixed or naturally present with the starch as in a potato) until the granules are ruptured and the material more or less dispersed is very much more rapidly hydrolyzed by digestive ferments than is raw starch.

To colloids such as starch, the usual methods of determining molecular weight are not applicable. It is certain, however, from the chemical complexity of some of the dextrins which result from hydrolysis of starch, that the molecular weight of starch must be very high and its chemical constitution very complex. Probably the value of x in the formula (C6H10O5)x is very large, perhaps in the neighborhood of 200, corresponding to a molecular weight of about 32,000. For a full discussion of the more important facts bearing on the chemical constitution of starch, see the paper by Thomas cited in the list of references at the end of the chapter.

* A very detailed study of the starch granules of different species of plants has been made by Reichert and published by the Carnegie Institution of Washington. (See references at end of chapter).

Starch either in the solid or in the "soluble" (dispersed) form is colored intensely blue when treated with iodine. This well-known reaction is delicate and distinctive, but is now believed to be due to colloidal adsorption rather than to the formation of a definite chemical compound.

The term "starch," as we ordinarily use it, probably covers at least two substances. The more abundant of these, α-amylose (also called "amylopectin"), forms on heating in water a viscous opalescent paste, gives a somewhat purplish blue color with iodine, is evidently of great molecular complexity, and has recently been found to contain a small amount of phosphorus * as an essential constituent. The less abundant component of starch, ß-amylose (also called "amylose"), forms when heated in water a clear, limpid solution which gives a pure blue color with iodine. The starch-digesting enzymes hydrolyze both α-amylose and ß-amylose, but not always with equal facility.†

Starch on hydrolysis by means of acid gives first mixtures of dextrin and maltose, and finally glucose only as an end-product. The most satisfactory hydrolysis of starch to glucose is accomplished by boiling or heating in a boiling water bath with hydrochloric acid of a concentration of about 2.5 per cent. When brought in contact with saliva, starch is hydrolyzed by the ptyalin, with the formation of dextrin and maltose. A similar hydrolysis is affected by "amylopsin," the starch-splitting enzyme of the pancreatic juice, preferably known as pancreatic amylase (see terminology of enzymes, Chapter IV (Enzymes And Digestion)).

* In the case of potato starch about 0.06 per cent. See papers by Northrup and Nelson and by Thomas referred to at the end of the chapter.

† See paper by Sherman and Baker referred to at the end of the chapter.

"Soluble starch," largely used for laboratory experiments, is usually prepared by soaking raw starch in cold hydrochloric acid (about 7 per cent HC1) for several days, and then washing with cold water.

Dextrins, (C6H10O5)x or (C6H10O5)xH2O, are formed from starch by the action of enzymes, acids, or heat. Small amounts of dextrin are found in normal, and larger amounts in germinating, cereals. Malt diastase, acting for some time upon starch in fairly concentrated solution, yields usually about one part of dextrin to four of maltose. During acid hydrolysis, dextrin is formed as an intermediate product between soluble starch and maltose. Commercial dextrin, the principal constituent of "British gum," is obtained by heating starch, either alone or with a small amount of dilute acid.

The dextrins are much more soluble than the starches; and dextrin molecules while doubtless very large and complex are probably not over one fifth the size of starch molecules.