~Maltose~ (Malt sugar) is an important constituent of germinating grains--malt and malt products being formed as the result of enzymic action (amylases) on starch. A similar action takes place in the mouth as the result of the ptyalin in the salivary juices and in the intestines from the action of the starch-splitting enzyme, amylopsin, in the pancreatic juice. The maltose thus formed is further converted into glucose by the sugar-splitting enzyme in the intestinal juice, and in this form it is chiefly absorbed. Maltose is also an intermediate product formed during the manufacture of commercial glucose as the result of the boiling of starch with dilute acids.
~Lactose~ (sugar of milk) is one of the most important constituents in the milk of all mammals. In freshly secreted human milk, lactose occurs in quantities ranging from 6 to 7%, and in the milk of cows and goats from 4 to 5%. Lactose is much less soluble than sucrose, and decidedly less sweet; hence, owing to this latter property, as well as to its lack of susceptibility to fermentation, lactose is frequently used to bring up the sugar content of infant formulas to the desired percentage, and the diets used in the abnormal conditions when additional energy material is needed. During the process of digestion, lactose is hydrolized by the lactase in the intestinal juice, yielding one molecule of glucose and one of galactose. Like maltose, little if any of this sugar is absorbed in its original form, since experiments made with injections of lactose into the blood result in the rapid and almost complete elimination by way of the kidneys. No such results are obtained when even large amounts of lactose are taken by way of the mouth.
~Polysaccharides.~--This group of carbohydrates is complex in character, built up of many sugar molecules, and upon digestion must be broken down into simple sugars before they can be utilized by the body.
~Starch~ is the form in which the plant stores her supply of carbohydrates. It is found in this form in roots and (mature) tubers, three-fourths of the bulk of which is made up of this material. From one-half to three-quarters of the solids of grains is made up of starch also. Pure starch is a fine white powder, odorless and almost tasteless. It is insoluble in cold water and alcohol, but changes from an insoluble substance to a more soluble one upon the application of heat. Upon hydrolysis starch gives first a mixture of dextrin and maltose, then glucose alone as an end-product. This hydrolysis may be the result of enzymic action, as occurs upon bringing starch in contact with the ptyalin in the saliva, or with the amylopsin in the pancreatic juice; or it may be the result of boiling starch with acid, as is seen in the manufacture of commercial glucose.