The three disaccharides here considered are di-hexoses or hexo-bioses of the formula C12H22O11, and are crystallizable and diffusible. Sucrose crystallizes anhydrous; maltose and lactose, each with one molecule of water, which can be removed by drying at temperatures of 100° and 1300 respectively. They are soluble in water; less soluble in alcohol. Lactose is much less soluble than sucrose and maltose. These disaccharides are important constituents of food and are changed to monosaccharides during the process of digestion.

Sucrose (saccharose, cane sugar) is widely distributed in the vegetable kingdom, being found in considerable quantity, generally mixed with glucose and fructose, in the fruits and juices of many plants. The commercially important sources of sucrose are the sugar beet, the sugar and sorghum canes, the sugar palm, and the sugar maple; but many of the common fruits and vegetables contain notable amounts. For example, sucrose is said to constitute at least half the solid matter of pineapples and of some roots such as carrots.

On hydrolysis each molecule of sucrose yields one molecule each of glucose and fructose. These sugars all rotate the plane of vibration of polarized light, sucrose and glucose to the right (+), and fructose to the left (-). The terms "dextrose" and "levulose," synonyms for glucose and fructose respectively, arose from this behavior of the sugars in rotating the plane of polarized light to the right and left. Since at ordinary temperatures the fructose rotates more strongly to the left than the glucose does to the right, the result of the hydrolysis of sucrose is to change the sign of rotation from + to -. For this reason the hydrolysis of cane sugar is often called " inversion," and the resulting mixture of equal parts glucose and fructose is known as "invert sugar."

Sucrose is very easily hydrolyzed either by acid or by the sucrase ("invertase" or "inverting" enzyme) of yeast or of intestinal juice. So far as known neither the saliva nor the gastric juice contains any enzyme capable of hydrolyzing cane sugar, and the slight amount of hydrolysis which takes place in the stomach is believed to be due simply to the presence of hydrochloric acid. Under normal conditions the sucrose of the food passes mainly into the intestine unchanged and is there split by the sucrase of the intestinal juice, and the resulting glucose and fructose are absorbed into the portal blood.

When large amounts of sucrose are fed, some absorption takes place in the stomach; but the unchanged sucrose thus absorbed appears to be largely, if not wholly, lost through the kidneys, as it is when injected directly into the blood current. Sugar eaten in concentrated form or in considerable quantities at a time is apt to cause irritation of the stomach either directly, or as the result of undergoing an acid fermentation, or in both of these ways. According to Herter sucrose and glucose are more likely to ferment in the stomach than is lactose. In cases where fermentation does not occur and the sucrose itself has no irritating effect, it may be especially useful as a rapidly available foodstuff. However, it is not known that sucrose has any advantage over maltose and lactose in this respect, and the latter are less apt to irritate the stomach and cause indigestion.

Lactose (milk sugar) occurs in the milk of all mammals, constituting usually from 6 to 7 per cent of the fresh secretion in human milk and 4.5 to 5 per cent in the milk of cows and goats. At the time of parturition, or if the milk is not withdrawn from the udder, some lactose may occur in the urine. If in such a case the mammary glands are removed, the percentage of glucose in the blood increases, and glucose (but no lactose) may appear in the urine (Abderhalden). These observations indicate that lactose is formed in the mammary gland and probably from the glucose brought by the blood.

Lactose is less sweet and much less soluble than sucrose, dissolving only to the extent of about 1 part in 6 parts of water.

When hydrolyzed either by heating with acids or by an enzyme, such as emulsin or the lactase of the intestinal juice, each molecule of lactose yields one molecule of glucose and one of galactose. In normal digestion, probably none of the lactose eaten is absorbed as such, for lactose injected into the blood is eliminated quickly and almost completely through the kidneys, whereas large amounts of lactose can be taken by the mouth without any such loss. As already noted, Herter found lactose to be less subject to fermentation in the stomach than is sucrose. Also, because of the much lower solubility, there is less danger of direct irritation of the stomach membrane by lactose than by sucrose. Recently Mathews has suggested that the occurrence in milk of lactose, a sugar having the galactose radicle, may be of special significance as a source of material for the synthesis of the galactosides of the brain and nerve tissues of the rapidly growing young mammal.

Maltose (malt sugar) is formed from starch by the action of diastatic enzymes (amylases) and is therefore an important constituent of germinating cereals, malt, and malt products. It is also formed as an intermediate product when starch is hydrolyzed by boiling with dilute mineral acids, as in the manufacture of commercial glucose.

In animal digestion maltose is formed by the action of the ptyalin of the saliva or the amylopsin of the pancreatic juice upon starch or dextrin. The maltose-splitting enzyme of the intestinal juice readily hydrolyzes maltose to glucose. Maltose is also readily and completely hydrolyzed by boiling with dilute mineral acids. In either case each molecule of maltose yields two molecules of glucose.

While it is probable that little if any maltose is absorbed as such from the digestive tract under ordinary conditions, it is possible that such absorption may occur and that maltose as such may play a part in the normal carbohydrate metabolism; for when injected into the blood it appears to be utilized to better advantage than either sucrose or lactose, and it may be obtained from glycogen by the action of diastatic enzymes in much the same way as from starch and dextrin.