These carbohydrates are all sugars which may be decomposed into two molecules of a simple sugar, or one molecule of each of two simple sugars. They are only three in number, - saccharose or sucrose (cane sugar), maltose (malt sugar), and lactose (milk sugar). When acted upon by weak acids or certain ferments, they break by cleavage (hydrolysis) as follows:

Saccharose + water = dextrose + levulose Maltose + water = dextrose + dextrose Lactose + water = dextrose + galactose

These are the changes that occur during the digestion of food.

72. Saccharose

The most important of these, commercially considered, is saccharose, which is the ordinary crystallized sugar of the markets. As a human food it is widely used, is especially valuable, and its manufacture and sale constitute a prominent industry. This sugar is obtained mostly from two plants, sugar cane and the sugar beet. It also exists abundantly in sorghum, pineapples, carrots, and in considerable proportions in the stalk of ordinary field corn. The first spring flow of sap in one species of maple tree is richly charged with it, and in a few states large quantities of maple sirup and sugar are manufactured.

Saccharose is not a prominent constituent of unmodified human foods. While it occurs in sweet-potatoes and in roots, and perhaps in minute proportions in certain seeds, it is only in the fresh corn plant, sorghum, pineapples, and sugar beets that it constitutes a material part of the food substance.

The fruits generally contain saccharose, mixed with other sugars and organic acids, and upon the relative proportions of these compounds depends the character of the fruit as to acidity or sweetness.

73. Maltose

A sugar that is intimately related to the first growth which occurs in the germination of seeds is maltose, for it stands as an intermediate product between the store of starch in the seed and the new tissues of the sprout. The solution that the brewer extracts from the malted grains contains this compound as the principal ingredient, and through succeeding fermentations in the beer vats it is broken up into alcohol and other compounds. It sustains an important relation, therefore, to the production of beers and other alcoholic liquors. The glucose sirups found in the markets sometimes contain small quantities of this sugar. It is also found abundantly in the intestinal canal during the digestion of food, being derived from starch and other carbohydrates through the action of ferments. Maltose is similar to cane sugar in ultimate composition, but not in constitution, though as a nutrient it evidently has an equivalent value. So far as known, however, it does not appear to occur in material quantities in foods.

74. Lactose

The only sugar of animal origin which is abundant in farm life is the lactose that is found in milk and which is known in commerce as milk sugar. The milk of all mammals contains sugar, which appears to be the same compound with every species so far investigated.

When they are fed wholly from the mother, this is the only carbohydrate which young mammals receive in their food. The average proportion of sugar in the milk of domestic animals varies from three to six parts in a hundred, cow's milk containing about five parts. When the cream is removed, much the larger part of sugar remains in the skimmed milk, and in cheese-making it is nearly all found in the whey, from which the milk sugar of commerce is obtained. Very soon after milk is drawn, unless it is heated to the point of sterilization, or is treated with some antiseptic, the lactose begins to diminish in quantity, being converted into lactic acid through the action of what is known as lactic acid organisms (bacteria). Sour milk, therefore, is different from sweet in at least one compound, and this change causes at least a slight modification of food value.