of the most desirable species of bacteria in the intestine. See brief discussions in Chapters III and IV and sources of fuller information among the references at the end of the latter chapter.

Maltose occurs in malted or germinated grains, in malt extracts, etc., but the amount of maltose eaten as such is not likely to be large. It is formed in quantity by the digestion of starch by the saliva or the pancreatic juice. Maltose, however, whether eaten or formed in the course of digestion, is not absorbed as such to any important extent, but is split by a digestive ferment of the intestinal juice, each molecule of maltose yielding two molecules of glucose.

Starch is the chief form in which most plants store their reserve supply of carbohydrate material. It constitutes over one half of the solid matter of the cereal grains and an even larger proportion of the total solids of some other starchy foods such as potatoes, bananas, and chestnuts. In the processes of digestion, starch (especially when it has been cooked) is changed to maltose and the latter (as stated above) into glucose. In addition to the direct use of starchy materials as food, much starch is separated on an industrial scale (Chapter VIII) and used as such or as a source of dextrin, maltose, commercial glucose, or fermentation products.

Raw starch is easily seen under the microscope to consist of distinct granules, the size and shape of which differ greatly in the starches formed in different types of plants. Figure 3 represents starch granules from potato, wheat, and corn (maize), all magnified in the same proportion.

Dextrins are formed from starch by the action of ferments, acids, or heat. Although usually represented by the same empirical formula as starch, the dextrins appear in general as intermediary products in the hydrolysis of starch to maltose or glucose; hence no further discussion is required here.

Glycogen is the chief reserve form of carbohydrate in animals as starch is in plants. For this reason and because of its physi

cal properties and its chemical relationship to maltose and glucose, it is often called " animal starch." It is stored principally in the liver and to a small extent in the muscles.

Inulin is a substance, found in a few vegetables, which on hydrolysis yields fructose. It is of little practical importance as food. Cellulose is familiar as a woody or fibrous material occurring in the cell walls of all vegetable tissues. It yields glucose on hydrolysis, but is not digested to a sufficient extent to make it

of much nutritive value to man, though it is often of value in giving proper bulk to the diet.

Hemicelluloses are substances which botanically seem to resemble cellulose in belonging to the walls rather than to the contents of plant cells. Chemically they resemble starch in being rather easily hydrolyzed by acids, but differ from it in that they often yield sugars other than glucose. Thus in any given plant tissue the material called hemicellulose may consist chiefly of galactan yielding galactose, mannan yielding mannose, or pentosan yielding one or both of the pentose sugars xylose and arabinose. These hemicelluloses do not appear to be digested by any of our digestive juices and are therefore presumably of little practical importance as food.

Carbohydrates in nutrition. From what has been said above it will be clear that the various digestible carbohydrates of the food, having been split by the digestive ferments to monosaccharides, are absorbed into the blood. Any surplus is stored temporarily in the form of glycogen, chiefly in the liver, though to some extent in the muscles. The glucose which circulates in the blood is burned in the muscles and other active tissues as fuel, the burned glucose being constantly replaced by new glucose derived from the stored glycogen, so that under ordinary conditions the carbohydrate of the food is entirely burned as fuel. When more carbohydrate is received than is burned, the surplus is stored as glycogen, but only to a limited extent, the total amount of glycogen which the human body can store being estimated at less than one pound or only about as much carbohydrate as might be contained in the food of one day. A surplus of carbohydrate, in addition to being stored as glycogen, may also be converted into fat, and this transformation of carbohydrate into fat can be carried on to a very large extent and with almost no loss of energy. The energy value to the body of average carbohydrate in the food is 4.0 Calories per gram, or 1814 Calories per pound.

Organic Acids

Some foods contain considerable quantities of organic acids or their salts. Oranges and lemons, for instance, are rich in citric acid; grapes contain potassium acid tartrate; apples and other fruits contain malic acid; and many fruits contain succinic acid. Fermented foods may contain appreciable quantities of lactic acid as in sauerkraut and sour milk, buttermilk, etc., or acetic acid as in vinegar. A few foods contain oxalic acid or oxalates, but these are probably of little if any food value and may be injurious.

With the exception of oxalic acid, these organic acids appear normally to be burned in the body, and doubtless their energy is used in practically the same way as the energy of the carbohydrates. The fuel values of some of these acids have been determined as follows: acetic acid, 3.5 Calories per gram; citric acid, 2.5 Calories per gram; lactic acid, 3.7

Calories per gram; succinic acid, 3.0 Calories per gram; tartaric acid, 1.7 Calories per gram. While these values are somewhat lower than those of the carbohydrates, it is not uncommon in reckoning the fuel value of a food to count the organic acid as carbohydrate, especially as in routine analyses the acids are often not sought nor are the carbohydrates determined directly, but all of the material not found to be moisture, protein, fat, or ash is often considered to be carbohydrate for the purposes of ordinary estimations of food values.

Fats

The fats are all glycerides; that is, substances consisting of combinations of glycerol (commercially called "glycerin ") with fatty acids. Many of these fatty acids belong chemically to the same series with acetic acid. The chief members of this series occurring naturally in fats are butyric acid, C4H8O2; caproic acid, C6H12O2; caprylic acid, C8H16O2; capric acid, C10H2002; lauric acid, C12H24O2; myristic acid, C14H28O2; palmitic acid, C16H3202; stearic acid, C18H36O2.

Butyric acid is a liquid which mixes in all proportions with water, alcohol, and ether, can be boiled without decomposition, and is readily volatile in steam.

With increasing molecular weight, the acids of this series regularly show increasing boiling or melting points, decreasing solubility, and become less volatile. Those up to capric acid are liquids at ordinary temperatures; those above are solids. The higher the molecular weight, the harder the solid. Stearic acid is a hard paraffin-like crystalline solid insoluble in water and only moderately soluble in alcohol and ether.

The properties of the fats themselves depend upon and run parallel with those of the fatty acids.

In addition to the fatty acids of the series to which acetic, butyric, and stearic acids belong, all of which are saturated compounds, there are several unsaturated fatty acids, capable of combining chemically with hydrogen, oxygen, or halogens by direct addition. The most important of these contain eighteen

carbon atoms to the molecule and therefore resemble stearic acid in molecular size.

The most important of these unsaturated fatty acids are: oleic acid, C18H3402; linoleic acid, C18H32O2; linolenic acid, C18H3002. All of these acids and their glycerides are liquid at ordinary temperatures. Commercial fats consisting mainly of the glycerides of these acids are therefore liquids and are usually called oils. The chief chemical difference between olive oil and lard is that the former contains more olein (glyceride of oleic acid) and the latter more of palmitin and stearin (glycerides of palmitic and stearic acids). Olein or linolein (glyceride of linoleic acid) may be converted into stearin by direct chemical union with hydrogen, and this is now done on a commercial scale for the hardening of fatty oils so as to give them the consistency of lard (Chapter X).

The body fat of man and of the animals commonly used as food consists of glycerides of palmitic, stearic, and oleic acids. Since palmitin and stearin are solids, while olein is a liquid, the hardness or softness of these fats is principally due to the proportion of olein which they contain. Butter fat contains all of the fatty acids listed above in the series from butyric to stearic acid and is distinguished from the other food fats principally by this fact. Olive oil consists chiefly of palmitin, stearin, and olein, but contains much more olein and much less stearin than the ordinary solid fats. In cottonseed oil, sesame oil, and other seed oils used as food, the quantities of palmitin and stearin are still smaller and, in addition to large quantities of olein, considerable quantities of linolein and in some cases even linolenin may occur.

"Simple" and "mixed" glycerides. For convenience we speak as though the oleic acid radicles in a fat were present simply in the form of olein; the stearic in the form of stearin; the palmitic as palmitin; etc. As a matter of fact this may be true, and glycerides which contain only one kind of fatty acid