Sidebilder
PDF
ePub

this use of the term “principles" is now generally avoided, but there is frequent occasion to use the terms “ ultimate " and “proximate” in speaking of the composition and analysis of foods, and it is well to keep the exact chemical significances of these terms in mind. The word "proximate ” must not be confused with “ approximate.”

Food materials and foodstuffs. The term “ food materials” is synonymous with the expression “ articles of foods.” Thus bread, meat, eggs, milk, are spoken of as food materials. The term “ foodstuffs ” as a scientific term, and as it will be used in this work, means the stuffs that foods are made of, or, in the terms which we have been using, the substances of which the food materials are composed. Thus, the proteins, fats, and carbohydrates and the various organic and inorganic compounds of phosphorus, potassium, iron, etc., which occur in food materials are foodstuffs.

The chemistry and nutritive significance of the foodstuffs, both organic and inorganic, have been discussed by the writer in another volume (Chemistry of Food and Nutrition, Revised Edition) and cannot be considered here in any detail. A brief summary of some of the facts having most relation to what follows in later chapters may, however, be advantageous at this point.

Carbohydrates The carbohydrates include the simple sugars and all the substances which can be split (by hydrolysis) into simple sugars. The simple sugars, having only one sugar radicle in the molecule, are called “monosaccharides.” Sugars whose molecules contain two sugar radicles, and from each molecule of which two molecules of monosaccharide can be obtained by hydrolysis, are called disaccharides. Substances like starch and dextrin which can be hydrolyzed to simple sugars but which are of high molecular weight, each molecule containing many monosaccharide radicles, are called polysaccharides.

The classification which follows includes only a few of the most important carbohydrates. Fuller information may be obtained from the books listed at the end of this chapter. Monosaccharides

Glucose (dextrose, grape sugar, starch sugar)
Fructose (levulose, fruit sugar)

Galactose
Disaccharides

Sucrose (cane sugar, saccharose)
Lactose (milk sugar)

Maltose (malt sugar)
Polysaccharides

Starch
Dextrin
Glycogen
Inulin
Cellulose
Hemicelluloses

Glucose is widely distributed in nature, occurring abundantly in many fruits and plant juices, often mixed with other sugars. Since most of the other carbohydrates yield glucose when split by the digestive ferments the total amount of glucose which is absorbed into the body is much larger than that of any other sugar. Normal blood always contains glucose (usually about 0.1 per cent) which is constantly being burned to yield energy to the body. Any surplus of glucose absorbed from the digestive tract is normally stored in the body in the form of glycogen which later is converted back into glucose as needed to replace that which has been burned. Commercially glucose is made by hydrolysis of starch as explained in Chapter VIII.

Fructose occurs with glucose in plant juices and especially in fruits and honey. It is formed along with an equal weight of glucose when cane sugar is hydrolyzed; hence its occurrence in molasses and sirups as well as honey. (See Chapter XI.)

[ocr errors]

When cane sugar is eaten, it is not absorbed as such, but is changed into equal parts of glucose and fructose in digestion. The fructose absorbed into the body serves the same purposes as glucose and like glucose may be changed into glycogen for storage. Glucose and fructose are the only monosaccharides which occur as such in foods, at least in any important quantity.

Galactose does not occur free in nature or in commercial food products, but as a product of digestion of milk sugar it is of some importance in nutrition. It is utilized like glucose in the body.

Sucrose occurs commonly in the vegetable kingdom, being found in considerable quantity in many familiar fruits and vegetables. Usually these sweet fruits and plant juices contain glucose and fructose along with the sucrose, and also other substances which make it difficult to separate the sucrose in crystalline form. The juices of the sugar cane, the sugar beet, and to a less extent certain maple and palm trees, contain enough sucrose and little enough of other substances to make it practicable to manufacture sugar from them commercially. (See Chapter XI.) On hydrolysis a molecule of sucrose yields one molecule each of glucose and fructose. The process is often called “inversion ” and the product “invert sugar.” When eaten, sucrose is digested into glucose and fructose, the nutritive functions of which have been mentioned above.

Lactose occurs in milk and is made commercially from the whey of milk used in the manufacture of cheese or casein. In the body lactose is digested into equal parts of glucose and galactose, the nutritive functions of which have been noted above. Lactose has special interest for the student of nutrition for at least two reasons. It is not found in the blood or body tissues generally, but is evidently formed only in the mammary gland for secretion in the milk, which suggests its especial importance in the nourishment of the young. It also appears to be unique among the sugars in its property of favoring the development 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

[graphic][graphic][graphic][ocr errors]

Potato starch.
Wheat starch.

Corn starch.
FIG. 3. — Starch granules magnified 300 diameters.

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.

« ForrigeFortsett »