Part 107

The method of constructing a bag for attachment to a sheep is shown in Fig. 118. It is made according to the directions given by Gay, of heavy cloth and in such a way as to fit closely the posterior parts of the animal. When attached, its appearance is shown in Fig. 119.

Healthy animals in the prime of life are used, and the feeding experiments are conducted with as large a number of animals as possible, in order to eliminate the effects of idiosyncrasy. The food used is previously prepared in abundant quantity and its composition determined by the analysis of an average sample.

The feeding period is divided into two parts. In the first part the animal is fed for a few days with the selected food until it is certain that all the excreta are derived from the nutrients used. In the second part the same food is continued and the excreta collected, weighed, the moisture determined, and the total weight of the water-free excreta ascertained. The first part should be of at least seven and the second of at least five days duration. The urine and dung are analyzed separately. Males are preferred for the digestion experiments because of the greater ease of collecting the urine and feces without mixing. For ordinary purposes the feces only are collected. The methods of analysis do not differ from those described for the determination of the usual ingredients of a food.

Example.—The following data taken from the results of digestive experiments, obtained at the Maine Station, will illustrate the method of comparing the composition of the food with that of the feces and of determining the degree of digestion which the proteids and other constituents of the food have undergone.

COMPOSITION OF MAIZE FODDER AND OF FECES THEREFROM AFTER FEEDING TO SHEEP.

BEFORE DRYING. Water, Ash, Proteid, Fiber, Fat, Undetermined, per per per per per per Food. cent. cent. cent. cent. cent. cent. Sweet maize 83.85 1.13 2.18 4.14 0.62 8.08 Feces 72.01 ... ... ... ... ...

DRY. Ash, Proteid, Fiber, Fat, Undetermined, per per per per per Food. cent. cent. cent. cent. cent. Sweet maize 7.01 13.52 25.63 3.86 49.98 Feces 14.42 17.52 19.34 2.68 46.04

DAILY WEIGHTS. Green, Dry, Food. grams. grams. Sweet maize 2521 407 Feces 445 125

PER CENT DIGESTED. Food. Ash, Proteid, Fiber, Undetermined, Fat, Sweet maize 37.0 60.2 76.9 71.8 78.3

In the above instance it is seen that the coefficient of digestibility extended from 37.0 per cent in the case of the mineral components of the food, to 78.3 per cent in the case of the fats. These data are taken only from the results obtained from a single sheep and one article of food. The mean data secured from two animals and three kinds of maize fodder show the following per cents of digestibility: Ash 39.4, proteid 61.8, fiber 76.7, undetermined matters 72.1, fat 76.4. The undetermined matters are those usually known as nitrogen free extract and composed chiefly of pentosans and other carbohydrates.

=554. Natural Digestibility of Pentosans.=—The digestibility of pentosan bodies in foods under the influence of natural ferments has been investigated by Lindsey and Holland. The feeding and collection of the feces is carried on as described above and the relative proportions of pentosan bodies in the foods and feces determined by estimating the furfuraldehyd as prescribed in paragraph =150=.

PRESERVED MEATS.

=555. Methods of Examination.=—In general the methods of examination are the same as those applied in the study of fresh meats. The contents of water, salt and other preservatives, fat and nitrogenous matters are of most importance. When not already in a fine state, the preserved meats are run through meat cutters until reduced to a fine pulp. Most potted meats are already in a state of subdivision well suited to analytical work. The composition of preserved meats has been thoroughly studied in this laboratory by Davis.

=556. Estimation Of Fat.=—Attention has already been called to the difficulty of extracting the fat from meats by ether or other solvents. In preserved meats, as well as in fresh, it is preferable to adopt some method which will permit of the decomposition of the other organic matters and the separation of the fat in a free state. The most promising methods are those employed in milk analyses for the solution of nitrogenous matters. Sulfuric or hydrochloric acid may be used for this purpose, preference being given to sulfuric. The separated fats may be taken up with ether or separated by centrifugal action. A method of this kind for preserved meats, suggested by Hefelmann, is described below.

About six grams of the moist preserved meat are placed in a calibrated test tube and dissolved in twenty-five cubic centimeters of fuming hydrochloric acid. The tube is placed in a water bath, quickly heated to boiling and kept at that temperature for half an hour. About twenty cubic centimeters of cold water are added and the temperature lowered to 30°, then twenty cubic centimeters of ether and the tube gently shaken to promote the solution of the fat. When the ether layer has separated, its volume is read and an aliquot part removed by means of a pipette, dried and weighed. The separation of the ethereal solution is greatly promoted by whirling.

The mean proportions of the ingredients of preserved meats are about as follows:

Per cent.

Water 67.0 Dry matter 33.0

Of which

Nitrogenous bodies 19.0 Fats 10.5 Ash and undetermined 3.5

=557. Meat Preservatives.=—Various bodies are used to give taste and color to preserved meats and to preserve them from fermentation. The most important of these bodies are common salt, potassium and sodium nitrates, sulfurous, boric, benzoic and salicylic acids, formaldehyd, saccharin and hydronaphthol. A thorough study of the methods of detecting and isolating these bodies has been made in this laboratory by Davis and the results are yet to be published as a part of Bulletin 13.

DETERMINATION OF NUTRITIVE VALUES.

=558. Nutritive Value of Foods.=—The value of a food as a nutrient depends on the amount of heat it gives on combustion in the tissues of the body, i. e. oxidation, and in its fitness to nourish the tissues of the body, to promote growth and repair waste. The foods which supply heat to the body are organic in their nature and are typically represented by fats and carbohydrates. The foods which promote growth and supply waste are not only those which preeminently supply heat, but also include the inorganic bodies and organic nitrogenous matters represented typically by the proteids. It is not proper to say that one class of food is definitely devoted to heat forming and another to tissue building, inasmuch as the same substance may play an important rôle in both directions. As heat formers, carbohydrates and proteids have an almost equal value, as measured by combustion in oxygen, while fat has a double value for this purpose. The assumption that combustion in oxygen forms a just criterion for determining the value of a food must not be taken too literally. There are only a few bodies of the vast number which burn in oxygen that are capable of assimilation and oxidation by the animal organism. Only those parts of the food that become soluble and assimilable under the action of the digestive ferments, take part in nutrition and the percentage of food materials digested varies within wide limits but rarely approaches 100. It may be safely said that less than two-thirds of the total food materials ingested are dissolved, absorbed, decomposed and assimilated in the animal system. We have no means of knowing how far the decomposition (oxidation) extends before assimilation, and therefore no theoretical means of calculating the quantity of heat which is produced during the progress of digestion. The vital thermostat is far more delicate than any mechanical contrivance for regulating temperature and the quantity of food, in a state of health, converted into heat, is just sufficient to maintain the temperature of the body at a normal degree. Any excess of heat produced, as by violent muscular exertion, is dissipated through the lungs, the perspiration and other secretions of the body.

Pure cellulose or undigestible fiber, when burned in oxygen, will give a thermal value approximating that of sugar, but no illustration is required to show that when taken into the system the bodily heat afforded by it is insignificant in quantity.

Thermal values, therefore, have little comparative usefulness in determining nutritive worth, except when applied to foods of approximately the same digestive coefficient.

=559. Comparative Value of Food Constituents.=—It has already been noted that, judged by combustion in oxygen, carbohydrates and proteids have about half the thermal value possessed by fats. Commercially, the values of foods depend in a far greater degree on their flavor and cooking qualities than upon the amount of nutrition they contain. Butter fat, which is scarcely more nutritious than tallow, is worth twice as much in the market, while the prices paid for vegetables and fruits are not based to any great extent on their food properties. In cereals, especially in wheat, the quantity of fat is relatively small, and starch is the preponderating element. In meats, carbohydrates are practically eliminated and fats and proteids are the predominating constituents.

In the markets, fats and proteids command far higher prices than sugars and starches. The relative commercial food value of a cereal may be roughly approximated by multiplying the percentages of fat and protein by two and a half and adding the products to the percentage of carbohydrates less insoluble fiber. This method was adopted in valuing the cereals at the World’s Columbian Exposition.

=560. Nutritive Ratio.=—In solid foods the nutritive ratio is that existing between the percentage of proteids and that of carbohydrates, increased by multiplying the fat by two and a half and adding the product. In a cereal containing twelve per cent of protein, seventy-two of carbohydrates, exclusive of fiber, and three of fat, the ratio is 12: 72 + 3 × 2.5 = 6.5. Instead of calculating the nutritive ratio directly from the data obtained by analysis, it may be reckoned from the per cents of the three substances in the sample multiplied by their digestive coefficient. Since the relative amounts of proteids, fats and carbohydrates digested do not greatly differ, the numerical expression of the nutritive ratio is nearly the same when obtained by each of these methods of calculation.