The methods of analyses followed by Atwater and Woods are given below:[544]
Water and Water-Free Substance.—The drying is done in ordinary water ovens at a temperature of nominally 100°, but actually at 96° and 98°. For each analysis of animal tissues (flesh) one or more samples of from fifty to one hundred grams of the freshly chopped substance are weighed on a small plate, heated for from twenty-four to forty-eight hours, cooled, allowed to stand in the open air for about twenty-four hours, weighed, ground, sifted through a sieve with circular holes one-half millimeter in diameter, bottled and set aside for analysis. In case of fat samples which cannot be worked through so fine a sieve, either a coarser sieve is used or the substance crushed as finely as practicable and bottled without sifting.
For the complete desiccation, about two grams of material are dried for three hours. It is extremely difficult to get an absolutely constant weight, though it is found that this is in most cases approximately attained in four hours.
Nitrogen, Protein, Albuminoids etc.—The nitrogen is determined in the partly dried substance by the method of Kjeldahl. The protein is calculated by multiplying the percentage of nitrogen by 6.25. The nitrogenous matters in meats and fish, i. e., in the materials which have practically no carbohydrates, are also estimated by subtracting the sum of ether extract and ash from the water-free substance, or the sum of water, ether extract and ash from the fresh substance, the remainder being taken as proteids, albuminoids etc., by difference. While this is not an absolutely correct measure of the total nitrogenous matter, it is doubtless more nearly so than the product of the nitrogen multiplied by 6.25.
Fat (Ether Extract).—The fat is extracted with ether in the usual manner. The point at which the extraction is complete is not always easy to determine. For the most part, the extraction is continued for such time as experience indicates to be sufficient, and then the flask is replaced by another and the extraction repeated until the new flask shows no increase in weight.
According to experience, the fat of many animal tissues is much more difficult to extract than that of most vegetable substances. In general, the greater the percentage of fat in a substance the more difficult is the removal of the last traces. Dried flesh is frequently so hard that the fineness of the material to be extracted seems to be a very important matter.
Ash.—Ash is determined by the method recommended by the Association of Official Agricultural Chemists.
Food Value—Potential Energy.—The food materials are not necessarily burned in the calorimeter, but the fuel value of a pound of each of the foods, as given in the tables, is obtained by multiplying the number of hundredths of a pound of protein and of carbohydrates by 18.6 and the number of hundredths of a pound of fat by 42.2, and taking the sum of these three products as the number of calories of potential energy in the materials.
More reliable results are obtained by using the factors obtained by Stohmann; viz., 5731 calories for proteids, 9500 calories for common glycerids, 9231 calories for butter fat, 3746 calories for pentose sugars, 3749 calories for dextrose and levulose and 3953 calories for sucrose and milk sugar.[545]
540. Further Examination of Nitrogenous Bodies.—It is evident that both of the methods proposed above for the examination of the nitrogenous constituents of meats are unreliable. If the total nitrogen be determined and multiplied by 6.25 the product does not by any means represent the true quantity of nitrogenous matter since the flesh bases contain in some instances more than twenty-five per cent of nitrogen.