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.[574] 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.[575]
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.
Where the proportion of protein is relatively large the ratio is called narrow, 1: 4 ... 6. When the proportion of protein is relatively small the ratio is called broad 1: 8 ... 12. In feeding, the nutritive ratio is varied in harmony with the purpose in view, a narrow ratio favoring the development of muscular energy, and a wide one promoting the deposition of fat and the development of heat. These principles guide the scientific farmer in mixing rations for his stock, the work horses receiving a comparatively narrow and the beeves a relatively wide ratio in their food.
561. Calorimetric Analyses of Foods.—The general principles of calorimetry have been already noticed. The theoretical and chemical relations of calorimetry have been fully discussed by Berthelot, Thomsen, Ostwald and Muir.[576] In the analyses of foods the values as determined by calculation or combustion are of importance in determining the nutritive relations.
Atwater has presented a résumé of the history and importance of the calorimetric investigations of foods to which the analyst is referred.[577]
In the computation of food values the percentages of proteids, carbohydrates and fats are determined and the required data obtained by applying the factors 4100, 5500 and 9300 calories for one gram of carbohydrates, proteids and fats respectively.
For most purposes the computed values are sufficient, but it is well to check them from time to time by actual combustions in a calorimeter.
562. Combustion in Oxygen.—The author made a series of combustions of carbonaceous materials in oxygen at the laboratory of Purdue University in 1877, the ignition being secured by a platinum wire rendered incandescent by the electric current. The data obtained were unsatisfactory on account of the crudeness of the apparatus. The discovery of the process of burning the samples in oxygen at a high pressure has made it possible to get expressions of thermal data which while not yet perfect, possess a working degree of accuracy. The best form of bomb calorimeter heretofore employed is that of Hempel, as modified by Atwater and Woods.[578]