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.[572]
556. Estimation Of Fat.—Attention has already been called to the difficulty of extracting the fat from meats by ether or other solvents.[573] 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.