PROTEID BODIES IN MILK.
476. Kinds of Proteid Bodies in Milk.—The proteid bodies in milk are all found in at least partial solution. Some authorities state that a portion of the casein is present in the form of fine particles suspended after the manner of the fat globules.[472] The number and kind of proteid bodies are not known with definiteness. Among those which are known with certainty are casein, albumin, peptone and fibrin. The latter body was discovered in milk by Babcock.[473] Lactoglobulin and lactoprotein are also names given to imperfectly known proteid bodies in milk. Lactoprotein is not precipitated either by acids or by heat and is therefore probably a peptone. By far the greater part of the proteid matters in milk is casein. Casein has been called caseinogen by Halliburton,[474] and paracasein by Schulze and Röse.[475] Casein has intimate relations to the mineral matters in milk, and is probably itself made up of several proteid bodies of slightly differing properties. In general all that class of proteid matter contained in milk which is precipitated by rennet or a weak acid, or spontaneously on the development of lactic acid, is designated by the term casein, while the albumins and peptones in similar conditions remain in solution. Casein contains phosphorus, presumably as nuclein. Fibrin is recognized in milk by the reactions it gives with hydrogen peroxid or gum guiacum. The decomposition of hydrogen peroxid is not a certain test for fibrin, inasmuch as pus and many other bodies will produce the same effect. If the milk decompose hydrogen peroxid, however, before and not after boiling, an additional proof of the presence of fibrin is obtained, since boiled fibrin does not act on the reagent.[476] The gum guiacum test is applied by dipping a strip of filter paper into the milk and drying. The solution of gum guiacum is applied to the dried paper and the presence of fibrin is recognized by the blue color which is produced. The fibrin is probably changed into some other proteid during the ripening of cream in which the fibrin is chiefly found. The albumin in milk is coagulated by boiling, while the casein remains practically unaffected when subjected to that temperature.
477. Estimation of Total Proteid Matter.—The total proteid matter in milk is determined by any of the general methods applicable to the estimation of total nitrogen, but the moist combustion method is by far the most convenient. From the total nitrogen, that which represents ammonia or other nonproteid nitrogenous bodies, is to be deducted and the remainder multiplied by an appropriate factor. Practically all the nitrogen obtained is derived from the proteid matters and, as a rule, no correction is necessary. The factors employed for calculating the weight of proteid matter from the nitrogen obtained vary from 6.25 to 7.04. It is desirable that additional investigations be made to determine the magnitude of this factor. It is suggested that provisionally the factor 6.40 be used. In the method adopted by the Association of Official Agricultural Chemists it is directed that about five grams of milk be placed in the oxidizing flask and treated without previous evaporation exactly as described for the estimation of total nitrogen in the absence of nitrates. The nitrogen obtained is multiplied by 6.25 to get the total proteid matter.[477] In order to prevent the too great dilution of the sulfuric acid, the milk may be evaporated to dryness or nearly so before oxidation. In this laboratory it is conveniently done by placing the milk first in the oxidizing flask, connecting this with the vacuum service and placing the flask in hot water. The aqueous contents of the milk are quickly given off at a temperature not exceeding 85°, and the time required is only a few minutes.
The milk may also be dried in dishes made of thin glass or tin foil and, after desiccation, introduced with the fragments of the dishes into the oxidizing flask.
The preliminary drying in the oxidizing flask is recommended as the best.
Söldner oxidizes the nitrogen in human milk by boiling ten cubic centimeters thereof for three hours with twenty-five of sulfuric acid, fifty milligrams of copper oxid and three drops of a four per cent platinic chlorid solution, and, after distilling the ammonia, uses the factor 6.39 for calculating the proteid matter. According to this author human milk is much less rich in nitrogenous constituents than is generally supposed, containing not more than one and a half per cent thereof in average samples collected at least a month after parturition.[478]
478. Precipitation of Total Proteids with Copper Sulfate.—This method of throwing out the total proteids of milk is due to Ritthausen.[479] The proteids and fat are precipitated together by the addition of a measured volume of copper sulfate solution, containing 63.92 grams of the crystallized salt in one liter. The process, as modified by Pfeiffer, is conducted as follows:[480]
Ten grams of milk are diluted with ten times that much water, five cubic centimeters of the copper sulfate solution added and then soda lye solution drop by drop until the copper is just precipitated. This is determined by testing a few drops of the filtrate with soda lye, which, when the copper is precipitated, will give neither a turbidity nor a blue color.
The mixture is poured into a dry tared filter, the precipitate washed with hot water, dried to constant weight and weighed. The fat is removed from the dry pulverized mass by extraction with ether and the residue dried and weighed.
The quantity of copper oxyhydrate contained in the precipitate is calculated from the quantity of the copper solution used and amounts to 0.2026 gram. The casein thus prepared contains not only the copper compound named, but also some of the sodium sulfate formed on the addition of the soda lye and other mineral salts present in the milk and from which it is quite impossible to completely free it. There are also many other objections to the process, and the product is of such a nature as to render the data obtained by the method very doubtful.