523. Proteids.—The total proteids are most easily estimated by the official kjeldahl method.[530] The separation of the proteid bodies is accomplished by the methods described in paragraphs [475-489].
In addition to the methods already described for separating the soluble and suspended proteid bodies in milk, and which may be used also for koumiss, the following should also be mentioned as of especial worth:
Separation by Filtration through Porous Porcelain.—A purely physical method, and one which is to be recommended by reason of the absence of any chemical action upon the different proteid matters, is that proposed by Lehmann, depending upon the principle that when milk is forced through porous porcelain, the albumin passes through together with the milk, sugar and other soluble constituents as a clear filtrate, while the casein and fat are perfectly retained.[531]
By this method it is quite certain that the albumin and other perfectly soluble proteids of milk may be obtained in the purest form.
Separation by Precipitation with Alum.—Probably the best chemical method of separating the two classes of proteid matters is that proposed by Schlosmann, which is effected by means of precipitating the casein with a solution of alum.[532]
The principle of this separation rests upon the fact that a solution of potash alum, when added to milk diluted with four or five times its volume of water, will completely separate the casein without affecting the albumin or globulin. The operation is conducted as follows:
Ten cubic centimeters of the milk are diluted with from three to five times that quantity of water and warmed to a temperature of about 40°. One cubic centimeter of a concentrated solution of potash alum is added, the mixture well stirred and the coagula which are formed allowed to subside. If the coagulation of the casein does not take place promptly, a small addition of the alum solution is made, usually not exceeding half a cubic centimeter, until the precipitation is complete. The temperature during the process should be kept as nearly as possible 40°. After a few minutes, the mixture is poured upon a filter and the filtrate, if not perfectly clear, is poured back until it is secured free of turbidity. In difficult cases the filtration may be promoted by the addition of some common salt or calcium phosphate, the latter acting mechanically in holding back the fine particles of casein. The precipitate is washed with water at a temperature of 40°, and afterwards with alcohol, not allowing the alcohol wash water to flow into the filtrate. When the water has been chiefly removed from the precipitate by washing with alcohol, the fat of the precipitated casein is removed with ether and the residue used for the determination of nitrogen in the usual way. The albumin is removed from the filtrate by a tannin solution in the manner already described ([480]). If it be desired to separate the albumin and globulin, the methods described in paragraph [399] may be used.
524. Mercurial Method.—A volumetric method for determining the total proteid matter in milk has lately been proposed by Deniges.[533] It is based upon the observation that in the precipitation of proteid matter by mercury salts, a definite quantity of mercury in proportion to the amount of proteid, is carried down therewith. The precipitation is made with a mercurial salt of known strength and the excess of the mercurial salt in the filtrate is determined by titration. For the details of the manipulation, the paper cited above may be consulted.
525. Water and Ash.—From two to five grams of the koumiss are dried to constant weight in a flat platinum dish over ignited sand, asbestos or pumice stone, and the dried residue incinerated.
526. Composition of Koumiss.—The composition of koumiss varies with the character of the milk used and the extent of the fermentation. Some of the data obtained by analysts are given below:[534]