The cubic content of a tube 100 scale divisions in diameter and fifty in length is 0.0031416 cubic millimeter. The number of globules found in fifty divisions of the tubes used is 40. Then the number which would be contained in a tube of a diameter of 100 divisions of the micrometer scale and a length of fifty divisions thereof is
| N = | 10000n | = | 400000 | = 160. |
| d² | 2500 |
Since the milk is diluted fifty times, the actual number of globules corresponding to the volume given is 8000. It is convenient to reduce the observations to some definite volume, exempla gratia, 0.0001 cubic centimeter. The equation for this in the above instance is 0.0031416: 0.0001 = 8000: x, whence x = 223, = number of fat globules in 0.0001 cubic millimeter.
In one cubic millimeter of milk there are therefore 2,230,000 fat globules, and in one cubic centimeter 2,230,000,000 globules. In a single drop of milk there are from one to two hundred million fat globules.[434]
453. Classification of Methods of Determining Fat in Milk.—The fat, being the most valuable of the constituents of milk, is the subject of a number of analytical processes. An effort will be made here to classify these various methods and to illustrate each class with one or more typical processes. In general the methods may be divided into analytical and commercial, those of the first class being used for scientific and of the other for trade purposes. For normal milk, some of the trade methods have proved to be quite as accurate as the more chronokleptic analytical processes to which, in disputed cases, a final appeal must be taken. When the analyst is called upon to determine the fat in a large number of samples of milk some one of the trade methods may often be adopted with great advantage.
454. Dry Extraction Methods.—Among the oldest and most reliable methods of determining fat in milk, are included those processes based on the principle of drying the milk and extracting the fat from the residue by an appropriate solvent. The solvents generally employed are ether and petroleum spirit of low boiling point. The methods of drying are legion.
In extracting with ether, it must not be forgotten that other bodies than fat may pass into solution on the one hand and on the other any substituted glycerid, such as lecithin or nuclein, which may be present may escape solution, at least in part. Perhaps petroleum spirit, boiling at from 45° to 60°, is the best solvent for fat, but it is almost the universal custom in this and other countries to use ether.
455. The Official Methods.—In the methods adopted by the Association of Official Agricultural Chemists two processes are recommended.
(1) The Asbestos Process: In this process it is directed to extract the residue from the determination of water by the asbestos method ([447]) with anhydrous pure ether until the fat is removed, evaporate the ether, dry the fat at 100° and weigh. The fat may also be determined by difference, after drying the extracted cylinders at 100°.
(2) Paper Coil Method: This is essentially the method proposed by Adams as modified by the author.[435] Coils made of thick filter paper are cut into strips 6.25 by 62.5 centimeters, thoroughly extracted with ether and alcohol, or the weight of the extract corrected by a constant obtained for the paper. If this latter method be used, a small amount of anhydrous sodium carbonate should be added. Paper free of matters soluble in ether is also to be had for this purpose. From a weighing bottle about five grams of milk are transferred to the coil by a pipette, taking care to keep dry the lower end of the coil. The coil, dry end down, is placed on a piece of glass, and dried at a temperature of boiling water for one hour, or better, dried in hydrogen at a temperature of boiling water, transferred to an extraction apparatus and extracted with absolute ether or petroleum spirit boiling at about 45°. The extracted fat is dried in hydrogen and weighed. Experience has shown that drying in hydrogen is not necessary. The fat may be conveniently dried in partial vacuo.