ESTIMATION OF FAT.
450. Form of Fat in Milk.—The fat in milk occurs in the form of globules suspended in the liquid, in other words in the form of an emulsion. Many authorities have asserted that each globule of fat is contained in a haptogenic membrane composed presumably of nitrogenous matter, but there is no convincing evidence of the truth of this opinion. The weight of experimental evidence is in the opposite direction. The supposed action of the membrane and the phenomena produced thereby are more easily explained by the surface tension existing between the fat globules and the menstruum in which they are suspended.
Babcock affirms that the spontaneous coagulation of the fibrin present in milk tends to draw the fat globules into clusters, and this tendency can be arrested by adding a little soda or potash lye to the milk as soon as it is drawn.[432]
The diameter of the fat globules is extremely variable, extending in some cases from two to twenty micromillimeters. In cow milk, the usual diameters are from three to five micromillimeters.
451. Number of Fat Globules in Milk.—The number of fat globules in milk depends on their size and the percentage of fat. It is evident that no definite statement of the number can be made. There is a tendency, on the part of the globules, to diminish in size and increase in number as the period of lactation is prolonged. To avoid large numbers, it is convenient to give the number of globules in 0.0001 cubic millimeter. This number may be found within wide limits depending on the individual, race, food and other local conditions to which the animal or herd is subjected. In general, in whole milk this number will be found between 140 and 250.
452. Method of Counting Globules.—The number of globules in milk is computed with the aid of the microscope. The most convenient method is the one devised by Babcock.[433] In carrying out this computation, capillary tubes, from two to three centimeters long and about one-tenth millimeter in internal diameter, are provided. The exact diameter of each tube, in at least three points, is determined by the micrometer attachment of the microscope, and from these measurements the mean diameter of the tube is calculated. This known, its cubic content for any given length is easily computed. Ten cubic centimeters of the milk are diluted with distilled water to half a liter and one end of a capillary tube dipped therein. The tube is quickly filled with diluted milk and each end is closed with a little wax to prevent evaporation. Several of these tubes being thus prepared, they are placed in a horizontal position on the stage of the microscope and covered with glycerol and a cover glass. The tubes are left at rest for some time until all the fat globules have attached themselves to the upper surfaces, in which position they are easily counted. The micrometer is so placed as to lie parallel with the tubes, and the number of globules, corresponding to each division of its scale, counted. The mean number of globules corresponding to each division of the micrometer scale is thus determined.
To compare the data obtained with each tube they are reduced to a common basis of the number of globules found in a length of fifty divisions of the micrometer scale in a tube having a diameter of 100 divisions, using the formula
| N = | 10000n |
| d² |
in which n = the number of globules found in the standard length of tube measured and d = the diameter of the tube. It is not difficult to actually count all the globules in a length of fifty divisions of the scale, but the computation may also be made from the mean numbers found in a few divisions. The usual number of globules found in a length of 0.1 millimeter in a tube 0.1 millimeter in diameter, varies from fifty to one hundred.
Example.—The length of one division of the micrometer scale is 0.002 millimeter, and the internal diameter of the tube 0.1 millimeter. The content of a tube, of a length of 0.002 × 50 = 0.1 millimeter, is therefore 0.0007854 cubic millimeter.