Part 90

L L t = ---- + 0.2F, and T = ----- + 1.2F, 4 4

in which L represents the reading of the lactometer. By the addition of the constant factor 0.14 the results calculated by the formula of Babcock are the same as those obtained by the method of Richmond.

In the following table are given the solids not fat in milks as calculated by Babcock’s formula. To obtain the total solids add the per cent of fat to solids not fat. To obtain total solids according to Richmond’s formula increase that number by 0.14.

TABLE SHOWING PER CENT OF SOLIDS NOT FAT IN MILK CORRESPONDING TO QUÉVENNE’S LACTOMETER READINGS AND PER CENT OF FAT.

Per cent Lactometer reading at 60° F. of fat. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36.

0.0 6.50 6.75 7.00 7.25 7.50 7.75 8.00 8.25 8.50 8.75 9.00 0.1 6.52 6.77 7.02 7.27 7.52 7.77 8.02 8.27 8.52 8.77 9.02 0.2 6.54 6.79 7.04 7.29 7.54 7.79 8.04 8.29 8.54 8.79 9.04 0.3 6.56 6.81 7.06 7.31 7.56 7.81 8.06 8.31 8.56 8.81 9.06 0.4 6.58 6.83 7.08 7.33 7.58 7.83 8.08 8.33 8.58 8.83 9.08 0.5 6.60 6.85 7.10 7.35 7.60 7.85 8.10 8.35 8.60 8.85 9.10 0.6 6.62 6.87 7.12 7.37 7.62 7.87 8.12 8.37 8.62 8.87 9.12 0.7 6.64 6.89 7.14 7.39 7.64 7.89 8.14 8.39 8.64 8.89 9.14 0.8 6.66 6.91 7.16 7.41 7.66 7.91 8.16 8.41 8.66 8.91 9.16 0.9 6.68 6.93 7.18 7.43 7.68 7.93 8.18 8.43 8.68 8.93 9.18 1.0 6.70 6.95 7.20 7.45 7.70 7.95 8.20 8.45 8.70 8.95 9.20 1.1 6.72 6.97 7.22 7.47 7.72 7.97 8.22 8.47 8.72 8.97 9.22 1.2 6.74 6.99 7.24 7.49 7.74 7.99 8.24 8.49 8.74 8.99 9.24 1.3 6.76 7.01 7.26 7.51 7.76 8.01 8.26 8.51 8.76 9.01 9.26 1.4 6.78 7.03 7.28 7.53 7.78 8.03 8.28 8.53 8.78 9.03 9.28 1.5 6.80 7.05 7.30 7.55 7.80 8.05 8.30 8.55 8.80 9.05 9.30 1.6 6.82 7.07 7.32 7.57 7.82 8.07 8.32 8.57 8.82 9.07 9.32 1.7 6.84 7.09 7.34 7.59 7.84 8.09 8.34 8.59 8.84 9.09 9.34 1.8 6.86 7.11 7.36 7.61 7.86 8.11 8.36 8.61 8.86 9.11 9.37 1.9 6.88 7.13 7.38 7.63 7.88 8.13 8.38 8.63 8.88 9.14 9.39 2.0 6.90 7.15 7.40 7.65 7.90 8.15 8.40 8.66 8.91 9.16 9.41 2.1 6.92 7.17 7.42 7.67 7.92 8.17 8.42 8.68 8.93 9.18 9.43 2.2 6.94 7.19 7.44 7.69 7.94 8.19 8.44 8.70 8.95 9.20 9.45 2.3 6.96 7.21 7.46 7.71 7.96 8.21 8.46 8.72 8.97 9.22 9.47 2.4 6.98 7.23 7.48 7.73 7.98 8.23 8.48 8.74 8.99 9.24 9.49 2.5 7.00 7.25 7.50 7.75 8.00 8.25 8.50 8.76 9.01 9.26 9.51 2.6 7.02 7.27 7.52 7.77 8.02 8.27 8.52 8.78 9.03 9.28 9.53 2.7 7.04 7.29 7.54 7.79 8.04 8.29 8.54 8.80 9.05 9.30 9.55 2.8 7.06 7.31 7.56 7.81 8.06 8.31 8.57 8.82 9.07 9.32 9.57 2.9 7.08 7.33 7.58 7.83 8.08 8.33 8.59 8.84 9.09 9.34 9.59 3.0 7.10 7.35 7.60 7.85 8.10 8.36 8.61 8.86 9.11 9.36 9.61 3.1 7.12 7.37 7.62 7.87 8.13 8.38 8.63 8.88 9.13 9.38 9.64 3.2 7.14 7.39 7.64 7.89 8.15 8.40 8.65 8.90 9.15 9.41 9.66 3.3 7.16 7.41 7.66 7.92 8.17 8.42 8.67 8.92 9.18 9.43 9.68 3.4 7.18 7.43 7.69 7.94 8.19 8.44 8.69 8.94 9.20 9.45 9.70 3.5 7.20 7.45 7.71 7.96 8.21 8.46 8.71 8.96 9.22 9.47 9.72 3.6 7.22 7.48 7.73 7.98 8.23 8.48 8.73 8.98 9.24 9.49 9.74 3.7 7.24 7.50 7.75 8.00 8.25 8.50 8.75 9.00 9.26 9.51 9.76 3.8 7.26 7.52 7.77 8.02 8.27 8.52 8.77 9.02 9.28 9.53 9.78 3.9 7.28 7.54 7.79 8.04 8.29 8.54 8.79 9.04 9.30 9.55 9.80 4.0 7.30 7.56 7.81 8.06 8.31 8.56 8.81 9.06 9.32 9.57 9.83 4.1 7.32 7.58 7.83 8.08 8.33 8.58 8.83 9.08 9.34 9.59 9.85 4.2 7.34 7.60 7.85 8.10 8.35 8.60 8.85 9.11 9.36 9.62 9.87 4.3 7.36 7.62 7.87 8.12 8.37 8.62 8.88 9.13 9.38 9.64 9.89 4.4 7.38 7.64 7.89 8.14 8.39 8.64 8.90 9.15 9.40 9.66 9.91 4.5 7.40 7.66 7.91 8.16 8.41 8.66 8.92 9.17 9.42 9.68 9.93 4.6 7.43 7.68 7.93 8.18 8.43 8.68 8.94 9.19 9.44 9.70 9.95 4.7 7.45 7.70 7.95 8.20 8.45 8.70 8.96 9.21 9.46 9.72 9.97 4.8 7.47 7.72 7.97 8.22 8.47 8.72 8.98 9.23 9.48 9.74 9.99 4.9 7.49 7.74 7.99 8.24 8.49 8.74 9.00 9.25 9.50 9.76 10.01 5.0 7.51 7.76 8.01 8.26 8.51 8.76 9.02 9.27 9.52 9.78 10.03 5.1 7.53 7.78 8.03 8.28 8.53 8.79 9.04 9.29 9.54 9.80 10.05 5.2 7.55 7.80 8.05 8.30 8.55 8.81 9.06 9.31 9.56 9.82 10.07 5.3 7.57 7.82 8.07 8.32 8.57 8.83 9.08 9.33 9.58 9.84 10.09 5.4 7.59 7.84 8.09 8.34 8.60 8.85 9.10 9.36 9.61 9.86 10.11 5.5 7.61 7.86 8.11 8.36 8.62 8.87 9.12 9.38 9.63 9.88 10.13 5.6 7.63 7.88 8.13 8.39 8.64 8.89 9.15 9.40 9.65 9.90 10.15 5.7 7.65 7.90 8.15 8.41 8.66 8.91 9.17 9.42 9.67 9.92 10.17 5.8 7.67 7.92 8.17 8.43 8.68 8.94 9.19 9.44 9.69 9.94 10.19 5.9 7.69 7.94 8.20 8.45 8.70 8.96 9.21 9.46 9.71 9.96 10.22 6.0 7.71 7.96 8.22 8.47 8.72 8.98 9.23 9.48 9.73 9.98 10.24

=449. Determination of Ash.=—In the determination of the solid residue obtained by drying milk, it is important to observe the directions already given (=28-32=).

In the direct ignition of the sample, a portion of the sulfur and phosphorus may escape oxidation and be lost as volatile compounds. This loss may be avoided by the use of proper oxidizing agents or by conducting the combustion as heretofore described. In the official method, it is directed to add six cubic centimeters of nitric acid to twenty of milk, evaporate to dryness and ignite the residue at a low red heat until free of carbon. It is doubtful if this precaution be entirely sufficient to save all the sulfur and phosphorus, but the method is evidently more reliable than the common one of direct ignition without any oxidizing reagent whatever.

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.

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. 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

10000n N = ---------, 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.

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

10000n 400000 N = -------- = ------ = 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.

=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. 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.