Part 44

The asbestos best suited for use in this process should be of a woolly nature, quite absorbent, and, previous to use, be ignited to free it of moisture and organic matter. A variety of serpentine, crysolite is sometimes used instead of asbestos. When the content of water alone is desired, it is accurately determined by drying in vacuo over pumice stone (page 33).

The methods above mentioned are typical and will prove a sufficient guide for conducting the desiccation, either as described or by any modification of the methods which may be preferred.

=448. Calculation of Total Solids.=—By reason of the ease and celerity with which the density of a milk and its content of fat can be obtained, analysts have found it convenient to calculate the percentage of total solids instead of determining it directly. This is accomplished by arbitrary formulas based on the data of numerous analyses. These formulas give satisfactory results when the samples do not vary widely from the normal and may be used with advantage in most cases.

Among the earliest formulas for the calculation may be mentioned those of Fleischmann and Morgen,[424] Behrend and Morgen,[425] Claus, Stutzer and Meyer,[426] Hehner,[427] and Hehner and Richmond.[428] Without doing more than citing these papers it will be sufficient here to give the formulas as corrected by the most recent experience.

In the formula worked out by Babcock the specific gravity of the sample is represented by _S_, the fat by _F_, and the solids not fat by _t_. The formula is written as follows:[429]

100_S_ - _FS_ _t_ = ( ----------------- - 1)(250 - 2.5 _F_). 100 - 1.0753_FS_

In this formula it is assumed that the difference between the specific gravity of the milk serum and that of water is directly proportional to the per cent of solids in the serum, but this assumption is not strictly correct. Even in extreme cases, however, the error does not amount to more than 0.05 per cent.

Since a given amount of milk sugar increases the density of a milk more than the same quantity of casein, it is evident that the formula would not apply to those instances in which the ratio between these two ingredients is greatly disturbed, as for instance, the whey.

The formula of Hehner and Richmond, in its latest form, is expressed as follows: _G_ _T_ = 0.2625 ---- + 1.2_F_, _D_

in which _T_ represents the total solids, _G_ the reading of the quévenne lactometer, _D_ the specific gravity, and _F_ the fat.

_Example._—Let the reading of the lactometer be 31, corresponding to _D_ 1.031, and the percentage of fat be three and five-tenths, what is the percentage of the total solids?

Substituting these values in the formulas we have

31 _T_ = 0.2625 ----- + 1.2 × 3.5 = 12.09. 1.031

To simplify the calculations, Richmond’s formula may be written

_G_ 6_F_ _T_ = ----- + ----- + 0.14. 4 5

Calculated by this shortened formula from the above data _T_ = 12.09, the same as given in the larger formula.

Calculating the solids not fat in the hypothetical case given above by Babcock’s formula, we get _t_ = 8.46, and this plus 3.5 gives 11.96, which is slightly lower than the number obtained by the richmond process.

The babcock formula may be simplified by substituting the number expressing the reading of the quévenne lactometer for that donating the specific gravity, in other words, the specific gravity multiplied by 100 and the quotient diminished by 1000.

The formulas for solids not fat and total solids then become

_L_ _L_ _t_ = ---- + 0.2_F_, and _T_ = ----- + 1.2_F_, 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.[430] 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.[431] 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.[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

10000_n_ _N_ = ---------, _d_²

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

10000_n_ 400000 _N_ = -------- = ------ = 160. _d_² 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.

=456. Variations of Extraction Method.=—The method of preparing the milk for fat extraction is capable of many variations. Some of the most important follow:

(1) _Evaporation on Sand_: The sand should be pure, dry and of uniform size of grain. It may be held in a dish or tube. The dish may be made of tin foil, so that it can be introduced with its contents into the extraction apparatus after the desiccation is complete. For this purpose, it is cut into fragments of convenient size after its contents have been poured into the extractor. The scissors used are washed with the solvent.

(2) _Evaporation on Kieselguhr_: Dry kieselguhr (infusorial earth, tripoli) may take the place of the sand as above noted. The manipulation is the same as with sand.

(3) _Evaporation on Plaster of Paris_,[436] (_Soxhlet Method_),[437] (4) _On Pumice Stone_, (5) _On Powdered Glass_, (6) _On Chrysolite_:[438] The manipulation in these cases is conducted as with sand and no detailed description is required.

(7) _Evaporation on Organic Substances_: These variations would fall under the general heading of drying on paper. The following materials have been used; _viz._, sponge,[439] lint,[440] and wood pulp.[441] In these variations the principal precautions to be observed are to secure the organic material in a dry state and free of any matter soluble in the solvent used.

(8) _Dehydration with Anhydrous Copper Sulfate_: In this process the water of the sample is absorbed by powdered anhydrous copper sulfate, the residual mass extracted and the butter fat obtained determined by saponification and titration.[442] In the manipulation about twenty grams of the anhydrous copper sulfate are placed in a mortar, a depression made therein in such a manner that ten cubic centimeters of milk can be poured into it without wetting it through to the mortar. The water is soon absorbed when the mass is ground with a little dry sand and transferred to the extractor.

Petroleum spirit of low boiling point is used as a solvent, successive portions of about fifteen cubic centimeters each being forced through the powdered mass under pressure. Two or three treatments with the petroleum are required. The residual butter fat, after the evaporation of the petroleum, is saponified with a measured portion, about twenty-five cubic centimeters, of seminormal alcoholic potash lye. The residual alkali is determined by titration with seminormal hydrochloric acid in the usual manner. From the data obtained is calculated the quantity of alkali employed in the saponification. The weight of butter fat extracted is then calculated on the assumption that 230 milligrams of potash are required to saponify one gram of the fat.

=457. Gypsum Method for Sour Milk.=—In sour milk, extraction of the dry residue with ether is attended with danger of securing a part of the free lactic acid in the extract. This may be avoided, at least in part, by making the milk neutral or slightly alkaline before desiccation. This method is illustrated by a variation of Soxhlet’s method of drying on gypsum proposed by Kühn.[443] The curdled milk is treated with potash lye of forty per cent strength until the reaction is slightly alkaline. For absorbing the sample before drying, a mixture is employed consisting of twenty-five grams of plaster of paris, four of precipitated carbonate of lime and two of acid potassium sulfate. To this mixture ten grams of the milk, rendered alkaline as above noted, are added in a desiccating dish, the excess of moisture evaporated at 100°, the residual mass finely ground and extracted with ether for four hours. A little gypsum may be found in the solution, but in such small quantities as not to interfere seriously with the accuracy of the results obtained.

=458. Estimation of Fat in Altered Milk.=—In altered milk the lactose has usually undergone a fermentation affording considerable quantities of lactic acid. If such milks be treated by the extraction method for fat, the results will always be too high, because of the solubility of lactic acid in ether.

Vizern[444] has proposed to avoid this error by first warming the soured milk for a few minutes to 40°, at which temperature the clabber is easily divided by vigorous shaking. Of the milk thus prepared, thirty grams are diluted with two or three volumes of water and poured onto a smooth and moistened filter. The vessel and filter are washed several times until the filtrate presents no further acid reaction. The filter and its contents are next placed in a vessel containing some fine washed sand. A small quantity of water is added, sufficient to form a paste. With a stirring rod, the filter is entirely broken up and the whole mass thoroughly mixed. Dried on the water bath the material is subjected to extraction in the ordinary way. Several analyses made on fresh milk and on milk kept for several months show that almost identical results are obtained.

In respect of this process there would be danger, on long standing, of the formation of free acids from butter glycerids, and these acids would be removed by the process of washing prescribed. In this case the quantity of fat obtained would be less than in the original sample.

=459. Comparison of Methods.=—An immense amount of work has been done by analysts in comparing the various types of extraction methods outlined above.[445]

The consensus of opinion is that good results are obtained by all the methods when properly conducted, and preference is given to the two methods finally adopted by the Association of Official Chemists. As solvents, pure ether and petroleum spirit of low boiling point are preferred. The direct extraction gravimetric processes are important, since it is to these that all the other quicker and easier methods must appeal for the proof of their accuracy.

=460. Wet Extraction Methods.=—It has been found quite impracticable to extract the fat from milk by shaking it directly with the solvent. An emulsion is produced whereby the solvent itself becomes incorporated with the other constituents of the milk, and from which it is not separated easily even with the aid of whirling. The disturbing element which prevents the separation of the solvent is doubtless the colloid casein, since, when this is previously rendered soluble, the separation of the solvent holding the fat is easily accomplished.