Part 89

Langlet has invented a lactoscope with a scale, showing the corrections to be applied for temperatures other than 15°. A detailed description of this instrument, as well as the one proposed by Pinchon, is unnecessary.

=445. Density of Sour Milk.=—Coagulated milk cannot be used directly for the determination of the specific gravity, both because of its consistence and by reason of the fact that the fat is more or less completely separated. In such a case, the casein may be dissolved by the addition of a measured quantity of a solvent of a known specific gravity, the density of the resulting solution determined and that of the original milk calculated from the observed data. Ammonia is a suitable solvent for this purpose.

=446. Density of the Milk Serum.=—The specific gravity of the milk serum, after the removal of the fat and casein by precipitation and filtration, may also be determined. For normal cow milk the number is about 1.027.

=447. Total Solids.=—The direct gravimetric determination of the total solids in milk is attended with many difficulties, and has been the theme of a very extended periodical literature. A mere examination of the many processes which have been proposed would require several pages.

The most direct method of procedure is to dry a small quantity of milk in a flat-bottom dish to constant weight on a steam-bath. The surface of the dish should be very large, even for one or two grams of milk; in fact the relation between the quantity of milk and the surface of the dish should be such that the fluid is just sufficient in amount to moisten the bottom of the dish with the thinnest possible film. The dish, during drying, is kept in a horizontal position at least until its contents will not flow. The water of the sample will be practically all evaporated in about two hours. The operation may be accelerated by drying in vacuo.

The drying may also be accomplished by using a flat-bottom dish containing some absorbent, such as sand, pumice stone, asbestos or crysolite. The milk may also be absorbed by a dried paper coil and dried thereon (=26=).

It is convenient to determine the water in the sample subsequently to be used for the gravimetric determination of the fat, and this is secured by the adoption of the paper coil method, as suggested by the author, or by the use of a perforated metal tube containing porous asbestos, as proposed by Babcock.

The process is conveniently carried out as follows:

Provide a hollow cylinder of perforated sheet metal sixty millimeters long and twenty millimeters in diameter, closed five millimeters from one end by a disk of the same material. The perforations should be about 0.7 millimeter in diameter and as close together as possible. Fill loosely with from one and a half to two and a half grams of dry woolly asbestos and weigh. Introduce a weighed quantity of milk (about five grams). Dry at 100° for four hours. During the first part of the drying the door of the oven should be left partly open to allow escape of moisture. Cool in a desiccator and weigh. Repeat the drying until the weight remains constant. Place in an extractor and treat with anhydrous ether for two hours. Evaporate the ether and dry the fat at 100°. The extracted fat is weighed and the number thus obtained may be checked by drying and weighing the cylinder containing the residue.

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, Behrend and Morgen, Claus, Stutzer and Meyer, Hehner, and Hehner and Richmond. 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:

100S - FS t = ( ----------------- - 1)(250 - 2.5 F). 100 - 1.0753FS

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 ---- + 1.2F, 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 6F 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