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.