Part 91

=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, (Soxhlet Method), (4) On Pumice Stone, (5) On Powdered Glass, (6) On Chrysolite: 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, lint, and wood pulp. 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. 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. 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 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.

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.

The principle on which the methods of wet extraction are based is a simple one; viz., to secure a complete or partial solution of the casein and subsequently to extract the fat with a solvent immiscible with water. The methods may be divided into three great classes; viz., (1) those in which the solvent is evaporated from the whole of the extracted fat and the residual matters weighed; (2) processes in which an aliquot part of the fat solution is employed and the total fat calculated from the data secured; (3) the density of the fat solution is determined at a definite temperature and the percentage of fat corresponding thereto determined from tables or otherwise. Methods (1) and (2) are practically identical in principle and one or the other may be applied according to convenience or to local considerations. The methods may be further subdivided in respect of the reagents used to secure complete or partial solution of the casein, as, for instance, alkali or acid.

=461. Solution in an Acid.=—A good type of these processes is the method of Schmid. In this process ten cubic centimeters of milk are placed in a test tube of about five times that content, graduated to measure small volumes. An equal quantity of hydrochloric acid is added, the mixture shaken, boiled until it turns dark brown, and cooled quickly. The fat is extracted by shaking with thirty cubic centimeters of ether. After standing some time the ethereal solution separates and its volume is noted. An aliquot part of the solution is removed, the solvent evaporated, and the weight of fat in the whole determined by calculation.

The schmid process has been improved by Stokes, Hill, and Richmond. The most important of these variations consists in weighing instead of measuring the milk employed, thus insuring greater accuracy. Dyer and Roberts affirm that the ether dissolves some of the caramel products formed on boiling condensed milk with hydrochloric acid, and that the data obtained in such cases by the process of Schmid are too high.

Since lactic acid is also slightly soluble in ether, sour milk should not be extracted with that solvent. In these cases petroleum spirit, or a mixture of petroleum and ether, as suggested by Pinette, may be used. Another variation consists in extracting the fat with several portions of the solvent and evaporating all the extracts thus obtained to get the total fat. This method is perhaps the best of those in which the fat is extracted from the residual liquid after the decomposition of the casein by an acid, and may be recommended as both reliable and typical within the limitations mentioned above.

=462. Solution in an Alkali.=—The casein of milk is not so readily dissolved in an alkali as in an acid, but the solution is sufficient to permit the extraction of the fat. Soda and potash lyes and ammonia are the alkaline bodies usually employed. To promote the separation of the emulsions, alcohol is added with advantage. The principle of the process rests on the observed power of an alkali to free the fat globules sufficiently to allow them to dissolve in ether or some other solvent. When the solvent has separated from the emulsion at first formed, the whole or a part of it is used for the determination of fat in a manner entirely analogous to that employed in the process with the acid solutions described above. There are many methods based on this principle, and some of the typical ones will be given below. Experience has shown that extraction from an alkaline solution is more troublesome and less perfect than from an acid and these alkaline methods are, therefore, not so much practiced now as they were formerly.

=463. Method of Short.=—Instead of measuring the volume of the separated fat, Short has proposed a method in which the casein is dissolved in an alkali and the fat at the same time saponified. The soap thus produced is decomposed by sulfuric acid and the volume of the separated fat acids noted. This volume represents eighty-seven per cent of the corresponding volume of fat.

The solvent employed is a mixture of sodium and potassium hydroxids, containing in one liter 125 and 150 grams, respectively, of these alkalies. The sample of milk is mixed with half its volume of the reagent and placed in boiling water for two hours. By this treatment the casein is dissolved and the fat saponified. After cooling to about 60°, the soap is decomposed by the addition of equal parts of sulfuric and acetic acids. The tubes containing the mixture are again placed in boiling water for an hour and they are then filled with boiling water to within one inch of the top. The tubes may either be furnished with a graduation or the column of fat be measured by a scale.

=464. Method of Thörner.=—The process of Short is conducted by Thörner as follows:

Ten cubic centimeters of milk measured at 15° are saponified, in tubes fitting a centrifugal, by the addition of one and a half cubic centimeters of an alcoholic potash lye, containing 160 grams of potassium hydroxid per liter, or one cubic centimeter of an aqueous fifty per cent soda lye. The saponification is hastened by setting the tubes in boiling water, where they remain for two minutes. The soap formed is decomposed with a strong acid, sulfuric preferred, the tubes placed in the centrifugal and whirled for four minutes, when the fat acids will be formed in the narrow graduated part of the tube and the volume occupied thereby is noted after immersion in boiling water. Thörner’s process is not followed in this country, but is used to a considerable extent in Germany.

=465. Liebermann’s Method.=—In this method, fifty cubic centimeters of milk, at ordinary temperatures, are placed in a glass cylinder twenty-five centimeters high and about four and a half internal diameter; five cubic centimeters of potash lye of 1.27 specific gravity are added, the cylinder closed with a well fitting cork stopper and thoroughly shaken. After shaking, fifty cubic centimeters of petroleum spirit, boiling point about 60°, are added. The cylinder is again stoppered and vigorously shaken until an emulsion is formed. To this emulsion fifty cubic centimeters of alcohol of ninety-five per cent strength are added, and the whole again thoroughly shaken. After four or five minutes the petroleum spirit, containing the fat, separates. In order to insure an absolute separation of the fat, however, the shaking may be repeated three or four times for about one-quarter minute, waiting each time between the shakings until the spirit separates.

Of the separated petroleum spirit twenty cubic centimeters are placed in a small weighed flask. The use of the flask is recommended on account of the ease with which the petroleum spirit can be evaporated without danger of loss of fat. Instead of the flask a weighed beaker or other weighed dish may be employed.

The petroleum spirit is carefully evaporated on a water-bath and the residue dried at 110° to 120° for one hour. The weight found multiplied by five gives the content of fat in 100 cubic centimeters of the milk. The percentage by weight can then be calculated by taking into consideration the specific gravity of the milk employed.

The results obtained by this method agree well with those obtained by the paper coil method, when petroleum spirit instead of sulfuric ether is used as the solvent for the fat. Sulfuric ether, however, gives an apparently higher content of fat because of the solution of other bodies not fat present in the milk.

=466. Densimetric Methods.=—Instead of evaporating the separated fat solution and weighing the residue, its density may be determined and the percentage of fat dissolved therein obtained by calculation, or more conveniently from tables. The typical method of this kind is due to Soxhlet, and until the introduction of modern rapid volumetric processes, it was used perhaps more extensively than any other proceeding for the determination of fat in milk. The reagents employed in the process are ether saturated with water and a potash lye containing 400 grams of potash in a liter. The principle of the process is based on the assumption that a milk made alkaline with potash will give up all its fat when shaken with ether and the quantity of fat in solution can be determined by ascertaining the specific gravity of the ethereal solution.