4. If the filtered fat is distilled with a mixture of alcohol and sulphuric acid, the distillate will possess the odour of butyric ether in a very marked degree, in case it consists of butter.[27]
5. The strained fat is treated with a solution of carbolic acid (1 part acid and 10 parts water): genuine butter dissolves to a clear solution; beef, mutton, and swine fat form two layers, the upper one becoming turbid upon cooling.
6. If the sample consists of butter or oleomargarine, and is mixed with about ten parts of glycerine and the emulsion digested with a mixture of equal parts of ether and alcohol, two layers of solution will be produced, without any deposit of solid matter between them; if, however, lard, suet, or starch is present it will become deposited between the layers.
It has already been mentioned that butter differs from some of its fatty adulterants in containing a considerable proportion of fatty acids which are soluble in hot water, the acids present in most foreign fats being, on the other hand, almost entirely insoluble. The estimation of the relative amounts of soluble and insoluble acids contained in a fat possesses therefore much importance; indeed, more significance attaches to this determination than to any other. The processes most frequently employed in the quantitative examination of butter fat are those of Koettstorfer, Hehner, and Reichert.
Koettstorfer’s method[28] is based upon the fact that, as butter fat contains the fatty acids, having a smaller molecular weight than those present in other fats, it must contain more molecules of acid, and will therefore require a greater amount of an alkali to effect saponication. The process is executed as follows:—One or two grammes of the filtered fat are weighed out in a narrow beaker and heated over a water-bath with about 25 c.c. of one-half normal alcoholic solution of potassium hydroxide. The saponification of the fat is assisted by repeated stirring; when it is completed the beaker is removed from the bath, a few drops of alcoholic phenol-phthaleine added for an indicator, and the excess of potash used titrated back with one-half normal hydrochloric acid. It has been found that pure butter fat requires from 221·4 to 232·4 milligrammes of potassium hydroxide for saponification. The following are the number of milligrammes of alkali necessary for the saponification of one gramme of various other fats:—
| mgr. | |
| Olive oil | 191·8 |
| Rape-seed oil | 178·7 |
| Oleomargarine | 195·5 |
| Beef tallow | 196·5 |
| Lard | 195·5 |
| Mutton suet | 197·0 |
| Dripping | 197·0 |
Taking 227 milligrammes as the average amount of potassium hydroxide required to saponify one gramme of pure butter fat, the following formula has been suggested for the estimation of the proportion of admixture in a suspected sample:—
(227 - n) × 3·17 = x,
in which n represents the number of milligrammes of potassium hydroxide used, and x the percentage of foreign fat added. In the Paris Municipal Laboratory, 221 milligrammes of K(OH) are regarded as a standard for the saponification of one gramme of genuine butter.
Cocoa-nut oil, unfortunately, requires a figure (250 mgr.) considerably above that of butter, and it is quite possible to prepare a mixture of this oil and oleomargarine, that by the foregoing test would show a result almost identical with that afforded by pure butter. Hehner’s process,[29] which is often employed for the determination of the insoluble fatty acids, is as follows:—About 4 grammes of the melted and strained sample are dissolved in 50 c.c. of alcohol, containing two grammes of potassium hydroxide in solution, and the mixture is heated until complete saponification takes place. The alcohol is removed by evaporation, the residue dissolved in 200 c.c. of water, and the fatty acids precipitated by adding dilute sulphuric acid to distinct acid reaction. The fatty acids are next melted by heating the liquid and are then allowed to cool, after which the insoluble acids are poured upon a tared filter and repeatedly washed with hot water until the washings cease to show acidity. The filter and contents are finally cautiously dried and weighed. In genuine butter the proportion of insoluble fatty acids ranges between 86·5 and 87·5 per cent.; it should not be above 88 per cent.[30] Oleomargarine, lard, mutton, beef, and poppy, palm, olive, and almond oils contain about 95·5 per cent. of insoluble acids.[31]