| Specific Gravity at 37°·5. | Per cent. Fixed Fatty Acids. | |
| Mutton suet | 0·90283 | 95·56 |
| Beef suet | 0·90372 | 95·91 |
| Fine lard | 0·90384 | 96·20 |
| Dripping (commercial) | 0·90456 | 94·67 |
| Mutton dripping (genuine) | 0·90397 | 95·48 |
| Oleomargarine butter | 0·90384 | 94·34 |
| „ „ | 0·90234 | 94·83 |
| „ „ | 0·90315 | 95·04 |
| „ „ | 0·90379 | 96·29 |
| „ „ | 0·90136 | 95·60 |
It will be noticed that the fats mostly used to adulterate butter are of a lower density. Blyth regards a gravity below 0·911 (at 37°·5) as clearly pointing to the presence of foreign fatty admixture.
The specific gravity determination is made by means of the areometer, or by the gravity bottle; numerous indirect methods have also been proposed. P. Casamajor[26] suggests a process for distinguishing genuine butter from oleomargarine which is based upon the fact that the density of a liquid in which a body remains in equilibrium is the density of the body itself. As the result of his investigations it was found that pure butter at 15° would be held in equilibrium by alcohol of 53·7 per cent. (sp. gr. = 0·926), and that oleomargarine would remain in equilibrium, at the same temperature, in alcohol of 59·2 per cent. (sp. gr. = 0·905). If equal volumes of alcohol of 53·7 per cent. and 59·2 per cent. (i.e. an alcohol of 56·5 per cent.) are taken, and a drop of melted butter and of oleomargarine are delivered upon its surface, the former will sink to the bottom and the latter will remain at the top, so long as the two globules are warm and liquid. In case the temperature of the alcohol is about 30°, the butter will solidify and also rise to the top, whereas the oleomargarine may remain liquid. On now keeping the alcohol for a short time at a temperature of 15° the oleomargarine becomes opaque, but remains at the top, while the solidified butter will sink to the bottom. If alcohol of 59·2 per cent. is employed, oleomargarine will remain at the surface and genuine butter fall to the bottom at all temperatures above 15°, and at this temperature oleomargarine will be in equilibrium. Since not over 33 per cent. of butter is usually added to oleomargarine, it is proposed to use alcohol of 55 per cent., and consider as oleomargarine any sample which does not sink at 15°.
The foregoing method can be applied quantitatively by determining the strength of the alcohol which will keep in equilibrium a drop of the fat under examination. Since the difference between 59·2 and 53·7 is 5·5, the difference between the strength of the alcohol used and 53·7, divided by 5·5 (or multiplied by 0·18), will give the proportion of oleomargarine present. For example, if the globule is held in equilibrium at 15° in 57 per cent. alcohol, the sample contains about 60 per cent. of oleomargarine, for (57 - 53·7) × 0·18 = 3·3 × 0·18 = 0·594 or, say, 6⁄10.
The melting-point of butter is below that of most of its fatty adulterants; as previously stated, it varies from 28° to 37°. The determination is made either in the ordinary manner by means of a fine tube, or a little of the chilled sample is attached to a looped platinum wire, placed near the thermometer-bulb, in water which is gradually heated until fusion takes place. Blyth gives the following table of the melting-points of various fats:—
| ° | |
| Butterine | 31·3 |
| Cocoa butter | 34·9 |
| Butter (average) | 35·8 |
| Beef dripping | 43·8 |
| Veal dripping | 47·7 |
| Mixed | 42·6 |
| Lard, from | 42 to 45 |
| Ox fat, from about | 48 „ 53 |
| Mutton fat, from about | 50 „ 51 |
| Tallow | 53·3 |
Numerous qualitative tests have been proposed by various authorities for the detection of foreign fats in butter, of which the following are perhaps sometimes of use. It should be added that the value of these tests, when applied to mixtures, is limited and very uncertain.
1. A little of the sample is heated in a test-tube: pure butter froths and acquires a brownish colour; with foreign fats there is but little foaming, and, although the caseine present darkens, the liquid itself remains comparatively clear.
2. If a sample containing oleomargarine is melted and the oil burned in an ordinary lamp-wick, a decided odour of burning tallow will be produced upon extinguishing the flame. Specimens of real butter, however, have been found to also emit a tallow-like odour.
3. The melted sample is filtered and treated with boiling ether; pure butter fat dissolves much more readily than do lard and tallow. Upon adding methylic alcohol to the solution the latter fats are precipitated, whereas pure butter will remain in solution.