Fig. 112. Thermometer for Butyrorefractometer.
497. Judgment of Suspected Butter or Lard by Refractive Power.—In discriminating between pure and adulterated butters by the aid of the butyrorefractometer ([301]), the absolute reading of the instrument is of less importance than the difference which is detected between the highest permissible numbers, for any degree of temperature, and the actual reading obtained at that temperature. These differences, within certain limits, do not perceptibly vary with the temperature, and heretofore they have been determined with the aid of a table, and in this respect the observations have been made the more laborious.
Wollny has rendered these tables unnecessary by constructing a thermometer in which the mercury column does not indicate degrees of temperature, but the highest permissible number for butter or lard at the temperature of observation. The scale of the instrument is so adjusted as to include temperatures of from 30° to 40°, which renders it suited to the examination of butter and lard. The oleothermometer is shown in [Fig. 112].
The side of the scale B is for butter and that marked S for lard. The use of the instrument is the simplest possible. The sample of fat is placed in the prisms in the usual manner. When the mercury in the thermometer is at rest, the scale of the instrument is read. In the case of a butter, if the reading of the scale give a higher number than that indicated by the thermometer, the sample is pronounced suspicious and the degree of suspicion is proportional to the difference of the two readings.
498. Estimation of Water, Fat, Casein, Ash and Salt.—The methods proposed by the author for conducting these determinations, with minor amendments, have been adopted by the Association of Official Agricultural Chemists.[501]
Water.—The sample held in a flat bottom dish is dried to constant weight at about 100°. The weight of the sample used should be proportional to the area of the bottom of the dish, which should be just covered by the film of melted fat. The dish may be previously partly filled with sand, asbestos or pumice stone. The drying may take place in the air, in an inert gas or in a vacuum.
Fat.—The fat in a sample of butter is readily determined by treating the contents of the dish after the determination of water with an appropriate solvent.
The process is conducted as follows:
The dry butter from the water determination is dissolved in the dish with ether or petroleum spirit. The contents of the dish are then transferred to a weighed gooch with the aid of a wash bottle containing the solvent, and washed till free of fat. The crucible and contents are heated at the temperature of boiling water till the weight is constant. The weight of fat is calculated by difference from the data obtained.