Glycerol is a product of fermentation and of the hydrolysis of the fats and oils in the substances fermented.
616. Specific Gravity.—In order to secure uniformity of expression, the specific gravity of fermented beverages is determined at about 15°.6, although that is a temperature much below the average found in American laboratories. The specific gravity may be determined by an alcoholometer, pyknometer or hydrostatic balance in harmony with the directions given in paragraphs [48-54] and [285]. By reason of the extractive matters held in solution, fermented beverages are usually heavier than water, even if the content of alcohol be twenty per cent or more. On the other hand distilled liquors are lighter than water.
617. Determination of Alcohol.—The determination of the percentage of alcohol present in a solution is based on two general principles. On the one hand, and this is the base of the methods in common use, the alcohol is secured mixed only with water and its amount determined by ascertaining the specific gravity of the mixture. On the other hand the quantity of alcohol in a mixture may be determined by ascertaining the temperature of the vapors produced on boiling. This is the principle involved in the use of the ebullioscope. The latter method is not employed to any extent in this country.
Use of the Alcoholometer.—The alcoholometer usually employed is known by the name of Gay-Lussac, who first made practical use of it in the determination of alcohol. It is constructed in such a way as to read directly the volume of absolute alcohol contained in one hundred volumes of the liquid at a temperature of 15°.6. The instruments employed should be carefully calibrated and thoroughly cleaned by washing with absolute alcohol before use. The stem of the instrument must be kept free from any greasy substance, and this is secured by washing it with ether. After this last washing the analyst should be careful not to touch the stem of the instrument with his fingers. It is most convenient to make the determination exactly at 15°.6, but when made at other temperatures the reading of the instrument is corrected by tables which may be found in works especially devoted to the analysis of wines.[632]
In this country the alcoholometer is used to some extent, but the official method is based upon the determination of the specific gravity by an instrument constructed in every respect like the alcoholometer, but giving the specific gravity of the liquor at 15°.6 instead of its percentage by volume in alcohol. The reading of the instrument having been determined at a temperature of 15°.6, the corresponding percentage of alcohol by volume or by weight is taken directly from the table given further on.
Fig. 123. Metal Distilling Apparatus.
Methods of Distillation.—The metal apparatus employed in the laboratory of the Department of Agriculture, for the distillation of fermented beverages in order to determine the percentage of alcohol by the method given above, is shown in the accompanying [figure]. The apparatus consists of a retort of copper carried on supports in such a way as to permit an alcohol or bunsen lamp to be placed under it. It is connected with a block tin condenser and the distillate is received in a tall graduated cylinder placed under the condenser in such a way as to prevent the loss of any alcohol in the form of vapor. Exactly 300 cubic centimeters of the wine or fermented beverage are used for the distillation. Any acid which the wine contains is first saturated with calcium carbonate before placing in the retort. Exactly 100 cubic centimeters of distillate are collected and the volume of the distillate is completed to 300 cubic centimeters by the addition of recently distilled water.[633] The cylinder containing the distillate is brought to a temperature of 15°.6, the alcoholometer inserted and its reading taken with the usual precautions.
Official Method.—The alcoholometers employed in the official methods are calibrated to agree with those used by the officers of the Bureau of Internal Revenue. They are most conveniently constructed, carrying the thermometer scale in the same stem with that showing the specific gravity. It is highly important that the analyst assure himself of the exact calibration of the instrument before using it. Inasmuch as the volume of the distillate may not be suited in all cases to the use of a large alcoholometer, it is customary in this laboratory to determine the specific gravity by means of the hydrostatic balance, as described further on. Attention is also called to the fact that, in the official method, directions are not given to neutralize the free acid of the fermented beverage before the distillation. Since the Internal Revenue Bureau is concerned chiefly with the determination of alcohol in distilled liquors, this omission is of little consequence. Even in ordinary fermented beverages the percentage of volatile acids, (acetic etc.,) is so small as to make the error due to the failure to neutralize it of but little consequence. In order, however, to avoid every possibility of error, it is recommended that in all instances the free acids of the sample be neutralized before distillation. In this laboratory, the distillations are conducted in a glass apparatus shown in the accompanying [figure]. The manipulation is as follow:[634]
