If it be desired to use this table for the examination of liquors containing a higher percentage of extract, Schultze’s table (intended originally for wort) may be consulted.
Gautier regards the fixed solids as the residue obtained on evaporating, in a flat platinum dish, ten cubic centimeters of wine at 100° for four hours and a half.[639]
The official French method is as follows: Twenty cubic centimeters of wine are placed in a flat bottom, platinum dish of such a diameter that the depth of the liquid therein does not exceed one millimeter. The dish should be immersed as totally as possible in the steam. The heating is continued for six hours.
The following method is used at the municipal laboratory of Paris:
Twenty-five cubic centimeters of wine are placed in a flat bottom, platinum dish seventy millimeters in diameter and twenty-five deep. The dish is placed on a water bath in such a manner that it just touches the surface of the water which is kept at a constant level. The heating is continued for seven hours.[640]
625. Determination in a Vacuum.—To avoid the changes and decomposition produced by heating, the fixed solids may also be determined by drying the sample in a vacuum over sulfuric acid. In this laboratory, it has been found that the process may be greatly facilitated by connecting the desiccating apparatus with the vacuum service of the working desks in which a vacuum corresponding to a mercurial column of 600 millimeters is obtained. The desiccator is provided with a valve whereby a minute current of dry air is allowed to flow through it. This current is not large enough to lessen the vacuum but is sufficient to greatly accelerate the rapidity of the evaporation. The evaporation is hastened also, in a marked degree, by absorbing the liquid with a piece of filter paper previously dried in a vacuum. When it is desired to examine the residue, however, it must be obtained in a flat dish exposing a large surface to evaporation.
626. Estimation of Water.—It is evident that the percentage of water in a fermented beverage is easily calculated when the percentage of alcohol by weight and that of the dry residue are known. In a given case, if the number of grams of alcohol in 100 of the sample be five and that of fixed solids four and a half, the quantity of water therein is 100 - (5.0 + 4.5) = 90.5 grams. In this case the volatile essences are counted as water, but these, at most, are so small in quantity as to be practically unweighable. Nevertheless, it must be admitted that direct drying, in many cases, may give erroneous results, especially when the sample contains an abundance of ethers and of glycerol. The loss which takes place on evaporation may be diminished by adding to the sample, before evaporation, a known weight of potassium sulfate in crystals, which serves to increase the surface of evaporation, to hasten the process and to obtain a quantity of residue in excess of that secured by direct evaporation in an open dish.
627. Total Acidity.—The acidity found in fermented beverages is due both to the natural acids of the materials from which they are made, and to those caused by fermentation. The typical acids also indicate the nature of the original materials, as malic in cider and tartaric in wine. The acids of beers are due almost exclusively to fermentation, and acetic is probably the dominant one. In determining total acidity, it is not always convenient to ascertain beforehand what acid predominates, nor to accurately distribute the acid among its various components. In the analytical work it is advisable, therefore, to estimate the total acid of cider as malic, of wines as tartaric and of beers as acetic. The process of titration is conducted as follows:
Expel any carbon dioxid that is present by continued shaking. Transfer ten cubic centimeters to a beaker and, in the case of white wines, add about ten drops of a neutral litmus solution. Add decinormal sodium hydroxid solution until the red color changes to violet. Then add the reagent, a few drops at a time, until a drop of the liquid, placed on delicate red litmus paper, shows an alkaline reaction.
One cubic centimeter of decinormal sodium hydroxid solution = 0.0075 gram tartaric, 0.0067 of malic and 0.006 gram of acetic acid.