Free Tartaric Acid and Potassium Bitartrate.—In the presence of a small amount of free acids, the detection of a considerable proportion of free tartaric acid may fairly be considered as strong evidence that the wine is artificial. Nessler recommends the following qualitative test:—20 c.c. of the sample are repeatedly shaken with a little freshly prepared and finely ground cream of tartar. After standing one hour, the solution is filtered, 3 or 4 drops of a 20 per cent. solution of potassium acetate are added, and the mixture is allowed to remain at rest for twelve hours, when, in presence of free tartaric acid, a precipitation will take place. The quantitative estimation of free tartaric acid and potassium bitartrate is made by Berthelot’s method, as follows:—Separate portions of the wine (20 c.c. each) are introduced into two flasks, a few drops of 20 per cent. solution of potassium acetate being added to the second flask. 200 c.c. of a mixture of equal parts of alcohol and ether are then added to both flasks, their contents repeatedly shaken and finally set aside for eighteen hours at a temperature between 0° and 10°. The separated precipitates are now removed by filtration, washed with the ether-alcohol mixture, and then titrated with 1⁄10th normal alkali solution. That formed in the first flask corresponds to the potassium bitartrate originally contained in the wine; the second represents the total tartaric acid present. The addition of a small quantity of clean sand will assist in the separation of the precipitates.
Malic Acid.—A slight excess of lime-water is added to 100 c.c. of the wine, and, after standing for some time the solution is filtered, concentrated by evaporation to one-half its original volume, and treated with an excess of absolute alcohol. The resulting precipitate (consisting of calcium malate and sulphate) is collected upon a filter, dried and then incinerated. The proportion of malic acid contained is now estimated by volumetrically determining the amount of calcium carbonate present by means of a normal acid solution: 1 part of calcium carbonate represents 1·34 parts of malic acid (C4H6O5).
Tannic Acid.—10 c.c. of the sample are taken, the free acids present neutralised with normal alkali solution, and a few drops of concentrated sodium acetate solution (40 per cent.) added. A solution of ferric chloride (10 per cent.) is then added, drop by drop, carefully avoiding an excess. A single drop of the iron solution represents 0·05 per cent. of tannic acid. The method of tannin determination described under Tea (see p. [22]) can also be applied.
Succinic Acid.—500 c.c. of the wine are decolorised with bone-black, filtered, the filtrate evaporated over the water-bath nearly to dryness, and the residue repeatedly treated with alcohol-ether. The solution thus obtained is concentrated, carefully neutralised with lime-water, evaporated to dryness, and the glycerine present removed by washing with the alcohol-ether mixture. The remaining residue is now treated with 80 per cent. alcohol, in order to dissolve the calcium succinate contained, every 100 parts of which represent 75·64 parts of succinic acid (H6C4O4). Thudichum and Dupré state that one litre of pure wine contains from 1 to 1·5 grammes of succinic acid.
Ethers.—The compound ethers in wine are volatile and fixed, and exist in but minute proportions. Of the former class, ethylic acetate C2H3(C2H5)O2 is the most important. As already mentioned, the aroma of wine is largely influenced by the presence of the ethers of the fatty acids, butyric, caprylic, etc. Dupré determines the proportion of both kinds of ethers indirectly as follows:—250 c.c. of the wine are distilled until 200 c.c. have passed over. Water is then added to the distillate to a volume of 250 c.c. 100 c.c. are first titrated with 1⁄10th normal soda solution. Another 100 c.c. of the distillate are next heated with a known quantity of alkali (by which the ethers are decomposed into their corresponding acids and alcohol), and the titration is repeated. The amount of volatile ethers is then calculated from the increased acidity shown by the second titration. In order to determine the proportion of fixed ethers, 500 c.c. of the sample are evaporated over the water-bath to a small volume which is made alkaline, and then subjected to distillation. The distillate is acidulated with sulphuric acid and again distilled. The alcohol present in the second distillate is now oxidised to acetic acid by means of potassium dichromate, and the amount of this acid found estimated by titration. According to Berthelot, the proportion of ethers in genuine wine bears a fixed relation to the amounts of alcohol and acids present: he suggests the following formula for calculating the amount of alcohol contained in the compound ether of one litre of wine, when etherification is complete:—
y = 1·17 A + 2·8
x = y × a 100 ,
where A is the percentage, by weight, of alcohol; a the amount of alcohol equivalent to the total free acid in one litre of wine (assuming this to be acetic acid); y, the proportion per cent. of a present as compound ether in one litre of wine, when the alcoholic strength of the wine is A; and x, the amount of alcohol present in the compound ether of one litre of wine.
The Ash.—100 c.c. of the wine are evaporated to dryness in a platinum dish, over the water-bath, and the residue is incinerated at a rather low temperature and weighed. By this process, the tartrates and malates contained in the wine are converted with carbonates. The ash of normal wine consists of potassium sulphate, carbonate, phosphate and chloride, sodium chloride, calcium carbonate, etc., but, in many samples, it will be found to be largely if not entirely composed of sulphates, which is due to the practice of sulphuring and plastering.[102] Generally speaking, the proportion of ash in genuine wine ranges from 0·15 to 0·30 per cent.
Chlorine.—100 c.c. of the sample are neutralised with sodium carbonate, evaporated to dryness, and the residue gently ignited. It is then extracted with boiling water, filtered, and the chlorine determined by means of silver nitrate, either volumetrically or gravimetrically.