Analysis of Wine.—The analysis of wine comprises the following estimations:—Specific gravity, alcohol, extract, sugar, polarisation, glycerine, total free acids, volatile acids, free tartaric acid, potassium bitartrate, malic acid, succinic acid, tannin, ethers, ash, chlorine, sulphuric and phosphoric acids, and colouring matters.
Specific gravity.—The density is determined by means of the gravity bottle, at a temperature of 15°.
Alcohol.—The proportion of alcohol is ascertained by the distillation of 50 or 100 c.c. of the wine in a suitable flask, which is connected with a Liebig’s condenser, until about half of the liquid has passed over. The distillate is made up to the original volume with water, and its specific gravity taken, from which the amount of alcohol (by weight) present is calculated by aid of the usual alcohol-metric tables (see p. [196]). The result (as well as the proportions of the other constituents) is preferably stated in grammes per 100 c.c. of wine. The determination may also be made by first removing the alcohol by evaporation, adding distilled water to restore the original volume, and then estimating the density of the liquid (see under Beer, p. [142]). In unfortified wines the alcoholic strength ranges from 6 to 12 per cent., and in wines which have received an addition of spirit, it may vary from 12 to 22 per cent.
Extract.—The extract is conveniently determined by evaporating 50 c.c. (measured at 15°), in a platinum dish over the water-bath, the residue being dried for 2½ hours in the steam-oven. In case a wine rich in sugar (containing, say, over 0·5 grammes per 100 c.c.) is under examination, 20 c.c. will suffice for the determination. The indirect method used in the estimation of the malt extract in beer may also be employed. According to Girardin and Pressier, it is possible to detect the watering of certain wines, the average composition of which is known, by means of the proportions of extract and alcohol present. For example, in genuine Bordeaux wines the proportion of extract ranges from 20 to 20·8 grammes per 1000 c.c., and the amount of alcohol is also very constant, it being a mean of 100 grammes per 1000 c.c. Should a sample of Bordeaux wine show an extract of 14·5 grammes per litre, the proportion of genuine wine present would be 72·5 per cent., for 1000 × 14·5 20 = 725·00, the remainder being water and alcohol. In order to estimate the amount of spirits artificially added, the alcohol contained in 72·5 parts of the wine is determined. If, for instance, it is found to be 11 parts, then, (11 - 7·25 = ) 3·75 parts of alcohol have been added.[101] The quantity of extract in pure natural wine varies from 1·5 to 3 per cent., but in sweet and fortified wines, it may reach 10 per cent. or more.
Sugar.—The sugar in wine consists of a mixture of fruit and grape sugar, usually in the proportion of 3 parts of the former to 1 part of the latter. The amount of sugar is best estimated by Fehling’s solution (see p. [111]). In the case of white wines, it is advisable to employ 100 c.c. for the determination; with sweet rich wines 25 c.c. are sufficient. The alcohol is first removed by evaporation over the water-bath, and the diluted liquid is next decolorised by means of bone-black or plumbic acetate, filtered, and made alkaline by addition of sodium carbonate. It is then made up to a volume of 200 c.c. and gradually added to 10 c.c. of Fehling’s solution. It is always well to test the wine by the polariscope, and, whenever the presence of cane sugar is indicated, to invert 100 c.c. of the sample by heating with a few drops of hydrochloric acid, and again make a sugar determination with Fehling’s reagent after neutralisation with sodium carbonate.
Polarisation.—The optical examination of wine is conducted by adding 20 c.c. of plumbic acetate solution to 100 c.c. of the sample, shaking the mixture, allowing it to stand for a short time, and passing it through a filter. If necessary the filtrate is further decolorised with animal charcoal and again filtered. The polariscope tube is then filled with the clear solution and the reading made. The majority of wines exhibit a left-handed polarisation, which is due to the fact that, as a rule, the proportion of fruit sugar present predominates over that of grape sugar; moreover, ½ part of fruit sugar will neutralise the dextro-rotary action of 1 part of grape sugar. In case the presence of an excess of grape sugar is indicated by the polariscopic examination, it is often assumed that this body has been directly added to the wine. It sometimes occurs, however, that, in the fermentation process, more grape sugar remains undecomposed than fruit sugar, under which circumstances the preponderance of the former body in the resulting wine would not prove sophistication; but, under ordinary conditions, the presence of an excessive proportion of grape sugar may safely be regarded as strongly pointing to the artificial addition of must syrup.
Glycerine.—100 c.c. of the wine are reduced by evaporation on the water-bath to 10 c.c., some pure sand added, and then milk of lime to decided alkaline reaction, after which the mixture is evaporated nearly to dryness. When cold, the residue is thoroughly agitated with 50 c.c. of 96 per cent. alcohol, next heated to boiling on the water-bath, and then passed through a filter. The insoluble residue is repeatedly washed with more hot alcohol, the washings being added to the first filtrate. The solution is now evaporated until it assumes a viscous consistency. The residue is taken up with 10 c.c. of absolute alcohol, and 15 c.c. of ether are added, the mixture being shaken and allowed to stand at rest in a well-stoppered flask until it becomes clear. The solution is subsequently filtered into a tared glass capsule, then carefully evaporated to a syrupy condition over the water-bath, and the residue dried in the steam-oven for one hour, and finally weighed. According to Pasteur, 112·8 parts of grape sugar yield 3·6 parts of glycerine; in natural wine, therefore, the glycerine should amount to about 1⁄14th part of the alcohol present.
Acids.—The acids in wine consist of acetic, tartaric, malic, tannic, succinic, racemic, formic, and propionic.
Total free Acids.—These are determined by titrating 10 c.c. of the sample with 1⁄10th normal soda solution, litmus paper or tincture of logwood being employed as the indicator. Wines containing free carbonic acid should be repeatedly well-shaken before making the estimation. The free acids are expressed in terms of tartaric acid (C4H6O6). If sulphuric acid or potassium bisulphate is present, a piece of filter paper will be rendered brittle when immersed in the wine for some time, and afterwards cautiously dried.
Volatile Acids.—The volatile acids are estimated by slowly evaporating 10 c.c. of the wine to the consistency of a syrup, and repeating the titration with 1⁄10th normal alkali solution. The difference in acidity represents the proportion of volatile acids present, which is stated in terms of acetic acid (C2H4O2). It is evident that the non-volatile acids can be calculated by deducting from the total amount of free acids, the tartaric acid corresponding to the acetic acid found. The proportion of volatile acid in genuine wine varies from 0·3 to 0·6 per cent. According to Dupré, in white wine, one-fourth of the total acidity should be due to volatile acids, and in fortified and red wine, they should not exceed a proportion of one-third.