This section is from the book "Alcohol, Its Production, Properties, Chemistry, And Industrial Applications", by Charles Simmonds. Also available from Amazon: Alcohol: Its Production, Properties, Chemistry, And Industrial Applications.
Deduct 2 from the extract percentage; the remainder is approximately the percentage of sugar. If this is not greater than 1, the wine is taken without dilution for the further operations. If greater than 1, dilute a convenient quantity so as to bring the percentage of sugar down to 1 or less. In either case take 100 c.c. of the wine, diluted if necessary, neutralise it very carefully with sodium hydroxide solution, add a drop of dilute acetic acid, and evaporate it in a porcelain basin to one-fourth its original bulk in order to expel the alcohol. Cool, and make up the liquid to the original volume in a flask which will hold more than 10 c.c. above the 100 c.c. mark. Add 10 c.c. of lead subacetate solution, shake, and filter. To 66 c.c. of the filtrate add 6 c.c. of a saturated solution of sodium sulphate, shake, and again filter: this filtrate serves for the sugar estimations. By the addition of the lead acetate and sodium sulphate solutions the volume has been increased, in effect, by one-fifth; this must, of course, be taken account of in the calculations, as well as the preliminary dilution of the wine, if any.
(1). Reducing sugars. - Twenty-five c.c. of the prepared wine are taken, and the reducing sugars determined gravimetrically or volumetrically by means of Fehling's solution in the usual way. The polarimetric value of the uninverted solution is also determined, using the 2-dcm. tube.
(2). Saccharose. - Twenty-five c.c. of the prepared filtrate are inverted by the Clerget method. The liquid is placed in a 50 c.c. flask, 25 c.c. of strong hydrochloric acid are added, a thermometer is placed in the liquid, and the flask heated in a beaker of water till the thermometer shows 68°. The temperature is kept between 68° and 70° for five minutes, when the flask is cooled, the solution nearly neutralised with caustic soda, and made up to 50 c.c. Of this, 40 c.c. may be taken for the gravimetric or volumetric determination of the invert-sugar by the Fehling process, and the remainder used for obtaining the polarimetric value after inversion, if this is required.
When the sugars are not estimated, we may proceed as follows. In the case of white wines, take 60 c.c, neutralise carefully, evaporate on the water-bath to two-thirds the original volume, restore the volume to 60 c.c., add 3 c.c. of lead subacetate solution, and filter. To 315 c.c. of the filtrate add 15 c.c. of a saturated solution of sodium sulphate or carbonate, filter, and polarise in a 200 mm. tube. The volume is, in effect, increased by one-tenth through the additions made, and the reading of the instrument must be corrected accordingly.
With red wines, 60 c.c. are evaporated to one-third after neutralisation, filtered, made up to the original volume, treated with 6 c.c. of lead subacetate, and re-filtered. To 33 c.c. of the filtrate, 3 c.c. of sodium sulphate solution are added, the liquid again filtered, and then polarised. In this case, the volume has been increased by one-fifth.
Red wines are sometimes difficult to "clear ' satisfactorily. If the treatment with lead acetate does not suffice, animal charcoal may be used, in the proportion of 5-6 grams per 100 c.c. of wine. The neutralised sample, evaporated to one-half, is digested or the bath for about a quarter of an hour, and filtered. The washing of the charcoal must be thorough: 200-250 c.c. of water may be required. The washings are kept apart from the main filtrate and evaporated to a small bulk, then added, and the whole made up with water to the original volume.
J. Laborde1 recommends treatment with potassium permanganate for decolorising red wines. A saturated solution of this salt is added drop by drop to 100 c.c. of the wine, kept stirred, until the red colour changes completely to brown, giving in general a precipitate of oxidised tannins. The volume required is noted - it is rarely more than 4 c.c.; then 5 c.c. of lead subacetate are added and the volume completed with water to 110 c.c. After being well
1 "Vins," p. 166, stirred, the liquid is filtered, giving a clear and colourless, or slightly yellow, filtrate.
If the extract has been determined by the evaporation method, the residue obtained may be used for the estimation of the ash. Otherwise take 25 c.c. of the sample for this determination. It is often convenient, with wines containing much sugar, to char and extract the mass with water, as described for Beer (p. 478).