In France the relations between alcohol, acid, and extract have been much studied, with the view of detecting such falsifications of wine as the addition of water or alcohol. Certain numerical values have been deduced to which much importance is attached. With wine, however, as with other products, natural variations in the proportions of the constituents may occur, and the data obtained from the study of normal wines must therefore always be used with circumspection.

In the following description the acidity, according to the French practice, is expressed in terms of sulphuric acid, and the weights of the constituents other than alcohol in grams per litre.

(1)

Ratio of alcohol to extract. - This is given by dividing (a), the total weight of alcohol, actual and potential, per litre, by (b) the weight of the extract in grams per litre, determined by drying at 100°, and suitably corrected as explained below.

(A)

The total weight of alcohol = the actual alcohol present plus what would be given by any sugar in excess of 1 gram still unfermented (1 gram of sugar per litre is regarded as the average quantity remaining in normally fermented wine). It is given by the formula: - Percentage of alcohol by volume X 8 + (wt. of sugar - 1) X 0.45.1

Example: - Alcohol 11.5 per cent. by vol., sugar 370 grams per litre. Then "total alcohol " = 11.5 X 8 + (370 - 10) x 045

= 108.2 grams per litre.

(B)

The "extract" or dried residue of wine will be increased in weight beyond its true value if non-volatile substances such as sugars, chlorides, and sulphates have been added to the wine. Hence to obtain the true value such additions must be deducted from the weight of the extract as actually determined. In the case of reducing sugars, chlorides, and sulphates, 1 gram per litre is regarded as normal to the wine, and only the excess above this is deducted. Besides these ingredients, the extract may contain, for example, added citric acid, added tartaric acid, or added glycerol. Occasionally also mannitol, due to the action of special microorganisms in the fermentation of the wine, may be present. In general, all added non-volatile matters revealed by the analysis should be deducted in calculating the corrected or "reduced' extract.

1 The factor 8 here is the round number used instead of the true factor 7.94 - i.e., the specific gravity of alcohol at 15° multiplied by 10.

Example: -

Hence the "reduced extract " is 60.4 . 42.4 = 18.0.

Then for red wines, the quotient total alcohol .÷ reduced extract falls between 25 and 4 5, and for white wines between 3.5 and 65. It lies outside these limits in exceptional cases only.

For instance, if the examples given above refer to the same wine, the value of the quotient would be 108.2/18.0 =61, which is within the permissible limits for genuine normal white wine, but would be outside them if the wine in question were a red one. An excessive value for the quotient is considered as presumptive evidence that either sugar has been added to the must or alcohol to the wine.

The excess of alcoholic strength clue to these additions can be calculated approximately from the formula

1/8 (total weight of alcohol - reduced extract X 45 or 65), taking the factor 4.5 if it is a question of red wine, or 65 in the case of a white wine.

(2). Sum of alcohol plus fixed acid (A. Gautier). - In a broad sense, the alcohol and the acid in wine are quantitatively complementary, so that the sum of the two, within limits, is constant. This sum lies, in general, between 13 and 17 when calculated as shown below.

Divide by 8 the "total weight of alcohol" obtained as described in the preceding section (1), (a). To the result add the amount of fixed acid and one-tenth of the volatile acid, both expressed in terms of sulphuric acid and in grams per litre. From this total, by way of corrections, the following deductions areto be made: -

(I)

0.2 for each gram of K2SO4 in excess of 2 grams found in the analysis of the wine.

(II)

The value, in terms of sulphuric acid, of the free tartaric acid in excess of 0.5 gram for red wines and 10 gram for white wines.

(III)

The value, in terms of sulphuric acid, of any added foreign acids shown by the analysis.

(IV)

The proportion of alcohol presumed to be due to added sugar or added alcohol when the alcohol-extract ratio exceeds 45 for red wines or 65 for white wines, according to the calculation in (1), above.

With both red and white wines, watering is presumed when the sum "alcohol + fixed acid" falls below 12.5, or in certain special cases, 11.5 (wines of the Aramon plains, in the south of France).

Exceptionally, lower values than 11.5 are found in genuine wine. Blarez1 gives three categories of wine, in which the minimum values of the sum "alcohol + fixed acid ' are respectively 125, 108, and 100.

Whichever minimum value is taken, the percentage of watering is calculated from the formula

where S is the value of the sum taken as standard, and S1 is the value found. Thus if 125 is the standard, and 105 has been found,

(3). Ratio of acid to alcohol (Halphen's rule). - This is another application of the fact noted in (2), that within limits the sum of the alcohol and the acid in normal wine is approximately constant. It follows that the acid varies inversely as the alcohol in quantity. In evaluating the ratio numerically, the acidity taken is the fixed acid plus 01 as the maximum for volatile acid in sound wine. The "alcohol ' is the percentage of alcohol by volume. The same corrections as in (2) are required for any increase of fixed acid or alcohol by added materials.