Example: - Alcohol 1275 per cent. by volume; fixed acid 205 grams per litre calculated as H1SO4; corrections nil. Then

1 Gay on and Laborde, "Vins," p. 214.

The value of the ratio decreases as the percentage of alcohol increases, since the acid varies inversely as the alcohol. With wines of the same kind, the value of the ratio does not vary much for the same alcoholic strength.

Now if water is added, it does not change the value of the ratio, since both acid and alcohol are diluted to the same extent. But as the alcoholic strength is reduced, this unchanged ratio is not the one which ought to be given, if the wine were genuine. The lower quantity of alcohol, if the wine were not watered, would mean a larger proportion of acid and therefore a greater value of the ratio. Hence a low value is presumptive evidence of the wine having been watered.

In practice, curves are constructed for different classes of wine, with ordinates showing the alcoholic strength, and abscissae the acidity per alcoholic degree. The value obtained for Halphen's ratio is then compared with the curve for wine of the same class. By way of illustration, a few values taken from the mean curve are given here: -

(4). Roos's ratio. - This is obtained by dividing the value obtained in (2) by that given in (1), any necessary corrections being included, as already explained. In other words, the Roos ratio is: alcohol + fixed acid / alcohol / reduced extract evaluated numerically as described in (2) and (1).

For red wines, the value of this ratio is ordinarily equal to or greater than 32, but exceptionally may be as low as 30. For white wines, the minimum value is 24. Values below these limits are presumptive evidence of watering. This ratio brings into the calculation all three of the chief elements in the composition of wine - alcohol, acid, and extract.

Examples: -

(a). Red wine. - Alcohol, 65 per cent.; total acidity, 625; volatile acid, 0 50; reduced extract, 1580.

Then, assuming that there are no other corrections except those included in the reduced extract, we have: alcohol + fixed acid = 65 + (625 - 0.50) = 1225, and alcohol/reduced extract = 6.5 x 8/15.8

= 3.28; whence Roos's ratio = 12.25 / 3.28 = 3.7.

This wine is therefore normal.

(b). Red wine. - Alcohol, 7.8 per cent.; total acidity, 46; volatile acidity, 095, reduced extract, 1560.

As before: alcohol + fixed acid = 78 .+ (4.6 - 095) = 1145; and alcohol extract = 7.8 x 8/15.6=4.0; whence Roos's ratio = 11.45/4= 2.8, and the wine is indicated as being watered.