Reducing Sugars

Sugar is an essential component in the production of wine. During alcoholic fermentation, yeast consume sugars found in the grape juice, or must, and converts it to ethyl alcohol and carbon dioxide. In the case of certain styles of wine such as semi-sweet or dessert wines, some sugar is allowed to remain post-fermentation. This residual sugar can serve to provide a sweeter character to the final blend or play a role in microbial stability.

The primary fermentable sugars found in grapes are glucose and fructose. These two simple sugars are also known as reducing sugars because they contain functional groups capable of being oxidized under certain conditions. After reaction with excess alkaline cupric tartrate (Fehling reagents), the content of reducing sugars can be determined colorimetrically. The Fehling method is not an exact determination but an index of the reducing sugar concentration, because the reaction depends upon the amount and type of reducing sugars present. When the reducing sugar content is known at the beginning of fermentation, the potential alcohol degree can be estimated by multiplying the sugar concentration (in g/L) by 0.06.

Reducing sugars can be measured photometrically with reagents. Photometric analysis is based on the Beer-Lambert principle of absorbance in which the intensity of the color produced is proportional to the concentration of reducing sugars in the sample.

Reducing sugars can also be measured by titration. The primary fermentable sugars, glucose and fructose, contain functional groups capable of being oxidized under certain conditions. It is possible to measure the residual sugar content by a redox titration.