The monosaccharide (simple sugar) is better known as glucose or dextrose. This type of sugar is formed first in the grapes. During vinification at the beginning of fermentation, it is present in the grape must together with fructose (fruit sugar) in a ratio of one to one. Both types of sugar belong to the so-called hexoses and are together known as invert sugar. They are very differently sweet, fructose sweetening about two and a half times as much as glucose. During fermentation, glucose is preferably converted into alcohol and carbon dioxide. That is why fructose dominates the residual sugar in the wine. In contrast to fructose, glucose cannot be broken down by the human body, or only with difficulty, in the case of diabetes (diabetes).
The French scientist Louis Pasteur (1822-1895) reported as early as 1861 that yeasts in an aerobic environment (i.e. presence of oxygen) consume much less sugar during fermentation than in an anaerobic environment. However, higher amounts of glucose in grape must from about 100 mg/l can also produce alcohol under aerobic conditions. This is called the Crabtree effect or "glucose effect" after the English biochemist Herbert Grace Crabtree. With small amounts of glucose, the yeasts in the presence of oxygen breathe in the sugar directly, so it is not converted into alcohol.
At the Geisenheim Research Institute, there are experiments using the enzyme glucose oxidase to convert the glucose in the grape must into gluconic acid and thus reduce the sugar. The gluconic acid cannot be converted into alcohol by the yeasts. This should make it possible to specifically produce wines with a balanced alcohol content to improve the taste. See also under alcohol reduction