D-gluconic acid (together with its cyclic form, D-Glucolactone) allows to evaluate the degree of firmness of the grape. It is produced from glucose by fungi and yeast and its concentration increases proportionally to the degree of over-ripening of the grapes as well as in grapes infected with fungi (for example, of the genus Botrytis). It is highly recommended to measure it when degree of humidity is high along the process of maturation of the grape to adapt the winemaking process accordingly.