Fast enzymatic method for gluconic acid determination
D-gluconic acid (together with its cycled form, D-Glucolactone) is a parameter to assess the degree of firmness of grapes produced from glucose by fungi and yeasts. Its concentration increases in proportion to the degree of over-ripening of the grape as well as in grapes infected by fungi (for example, of the genus Botrytis). Its determination is highly recommended when the degree of humidity is high during the grape ripening process to adapt the winemaking process properly.
The majority acids in the grape are tartaric, L-malic, which represent about 90% of the total, and citric acid. In addition, during the fermentation process, other acids such as L-lactic, succinic or acetic appear that also contribute to total acidity. The total content of acids is important because it gives the wine its organoleptic characteristics, resulting in harsh wines when it is excessively high or flat, when it is too low. In addition, the total content of acids has relevance in terms of the conservation of wine, inhibiting the development of microorganisms.
Most of the lactic acid present in the wine is produced during the malolactic fermentation with the transformation of L-malic acid into L-lactic acid, so that more than 75% of the lactic acid present in the wine is the L isomer. D-lactic acid, on the other hand, is associated with the metabolism of glucose (and other hexose sugars) by the same lactic bacteria (mainly leuconostoc and lactobacillus). A presence of D-lactic acid greater than 0.3 g/L is an indication of bacterial contamination, since these bacteria compete with the yeasts for sugars, until inhibiting alcoholic fermentation.
Tartaric acid is the main component that gives the wine its characteristic acidity, being present both in its acid form and in the form of potassium salt. From the point of view of production it is a critical element in the development of color and flavor during the wine ripening process, as well as its subsequent chemical stability. The determination of tartaric throughout the manufacturing process allows to keep track of its organoleptic evolution.
Citric acid is not very abundant in the grape compared to other organic acids. At the end of fermentation, it can be added to raise the acidity, which increases the efficiency of the sulphites present, and to prevent iron turbidity, since it forms soluble complexes with iron and copper, although this practice has legal restrictions. Citric acid also brings a feeling of freshness to the wine, but in excessive amounts it is unpleasant.