Acetic acid is the main indicator of wine deterioration although in small quantities (less than 300 mg/L) contribute to endow it with organoleptic characteristics through the formation of esters and other compounds. It is produced, mainly from the oxidation of ethanol, by certain bacteria (especially of the genus Acetobacter). The assessment of acetic acid allows monitoring possible deterioration situations along the elaboration process.
L-Lactic acid appears as a product of malolactic fermentation by the action of lactic acid bacteria in secondary fermentation. This process reduces perceived acidity as L-Malic is transformed into the softer L-Lactic, adding dairy flavour (milk, butter, cheese) and is desirable in many red wines, as it provides a feeling of mouth fullness.
Enzymatic method for glucose and fructose measurement
D-Glucose and D-Fructose are the main reducing sugars present in grapes and other fruits. Its determination in the grape allows to verify their state of maturity to establish the optimum moment of harvest. In the must, allows to estimate the amount of alcohol that will be produced during the fermentation. Finally, at the end of the fermentation, to assess the remaining sugar that could produce an undesired fermentation.
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.
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.