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.
Malic acid, both in free and esterified form, is usually present in ripening fruit and is responsible for the astringent taste of the unripe fruit. Its concentration is reduced as it matures. In the process of winemaking, more than 30% of the malic acid is transformed by a process of fermentation in lactic acid (malolactic fermentation) that helps to reduce the initial acidity of the must. Control of the malic acid level is necessary to maintain the desired taste, acidity and astringency characteristics of wine.
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.