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.
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.
Anthocyanins are primarily responsible for the color in red wine and especially contribute to the astringency sensation of the wine. At the beginning of the maturation process they form simple complexes with other compounds present in the wine giving rise to a progressive change in the color of the wine (turning to blue tones) but as maturation progresses these compounds tend to form other compounds (often polymers) a lot more stable in color with respect to pH and sulphites. The proportion between the different compounds present (simple and polymeric) is directly related to the final color obtained and its stability. This determination allows the concentration of non-polymeric (ionizable) anthocyanins to be established.