Author(s): Alice Vilela
As weak organic acids, carboxylic acids partially dissociate in aqueous systems, like wine, establishing equilibrium between uncharged molecules (undissociated form) and their anionic form, according to the medium pH and their pKa. This property influences yeasts cell-behaviour, particularly the mechanisms by which the molecules can cross biological membranes. Occasionally wines may present an excessive amount of organic acids. In the mouth they will seem unbalanced and sometimes excessive sourness diminishes their quality. Moreover, these acids originated from grapes or from the fermentation process itself, negatively affect wine yeasts, yeast fermentation process and the final wine quality. Two of those acids are L-malic acid and acetic acid. The first one affects the wine mainly in his tastiness, making it much to sour; the second one, being a volatile compound, besides the excessive sourness, also imprints the wine with an unpleasant vinegar flavour. One approach to solving this problem is biological deacidification by Saccharomyces and non-Saccharomyces wine yeasts. To these biological processes of wine acidity bio-reduction we can call wine bio-demalication (malic acid bio-degradation) and wine biodeacetification (acetic acid consumption by yeasts).
