WINETECH Technical Yearbook 2019

is converted to the weaker lactic acid during malolactic fermentation (MLF).

If tartaric acid is added to a juice with a pH lower than 3,65, a high percentage of the H 2 T is formed. This will liberate a free hydrogen in the wine, which causes the pH of the juice to decrease. An HT - ion will simultaneously be released, which reacts with the potassium to form K-bitartrate, which will precipitate. This precipitation will consequently remove tartaric acid from the solution, which causes a drop in the TA concentration of the solution. The tartaric acid equilibrium in the solution will then require a replacement of the precipitated K-bitartrate by dissociating more H 2 T in the hydrogen ion form, which will cause a further drop in the pH. If tartaric acid is added to a wine with a pH higher than 3,65, the acid occurs mainly

in the HT - form. This ion binds with the potassium and practically all the added tartaric acid precipitates effectively as K-bitartrate. This removes tartaric acid from the liquid and decreases the TA concentration of the wine. The tartaric acid equilibrium is consequently balanced by replacing the precipitated K-bitartrate by hydrogen and T -2 ions to form more HT - ions. This reaction leads to a pH increase. It is consequently important to lower the pH of grape juice as soon as possible to below 3,65 before the fermentation commences. The TA concentration must however not be increased to more than 7,5 g/L. This should lead to wines with a pH of 3,5 to 3,6 and a decrease of TA concentration if 1 g/L malic acid is converted during MLF. If

the TA concentration is above 7,5 g/L the pH will be lowered to 3,4, if it is accepted that much K-bitartrate precipitates during fermentation. In some cases a tartaric addition of more than 4 g/L is required. Such addition is still more cost-effective than alternative procedures like ion removal by resins. If K-bitartrate precipitates during cold stabilisation, the TA concentration will drop to more acceptable levels (Anonymous, 2018). REFERENCE Anonymou s , 2018 . A s k t he AWR I : Winemaking with high pH, high TA and high potassium fruit. Australian and New Zealand Grapegrower and Winemaker , October 2018: 80-81.

Tartaric acid is frequently added to juice to increase its TA concentration. Tartaric acid is however a weak organic acid. If it is added to a solution it dissociates in three different forms, namely tartaric acid (H 2 T), bitartrate (H 2 T - ) and tartrate (T -2 ). Different percentages of these three forms exist at different pHs. In a grape juice with a pH of 3,01, approximately 50% of each of the H 2 T and HT - forms are present. At a pH of 4,37, there is no H 2 T and approximately 50% of each of the HT - and T -2 forms are present. At the average of these two values, namely a pH of 3,65 there is a major shift between the percentages of these forms.