Effect of drying on tartaric acid and malic acid in Shiraz and Merlot berries

Background and Aims Grape berries are dried to concentrate sugar and aroma compounds to produce specific wine styles. This work aimed to characterise biochemical changes during drying of two different cultivars, considering in particular tartaric acid and malic acid evolution. Methods and Results Shiraz and Merlot grapes were dried at nine, 15, 21 and 27°C and berries were sampled every 2 to 3 days and sorted by density using NaCl solutions to account for berry heterogeneity. Mass loss of up to 45%, increase in sugar concentration of 71% and decrease in malic acid concentration of 64% were observed. The TA declined by up to 49%. Conclusion The decline in tartaric acid could be explained by enhanced potassium hydrogen tartrate precipitation inside the berry or during sampling. Significance of the Study The tartaric acid precipitation causes important analytical biases in physiological experiments. This study illustrates that common analytical methods are often inappropriate and are the reason for inconsistent tartaric acid values in many studies.