Can the post-véraison grape berry metabolize tartaric acid?

Abstract

In contrast to malic acid, few studies relate to the metabolism of tartaric acid (TA) under abiotic stress in ripening berries. TA biosynthesis is well known with its metabolism believed to be limited from berry set until véraison and therefore not impacted by ripening conditions. However in previous (unpublished) berry drying experiments, as well as in some studies in the literature TA content seems sometimes to decrease post-véraison.

To investigate this hitherto never reported phenomenon, experiments were conducted in climatic chambers at different temperatures (9, 15, 21, 27 °C) with two varieties (Syrah, Merlot) for 30 days. Samples were drawn every two to three days and sorted in a NaCl gradient to circumvent berry heterogeneity. Weight, sugars, organic acids, potassium and ethanol were determined by HPLC, enzymatically and by WineScan. Weight loss was highest at 27°C with 45% (Syrah) and 32% (Merlot). Sugar concentration increased 71% (Syrah) and 43% (Merlot) and malic acid per berry degraded by 64% (Syrah) and 62% (Merlot). Similar to earlier experiments a decrease in TA was observed for Syrah (49%) and Merlot (36%). However a potassium per berry decrease could be observed to a similar extent as TA decline. This indicates an enhanced potassium tartrate precipitation which would occur in intact berries or during sample processing. Precipitation in intact berries seems unlikely due to compartmentation of the metabolites, however an increasing cell death in drying berries could provoke a potassiumbitartrate precipitation.  A systematic almost linear increase in precipitation during sample processing appears also rather unlikely. These results together with other similar observation in the literature indicate that post-véraison TA metabolism and/or precipitation processes in the berry and during sample preparation are not fully understood so far and need further investigation.