Monitoring levels of ascorbic acid derivatives in Sauvignon blanc (Vitis vinifera L.) during berry development and in the wine

Abstract

The exposure of plants to unfavourable conditions such as drought, high or low temperatures, heavy metals, salt stress and/or pathogenic attack increases the production of reactive oxygen species (ROS), thus inducing oxidative stress. One of the most important non-enzymatic antioxidants is ascorbic acid, which is used by plants to protect themselves against these toxic oxygen intermediates. The biologically active intermediate of ascorbic acid metabolism, dehydroascorbic acid (DHA), have antioxidant properties of its own. Diketogulonic acid (DKG) can also act as an antioxidant. DHA, DKG and L-L-threonate are ascorbic acid derivatives that are found during ascorbic acid catabolism. Catabolism refers to the breakdown of ascorbic acid when exposed to oxygen, especially in grape juice/wine. The specific aims of the study were (i) to monitor and investigate the level of ascorbic acid derivatives in Sauvignon blanc (Vitis vinifera L.) during berry development until harvest; (ii) to monitor and investigate the effect of canopy management practices in the synthesis of ascorbic acid derivatives; (iii) to determine whether the trends in the development of ascorbic acid derivatives are the same for cooler and warmer areas; and (iv) to determine the influence of ascorbic acid on the pinking phenomena associated with Sauvignon blanc. Grape berries were sampled at different phenological stages according to the Eichorn-Lorenz scale (E-L 32 to E-L 38) from two grape-producing regions for the analysis of ascorbic acid and its derivatives DHA, DKG and L-threonate, with each region having north‒south and east‒west row orientations. Ascorbic acid derivative concentrations between the farms varied significantly. Both farms had high concentrations of L-threonate in all stages of ripeness, of the row direction. The DHA, DKG and L-threonate for Elgin were the highest at E-L 32 and the lowest for Wellington. No significant effect was observed for ascorbic acid derivatives between the row orientation comparisons on each farm block concerning the DHA, DKG or L-threonate content of the grapes. Ripening grape parameters such as pH, titratable acidity and sugar content of the berries were also measured at different grape ripening phenological stages on both farms. These parameters did not show any significant differences within the farm on the different vineyard row orientations (p > 0.05), but there were significant differences between the farms (p < 0.05) on different regions. Significant differences in pinking potential between the two climatic regions were observed in this study, with Elgin wines more susceptible to pinking and Wellington less susceptible. The study also added to the current knowledge that grapes grown in cool climates are more susceptible to pinking than grapes grown in warm climates.