Investigation of the Impact of Grapevine Red Blotch Virus (GRBV) on Phenolic Extractability and Winemaking Strategies to Mitigate
Since its identification in 2012, grapevine red blotch (RB) disease has been found to be widespread in the United States1 . This disease is caused by grapevine red blotch virus infection of grapevines2 . Previous research in the Oberholster lab indicates mostly a substantial impact on berry ripening in all varieties studied, along with variable impacts on primary and secondary metabolites depending on site and season, which had a larger impact than variety3–5 . RB diseased grapes show transcriptional suppression of primary and secondary metabolic pathways, specifically restricting the biosynthesis and accumulation of phenylpropanoids and derivatives6 . Our research indicates a clear trend of decreasing anthocyanin content in red grapes where the impact on sugar accumulation was severe (16 to 20% reduction). There were also significant differences in the volatile composition of RB(+) versus RB(-) grapes, with mostly a suppression in aroma compounds due to RB disease.
During grape ripening, multiple factors may influence phenolic extractability such as interactions with cell walls, cell integrity, individual phenolic concentrations and interactions with each other7–10. It is commonly accepted that pectolytic enzyme degradation of skin cell walls during grape ripening increases the extractability of anthocyanins. However, GRBV impact on cell wall composition has yet to be investigated. Studies have shown that grapes with increased anthocyanin and skin tannin concentrations resulted in wines with higher tannin concentration, deeper color and better ratings by wine judges11. The question is whether winemaking protocols used to increase phenolic extraction from diseased grapes such as maceration enzymes and extended maceration can compensate for extractability differences and compositional differences. Prior research focused mostly on determining the impact of RB disease on wine composition and the grapes when harvested when the healthy controls reached 25°Brix12. Subsequent, studies determined the impact of longer hangtime of RB(+) grape to reach 25°Brix and found that extractability improved greatly resulting in wine with improved phenolic content13,14. Therefore, further research is needed to understand whether the impact of GRBV decrease with increasing ripeness levels, such as 27ºBrix for healthy vines, at which most commercial harvests occur.
Thus, the aim of the current study is two-fold. The first is to investigate the impact of GRBV and ripening on phenolic extractability by determining the changes in grape cell wall composition and how this relates to the release of phenolics under winemaking conditions. The second is to determine whether winemaking protocols such as enzyme addition and extended maceration can increase the extractability of RB(+) grapes resulting in wines more similar to those made from healthy grapes. Characterization of grape cell walls is underway. Preliminary data indicate that both the use of maceration enzymes and extended maceration increased the tannin concentrations in RB(+) fermentations compared to the controls, although not for all ripeness levels. Wines were bottled in February. However, due to the coronavirus pandemic, both the wine phenolic composition and sensory characteristics have not been completed.