Runze Song,* Cristina Medina-Plaza, Arran Rambaugh, Mysore Sudarshana, and Anita Oberholster
*University of California, Davis, 403 Russell Park, Apt 06, Davis, CA, 95616 (rrzsong@ucdavis.edu)

Grapevine red blotch virus (GRBV) significantly impacts berry ripening and vari­ably influences primary and secondary metabolites, depending on site and seasonal factors. Recent studies showed wines made from red blotch (RB) diseased fruit left longer on the vine to reach a similar sugar level as their healthy counterparts achieved riper fruity flavors and better mouthfeel. Research showed this was partly due to increased phenolic extraction during winemaking, instead of higher grape phenolic content or ethanol production. The impact of RB disease on cell wall compo­sition is unknown. Therefore, this study investigated the impact of GRBV and ripening on cell wall composition and phenolic extractability during winemaking. In this work, phenolic extractability of Merlot from Napa County, CA, was investigated over two continuous vintages (2019 and 2020), with grapes harvested at 23, 25, and 27 Brix. The phenolic profile of samples obtained by exhaustive extractions and microfermen­tations of grapes were determined by Adam-Harbertson assay and RP-HPLC. Cell wall (CW) materials isolated from berry skins were subsequently characterized (lignin, lipids, uronic acids, protein, phenolic, total glucose, cellulose, and non-cellulosic glu­cose). With partial least square regression analysis, the relationship between phenolic extractability and cell wall composition was investigated. Results confirmed that longer hang time increased phenolic extractability of RB-diseased grapes, potentially because GRBV causes delayed ripening and reduced degradation of skin cell walls. This was further substantiated by the larger amount of CW found in GRBV grapes. Furthermore, CW characterization indicated that GRBV-infected fruit contained more soluble proteins and pectins, which could bind with phenolics and decrease extraction under winemaking conditions. Further study will investigate whether these are pathogen-related proteins as a result of the plant defense mechanism against the virus and ways to mitigate impacts of GRBV on grape and wine composition.

Funding Support: American Vineyard Foundation