Glutathione addition alters tannin composition and astringency of Petit Verdot wines

Abstract

To understand how phenolic composition and structural modifications impact astringency, research scale (120L) Petite Verdot wines were made to allow for a full-factorial design altering pH (3.25, 3.5, 3.75, 4.0) and oxidative and reductive adjuvants (glutathione, GSH, sulfur dioxide, SO2, and acetaldehyde) in triplicate. HPLC-MS/MS (phloroglucinolysis), HPLC-QToF, and chemical assays were used to assess phenolic chemistry. Descriptive analysis was used to sensorially evaluate a sub-set of the treated wines using a trained panel. Lower pH wines had significantly less anthocyanins while having no effect on protein precipitable tannins or iron reactive phenolics. Only glutathione additions significantly reduced protein precipitable tannins, independent of pH. The oxidative or reductive adjuvants either increased (acetaldehyde) or negated (GSH, SO2) the natural progression of polymeric pigment formation. However, only sulfur dioxide was able to significantly retain the anthocyanins compared to the other treatments and control. Phloroglucinolysis of the control wines, showed the mean degree of polymerization (mDP) was significantly lower in low pH wines likely due to the labile nature of the interflavan bonds. The SO2 treated wines had significantly greater mDPs (1.4-2.3x larger) than the other treatments indicating a preservation effect. The terminal units epicatechin and catechin were impacted by pH and treatment with little to no interaction. The glutathione treated wines had lower concentrations of both terminal and extension units. Further the GSH treated wines uniquely had significantly higher percentages of catechin as terminal unit and lower percentages epicatechin as a terminal unit, which amplified at higher pH. QToF analysis of glutathione-catechin adducts showed a significant increase (29x higher) compared to the control, independent of pH. Flavan-3-ols and procyanidin dimers were only impacted by acetaldehyde treatments with inconsistent effects from pH. Multifactor analysis of sensory and chemical data revealed that the wines were primarily separated by treatment. Astringency and tannin concentration were inversely related to glutathione-treated wines. Surprisingly neither SO2, or acetaldehyde treated wines were different in astringency from the control. Aroma characteristics were only affected by the SO2 treatment, which correlated with baking spice and earthy ortho-nasal attributes.