Armando Vega Osorno and Federico Casassa*
*Wine and Viticulture Department – Cal Poly San Luis Obispo, 1 Grand Avenue, San Luis Obispo, CA 93410 (lcasassa@calpoly.edu)

Whole-cluster additions at 100% rate (WC) and green (GS) and dry stems (DS; dried for one hour in an industrial convection oven at 95°C) added at 100% rate according to cluster fresh weight were investigated in triplicate fermentations of Syrah and Tannat wines from the Santa Maria Valley and Paso Robles AVAs, respectively. Fruit differed in basic chemistry and percentage of stems (Syrah: 5.16%, Tannat: 8.99%). The winemaking treatments did not affect volatile or titratable acidity, but WC, GS, and DS increased wine pH by ~0.11 units on average relative to Control wines, while alcohol levels decreased by 0.27% and 0.49% in GS and WC wines, respectively. WC and GS decreased anthocyanins and wine color in both varietal wines; DS did so to a lesser degree, even increasing anthocyanins in Syrah wines at pressing. This suggests reduced anthocyanin extraction in WC fermentations and dilution with vegetal water and/or electrostatic adsorption of anthocyanins into stems in GS fermentations. These effects were mitigated by DS. In Tannat, tannin extraction increased by an average of 66% in GS and DS wines, but in Syrah, tannin extraction increased by 191% (almost two-fold) and by 363% (almost 3.6-fold) in GS and DS wines, respectively, suggesting that Syrah stems are particularly rich in tannins and that drying favors tannin extraction and retention into wine. In spite of different anthocyanin to tannin ratios resulting from either the specific variety (0.52 in Syrah, 0.83 in Tannat) or winemaking technique, there were no effect of the winemaking treatments on total polymeric pigments. Overall, GS and DS generally doubled the tannin content of the wines while WC moderated the extraction and retention of both anthocyanins (relative to Control wines) and tannins (relative to DS and GS wines).

Funding Support: None