Effect of canopy manipulation and fermentation on grape aroma components
Several experiments were conducted to determine optimum methods for extraction, isolation and analysis of selected aroma components and the influence of grapevine canopy manipulation and fermentation on those components.
A polymeric styrene resin (XAD-2) was evaluated for its ability to absorb and desorb five monoterpene alcohols, three monoterpene hydrocarbons, four monterpene oxides, two aromatic alcohols and a glucopyranoside from White Riesling juice at two different pH values. The percent recovery and the coefficients of variation for each compound was compared with a continuous Freon 11 extraction system. The percent recovery averaged 90% or greater for both systems with the coefficient of variation being smaller with the resin extraction.
In two separate studies canopy manipulation was evaluated for the effect on aroma components using the XAD-2 resin isolation procedure. The influence of shoot topping to 10 or 20 nodes or ethephon application on grape aroma components was measured for three seasons. Canopy modification by both topping levels and ethephon treatment increased sunlight penetration into the canopy fruiting zone. Free volatile terpenes (FVT) were increased by ethephon in two of three seasons while shoot topping increased FVT and potentially free volatile terpenes (PVT) in one of three seasons.
In the second separate three-year study, two to four leaves were removed from the fruiting zone of grapes grown on two training systems. Selective leaf removal increased sunlight penetration into the grape canopy but generally did not influence FVT. However, PVT was frequently higher in the leaf-pulled fruit including four of six commercial harvest dates. The total quantity of the bound geraniol, nerol, linalool, and a terpineol was higher in fruit from the leaf-pulled vines at harvest.
Four strains of Saccharomyces cerevisiae were evaluated for their influences on free and conjugated aroma components of White Riesling grapes, immediately following and 45 days post-fermentation with lees or Sur lie. Fermentation generally reduced free terpenes except for «- terpineol, hotrienol, citronellol, and linalool oxides. Fermentation also increased free benzyl and 2-phenylethanol. In newly fermented and aged wines the concentrations of free volatiles were always below the sensory threshold for each compound. The potentially volatile terpenes (PVT) were Similar among treatments following fermentation, the exception being the Fermiblanc (FB) yeast strain. Additional hydrolysis of bound compounds occurred in each wine following lees storage, the exception being the wine fermented with the Fermiblanc (FB) strain.