The effects of UV-B and water deficit on grape amino acids and wine aroma compounds

Abstract

There are significant correlations between amino acid (AA) concentrations in grape must and wine aroma compounds. Solar radiation and water availability factors can affect fruit AA concentration and therefore influence the evolution of aroma compounds during alcoholic fermentation. This thesis investigates the effects of altering grapevine UV-B exposure and water availability on wine aroma compounds, focussing on the role of AAs in the process. Pinot noir vines growing in the Lincoln University vineyard and own-rooted, potted Pinot noir vines growing in a glasshouse were use in this study. In the glasshouse, different UV-B (+UV and -UV) and irrigation (+W and -W) treatments were established from veraison. A field trial water deficit treatment was not able to be put in place due to rainfall events, but fully exposed fruit (LR) was compared to fruit shielded from UV-B using PETG screens erected over the fruiting zone (PETG) and fruit shielded from full sun using shadecloth (SC). As small amounts of fruit were available for this study, micro-vinifications of 160g and 100g for the field and glasshouse study, respectively, were applied. HPLC analysis of wine AAs indicated UV-B radiation significantly decreased total AA concentrations and most individual AAs by a range of 41% to 93%. Water deficit in the glasshouse study increased total and some individual AAs by a range of 56% to 90%. This provided a good basis on which to investigate impacts of fruit AAs on wine aroma compounds. SPME-GCMS analysis of wine showed that concentrations of higher alcohols, fatty acids, esters and norisoprenoids were related to the consumption of AAs during yeast fermentation. From the pattern of results it can be concluded that direct catabolism of AAs into aroma compounds is less likely than their having an indirect effect on aroma compounds formation during alcoholic fermentation.

As relationships exist between AA consumption during fermentation and wine aroma compounds, the effects of UV-B and irrigation treatments on aroma compounds (via affecting grape AAs) deserved additional investigation. In the glasshouse trial higher alcohols, fatty acids, esters, monoterpenes and C13-norisoprenoids were affected. +UV decreased the concentration of phenylethyl alcohol, hexanoic acid, ethyl acetate, ethyl isobutyrate, ethyl butanoate, ethyl isovalerate, ethyl hexanoate, and ethyl decanoate, and isoamyl acetate in wine, but did not affect isoamyl alcohol. UV-B exposure increased the concentration of linalool and decreased that of citronellol. On the other hand, water deficit in the glasshouse experiment increased most esters. For the field experiment β-damascenone in wines was unaffected by LR and PETG, but decreased by SC. Principal Components Analysis showed distinct separation of treatments: for example -UV-W tended to be more dominated by compounds related to fruity and floral aromas.

These results suggest sunlight (and thus UV-B) exposure and water deficit have direct effects on fruit AAs, which leads to significant differences in some wine aroma compounds. Moreover, UV-B radiation and water deficit combined had interactive effects on wine aromas. Based on these results, water deficit could amplify the positive effects of UV-B exclusion on the formation of aroma compounds.