Impact of drought stress on concentration and composition of wine proteins in Riesling

Protein haze in white wines is a major technological and economic problem for the wine industry. In particular, an increased haze formation can be observed in wines from dry vintages. Therefore, field tests were carried out in steep slope vineyards planted with Riesling grapes over three dry growing seasons to study the effect of drought stress on the concentration of proteins in the resulting wines. Plots suffering from drought stress were compared with the surrounding drip-irrigated plots. Riesling grapes were processed into wines by conventional procedures. Protein amounts of the isolated wine colloids of the stressed samples (S) were always higher than those of the watered samples (W) (mean watered 13.8 ± 0.44, mean stressed 17.4 ± 0.40 g 100 g−1 isolated colloids). As a consequence, higher bentonite doses were needed to achieve protein haze stability of the drought-stressed treatments. Concerning the protein composition, there were no significant differences between stressed and watered treatments. Sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) revealed that the molecular weights of proteins ranged from 12 to 75 kDa with an accumulation of chitinases and thaumatin-like proteins between 20 and 30 kDa. Concerning the protein composition, minor differences became obvious only between vintages but not between the stressed and watered samples. In-solution digest of proteins from Riesling grapes 2008 followed by LC-MSn and database research identified 15 proteins originating from grapes and 10 from the yeast Saccharomyces cerevisiae. In conclusion, our study demonstrates the importance to develop new strategies to prevent increased haze formation in wine under the predicted conditions of climate change. Prevention of drought stress by drip irrigation can be considered as one strategy to decrease protein amounts in wines.