Impact of UV and powdery mildew on protein and phenolic content in Sauvignon Blanc grape and juice

Abstract

This study investigatedcompositional changes in New Zealand Sauvignon Blanc grapes in response to UV exclusion and powdery mildew infection, and any consequential influence on the extraction of haze forming pathogenesis-related (PR) proteins in juice which may influence bentonite requirement for wines.UV exclusion and powdery mildew infection treatments were applied to potted grapevines grown in a glasshouse. For all treatments, grape skin, grape pulp and extracted juice were analysed for changes in concentrations of total phenolics, tannin, total proteins and PR proteins. Lower concentrations of total phenolics, tannin, total proteins and PR proteins were measured in grape skin from the UV exclusion treatment, but no difference in PR proteins was determined in the pulp. Powdery mildew infection of grape berries resulted in a significant increase in PR proteins in grape skin and pulp but had no impact on tannin in skin. The lower concentration of PR proteins in juice from the control treatment of the UV exclusion experiment suggested a loss of PR protein during extraction, possibly due to interactions between phenolics/tannin and proteins. The significantly higher concentration of PR proteins in grapes from the high powdery mildew infection treatment was reflected in the resultant juice. In addition, the lower concentration of tannin in juice from this treatment suggested loss of tannin by interaction with proteins during the extraction.Thus, UV exclusion and powdery mildew infection have significant impacts on Sauvignon Blanc grape berry composition. The concentration of PR proteins in resultant juice is predominantly determined by the concentration of PR proteins in grape pulp, but the concentration of PR proteins in the skin and interactions with phenolics/tannin during juice extraction may modulate the final concentration of PR proteins in juice. As PR proteins are the main cause of haze formation in finished white wine. The findings presented here contribute to improved understanding of the variable levels of PR proteins in juice and resultant wine, and hence variations in bentonite requirement.