Pathogenesis-related proteins in grapes and their extraction as affected by UV radiation and powdery mildew infection

To keep the wine clarity, protein stabilization prior to bottling is important for white wine production. Pathogenesis-related (PR) proteins, originally derived from grape berries, are the main cause of protein haze formation. Thaumatin-like proteins (TLPs) and chitinases are two groups of PR proteins found in finished white wine. Thus, it’s important to understand the distribution of TLPs and chitinases in grape berries, as well as, how their extraction can be affected during juicing process. In this study, total proteins have been extracted from grape skin, pulp and seed respectively, and analysed by liquid chromatography electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). The extraction of TLPs and chitinases has been analyzed as grapes being affected by UV radiation and powdery mildew infection. The proteomic analysis indicated that TLPs and chitinases are present in both grape skin and pulp, but not in seed. The UV exclusion treatment resulted in lower concentration of phenolics, tannins and PR proteins in grape skin, but had no effect on PR proteins in pulp. Powdery mildew infection of grapes resulted in a significant increase in PR proteins in grape skin and pulp but had no impact on tannins in skin. A lower concentration of PR proteins was observed in the corresponding juice from the control of the UV exclusion experiment. The significantly higher concentration of PR proteins in grapes from powdery mildew infection treatment was reflected in the resultant juice. These results suggested the concentration of PR proteins in juice is predominantly determined by their concentration in grape pulp, but their concentration in grape skin and interactions with phenolics/tannins during the juicing process may modulate their final concentration in juice. The findings presented here contribute to an improved understanding of the variable concentration of PR proteins in juice and resultant wine, and hence the variation in bentonite requirement