Extraction of pathogenesis-related proteins and phenolics in Sauvignon blanc as affected by grape harvesting and processing conditions

Abstract

The composition of wine is largely determined by the composition of pre-fermentation juice, which is influenced by extraction of grape components. Different grape harvesting and processing conditions could affect the extraction of grape components into juice. Among these grape components, pathogenesis-related (PR) proteins are of great concern for white wine maker as they are the main cause of haze formation in finished white wine. If not removed before bottling, these PR proteins may progress into haze through the formation of complex with phenolics under certain conditions. Thaumatin-like proteins (TLPs) and chitinases are the main constituents of PR proteins found in protein haze. The aim of this study was to investigate the effects of grape harvesting and processing conditions on extraction of haze-forming PR proteins and phenolics, and the consequent impact on protein stability of wine. Sauvignon Blanc grapes were harvested by hand and by machine in 2011 and 2012 in Marlborough, New Zealand. Hand harvested grapes were processed by two different treatments: whole bunch pressing without skin contact, and pressing after destemming and crushing followed by 3 h skin contact; machine harvested grapes were processed by pressing after destemming and crushing followed by 3 h skin contact. For each treatment, the juice pressing procedure was carried out by gradually increasing the pressing pressure to three different levels (0.4 MPa, 0.8 MPa and 1.6 MPa), and juice samples were collected at each pressure point and bottle-fermented separately. Results of this study showed that pressing after destemming and crushing followed by 3 h skin contact can result in greater extraction of proteins and phenolics from grapes into juice compared with whole bunch pressing. Juice and wine obtained from machine harvested grapes showed a tendency of having lower concentration of proteins, as well as, PR proteins. Juice and wine obtained from higher pressing pressure showed the highest concentration of phenolics but the lowest concentration of proteins, suggesting that proteins in grapes are easily extracted at low pressing pressure, but greater extraction of phenolics in grapes requires higher pressing pressure. Analysis of wine protein stability showed a linear correlation between bentonite requirement and the concentration of chitinases in wine, indicating the importance of removal of chitinases to achieve protein stabilization. The findings presented here contribute to an improved understanding of the variable concentration of haze-forming PR proteins in juice as affected by grape harvesting and processing conditions, and hence the variation in bentonite requirement for resultant wine.