The effect of mechanical harvesting and long-distance transport on the concentration of haze-forming proteins in grape juice

The concentration and composition of soluble, haze-forming protein in juice of hand-harvested (HH) and mechanically harvested (MH) Pinot Noir and Sauvignon Blanc grapes was determined. The major soluble proteins in both varieties were pathogenesis-related (PR) proteins, specifically thaumatin-like proteins and chitinases. Comparison among HH berries, MH intact berries and the mixture of broken fruit and juice, the predominant form of MH fruit, indicated that little if any protein was produced as a result of stress caused by MH. Nevertheless, MH coupled with long-distance transport which caused a delay in pressing the mechanically damaged fruit, resulted in juice with a higher concentration of PR proteins compared to juice obtained from pressing HH fruit. Soluble PR proteins were detected in the berry skin of both varieties. Increases in protein content of juice from MH fruit are predominantly caused by extraction of protein from skins and solids of broken grapes during transport to the winery.