Proteolytic Activity of Yeast Strains During Grape Juice Fermentation

Secreted protease activity was investigated in several genera of yeast commonly associated with grapes during grape must fermentation. Members of two genera, Kloeckera and Hanseniaspora, produced significant amounts of protease activity and reduced the protein concentration of the juice by approximately one-third. The majority of this activity occurred by day 7 of the fermentation, after completion of exponential growth but before completion of the fermentation, suggesting that the proteolytic degradation products would be available for yeast nutrition during the non-proliferative phase. The amount of protease produced varied with different strains of the same species. Proteolytic degradation of protein did not favorably impact the heat/chill haze forming potential of the wines. Decreases in protein content were not associated with loss of specific protein peaks. Instead, it appeared that all protein peaks were sensitive to proteolytic attack, and that a fraction of each protein species was resistant to proteolytic degradation. Some of the non-Saccharomyces strains actually increased the haze forming potential of the wines which was not correlated with extracellular protein production.