Acid Washing and Serial Repitching a Brewing Ale Strain of Saccharomyces cerevisiae in High Gravity Wort and the Role of Wort Oxygenation Conditions†

Acid washing pitching yeast is an effective method for removing bacterial contamination, but if the yeast is washed incorrectly decreased fermentation performance and beer quality problems may result. Various factors can affect the acid resistance of yeast strains during brewery fermentations. Yeast from shaking flask experiments was more resistant to the combination of high gravity and acid washing conditions than yeast cropped from static fermentations. Yeast harvested from static high gravity wort (20° Plato; 1.083 OG) fermentations was more adversely affected by acid washing than yeast from standard gravity (12° Plato; 1.048 OG) wort. Wort oxygenation resulted in enhanced yeast fermentation performance and healthier yeast crops when yeast was serially repitched into 20° Plato wort. Yeast cropped from fermentations with air saturated high gravity wort responded poorly when acid washed. These results suggest that the structure of the plasma membrane particularly the sterol and fatty acid composition, may have an important role in tolerating high gravity wort and acid washing conditions.