High-Gravity Brewing: Influence of Pitching Rate and Wort Gravity on Early Yeast Viability

Poor yeast crop viability has been reported in high-gravity brewing. When traditional pitching rates are used in high-gravity worts of 19.8–39° Plato, yeast viability is a problem within the first 24 hr of the fermentation. Both immediate and continued losses of viability and fermentative ability were observed; the duration and severity increased with increasing wort gravity. Surprisingly, viability losses were more severe when assayed by the methylene blue stain, traditionally a method that gives higher viabilities than enumerations based on colony-forming units. Under conditions of high-gravity brewing (high osmotic pressure), the methylene blue stain might indicate cell stress as well as viability. By either method, however, early losses in viability in high-gravity worts were significantly reduced by pitching at higher than usual pitching rates. The significance of this to high-gravity brewing is that such drastic cell death reduces the real pitching rate. Such lower pitching rates have been said to be more inefficient in the utilization of oxygen for lipid synthesis. As a result, sluggish starts, protracted fermentations, and oxygen-deficient yeast crops are more likely to occur.