Effect of the concentration of propagation wort on yeast cell volume and fermentation performance

Changes in cell volume occur when yeast is propagated in worts of aincreasing original gravity (OG) in the range 7.5 to 17.5°P. These changes were quantified using image analysis techniques capable of measuring the volume of individual yeast cells. The mean individual cell volume of ale and lager yeast increased by up to 30% as OG increased to 17.5°P. Propagation of yeast in high-gravity wort (17.5°P) has a deleterious effect on yeast quality during subsequent high-gravity fermentations. Pitching of high-gravity fermentations (17.5°P) using yeast propagated in 7.5, 12.5, and 17.5°P wort resulted in typical ale yeast viabilities at the end of fermentation of 93, 90, and 85%, respectively, and typical lager yeast viabilities at the end of fermentation of 98, 95, and 89%, respectively. Wort used for propagation of ale and lager yeast with an OG greater than 12.5°P resulted in yeast with a viability of less than 90% at the end of high-gravity fermentations.