The Effect of Alcohol and Bitterness Levels on Brewing Yeast Viability

Abstract

Two of the most popular beer styles within the craft brewing industry are India Pale Ales (IPA's) and those with high alcohol by volume (ABV). Production of these styles requires high gravity fermentation and high amounts of bittering hops in order to reach the required values for ABV and International Bitterness units (IBU) respectively. The aim of this study was to determine the impact of high gravity fermentation and high IBU levels on yeast viability and repitching cycles.An initial experiment on high gravity fermentations was done in order to assess the effects this variable had alone. Successive five day fermentations employing serial repitching were performed on worts with low (10 °P), medium (14 °P), and high (18 °P) gravity levels. The minimum viability for repitching established for this study was 85%. Once the viability of a sample fell below this minimum, it would not be suitable for repitching. It was found that increasing gravity level led to lower viabilities at the end of the fermentation period. Viability decreased further as fermentation generation increased for the high gravity samples. Yeast harvested from low gravity fermentations could be repitched up to eight times. Medium and high gravity fermentations could be repitched up to five times.This study was repeated at single gravity levels with low (25), medium (50) and high (75) IBU levels. A loss in viability with increased IBU levels over serial re-pitching cycles in the low gravity wort (10 ºP) was observed. It was found that at the low gravity level, yeast could be repitched eight times at the low IBU level, five times at the medium IBU level, and four times at the high IBU level.When the experiment was repeated with medium and high gravity worts, the results indicated that the compound effects of increased gravity and IBU levels significantly reduced yeast viability throughout re-pitching cycles and thus limits the number of times that this yeast could be reused when compared to low gravity and low IBU fermentations. Medium gravity fermentations could be repitched three times at the low IBU level and twice at the medium and high IBU levels. High gravity fermentations could be repitched three times at all IBU levels. v ACKNOWLEDGMENTS I would like to thank the Food Science department at Cal Poly San Luis Obispo for this opportunity and the CSU-ARI for funding on this project. I wish to thank my family for their support through graduate school, especially my parents Eric and Toni Bryant, my sister Kerri Bryant, and my grandmother Sylvia Bocchario. Thank you to my grandfather Papa "HoHo". Thank you to my undergraduate advisor, Dr. Jackie Trischman for believing in me and inspiring me to further my studies with graduate school. Thank you Dr. Amin for the encouragement and wonderful food.I would like to thank Firestone Walker, Barrelhouse, and Barth-Haas group for their donation of materials to this project. I would also like to thank Dr. Reeves from the Nutrition department for lending me his UV/Vis spectrometer. Thank you to Dr. Smith from ...