28. Serial repitched fermentations: Two case studies

Maria Josey (1); (1) Heriot Watt University, Edinburgh, U.K.

Technical Session 8: Yeast Stress & Performance
Monday, August 15  •  8:15–9:30 a.m.
Tower Building, Majestic Level, Majestic Ballroom

Serial repitching is a universal practice used in the industry and involves yeast storage and reuse in subsequent fermentations. As propagating yeast takes both time and money, it is important to optimize the number of serial repitchings for a given yeast strain. While multiple studies have been performed on repitched yeast, they are controversial. Two studies observed that no significant (P < 0.05) differences in fermentations existed for up to 10 serial repitchings, while a third study showed fermentation changes after 8 serial repitchings. In an attempt to resolve this controversy, fermentations were examined in two breweries. In the first study, the wort density was monitored in industrial-sized ale fermentations where yeast was repitched up to 18 times. The ale yeast was skimmed post-fermentation and stored in a cooled yeast brink until the next fermentation. The density attenuation of the fermentations were analyzed using the four parameter logistic model of ASBC Yeast-14. The four parameters reflect the final and initial gravities, a function of the slope, and the time to the midpoint of the fermentation. These four parameters allow statistical analysis of the fermentation and, thus, do not rely on individual data points for conclusions. Results from this study showed that the midpoint of the fermentation decreased as the number of serial repitchings increased. This suggests that yeast that has been serially repitched multiple times will ferment quicker than freshly propagated yeast—a somewhat controversial finding. From a time-saving stand-point, this result suggests that it could be beneficial to ferment with yeast that has been repitched multiple times. While the ale from this trial was released commercially, subtle flavor changes to the beer were not examined. Ongoing studies are currently monitoring similar fermentations and gathering data on esters and higher alcohols produced to address this question. In the second study, a lager strain was examined in a 2-hL pilot brewery, repitching up to nine times. The attenuation was monitored along with flavor compounds at the end of each fermentation. Upon visual examination, no consistent trend was found between attenuation and number of repitches. There were slight differences in malt composition between the fermentations that had a larger impact on density attenuation than the repitching influence. Additionally, no consistent trends were found with the repitched yeast and flavors present at the end of fermentation. This suggests that ingredient variations may possibly have more effect than yeast repitching practices. We hope this report will help shed light on the question of the optimum serial repitching number in modern brewing operations.

Maria Josey received her BAS in food science at Dalhousie University in Nova Scotia, Canada, in 2014. Her final year research project investigated characteristics of various yeast strains during the beer fermentation process. During her BAS studies, she spent a semester on exchange at the University of Adelaide. After graduating from Dalhousie University, she began working in 2014 on her Ph.D. degree on quantification and improvement of fermentability during repeated yeast repitching at Heriot Watt University, Edinburgh, UK. She currently serves on the Institute of Brewing and Distilling Scottish Section Committee as a young member representative.

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