Quantification and understanding of the fermentative ability of re-pitched yeast
Josey, Maria Elizabeth
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Brewer’s collect yeast from one fermentation and re-pitch the cells into subsequent fermentations; this process is termed serial re-pitching. Deviations in the serial repitching process often impact the quality of the beer produced. Three variables within this process were investigated: the percentage of petite mutants in the pitched yeast, varying levels of dissolved oxygen in the wort and the length of yeast storage time between re-pitched fermentations. Results found that when petites in the culture increased from 3.7 % to 10.8 %, levels of esters and vicinal diketones were increased after the primary fermentation. The elevated concentrations of flavour compounds, however, would likely be undetectable to a consumer. When the yeast was subjected to 66 hour storage between fermentations, as opposed to 18 hour, the density attenuation rates during fermentation became less consistent. When investigating dissolved oxygen on serial re-pitching, the effect of low (3 mg/L) and high (16 mg/L) dissolved wort oxygen levels were not apparent on the flavours produced until the third fermentation; then, levels of propan-1-ol and 2,3-butanedione were increased with higher dissolved oxygen levels. Overall, these results could be used to help a brewer prioritize the controllable variables within the re-pitching process.