Long term serial repitching and the genetic and phenotypic stability of brewer's yeast

Two brewing yeast strains, one employed for the production of an ale type product and the other used solely for bottle conditioning. were serially repitched over a period of one year. Subsequently each yeast culture was compared to the original stocks for a variety of phenotypic and genotypic characteristics. Fermentation performance was assessed in terms of flocculation capacity and the time required to achieve attenuation in 150 barrel fermentation vessels, while the propensity for the population to accumulate variants was assessed by analysing giant colony morphology. Changes to the genome were monitored by DNA fingerprinting of each yeast culture using RAPD-PCR and RFLP. Although some colony morphology variation was observed between fresh and old ale yeast cultures, there were no detectable genetic changes or alterations in fermentation characteristics to either yeast strain over the course of serial repitching. It is suggested that although some brewing yeast strains are susceptible to genetic drift, others are more resilient and can remain stable over extended periods of time. The propensity to produce variants may therefore play a significant role in determining the number of times a strain may be serially repitched, or its suitability for beer fermentations.