SOME OBSERVATIONS ON YEAST MUTATION DURING CONTINUOUS FERMENTATION

The spontaneous genetic mutation of yeasts, including brewery yeasts, is a well‐known phenomenon. By analogy with other micro‐organisms it may be suspected that under the peculiar conditions of continuous fermentation mutant forms will tend to accumulate, involving the risk sooner or later of far‐reaching qualitative changes of the yeast mass. Since mutations usually imply the loss, rather than the gain, of desirable characters it cannot be excluded that such changes may be detrimental to beer quality. The observations reported here on a typical bottom fermenting strain of brewery yeast, Saccharomyces carlsbergensis , have shown that after cultivation for nine months in a continuous fermentor approximately one half of the cells had mutated. The mutations included loss of flocculence, reduction of fermentation efficiency, reduction of final attenuation, change of growth rate, and production of undesirable flavours. One mutant form of especial taxonomic interest even appeared to have lost its claim to membership of the parental species.