Introduction of respiratory-deficient mutants in brewery yeast.

Cowan, W.D., Hoggan, J. and Smith, J.E.

Abstract
Production, pilot and laboratory infusion mashes were made with 100 and 150 ppm of formaldehyde added at the beginning of mashing. The hopped worts were fermented with a bottom fermenting, nonhead forming ale yeast (strain AYA) or, in laboratory studies only, with a head forming ale strain (AYB) or a lager strain (LYA). Respiratory deficient mutants were detected by plating out and overlaying with triphenyl tetrazolium chloride. When strain AYA was carried through two series of 5 successive production fermentations, respiratory mutants developed progressively, especially when 150 ppm of formaldehyde was used. A similar development occurred in pilot fermentations with 100 ppm. In laboratory fermentations with 150 ppm, mutant development with strain AYA was slower, perhaps because there was less selection for sedimentary yeasts, but addition of 5 ppm, just before fermentation induced a much higher rate of mutant increase. In laboratory experiments, strains AYB and LYA behaved similarly, but the rate of increase of mutants was slower with AYB and mutants developed at a significant rate from LYA even in the formaldehyde free control fermentations. Formaldehyde was mot effective when added at the start of mashing. Acetaldehyde added to formaldehyde free wort just before pitching exerted no mutagenic affect. After mashing, levels of free formaldehyde were low (0.1 to 0.5 ppm): this, and the fact that respiratory deficient mutants developed more slowly and less markedly in a synthetic medium, indicated that some mutagenic reaction product(s) of formaldehyde with mash components was involved. This product appears to be a heat stable, dialysable, nitrogen compound of MW <10,000. Ten respiratory deficient mutants isolated from production and laboratory fermentations were further characterised. All were more sedimentary than the parent strain and showed considerable differences of growth and fermentation rates. Because of their lower growth rates, but unaltered fermentation times and increased sedimentation rates, two strains seemed to offer promise as brewing strains.
Keywords: brewers' yeast carbonyl compound genetics mashing mutation respiration