ABSTRACT
Two strains of Streptococcus faecium and one strain of Streptococcus faecalis were subjected to heat stress in a ham broth and recovered on all purpose tween agar; deMan, Rogosa, Sharpe agar; tryptone-dextrose-yeast extract-meat extract-peptone agar; and tryptic soy agar. Survival curves for the organisms recovered on each agar were constructed and D values (death rates) were calculated. Differences in death rates were noted for each organism between different media. The greatest recovery of cells that had received sub-lethal heat treatment occurred using all purpose tween agar.
ABSTRACT
A number of economic and product quality advantages exist in brewing when high-gravity worts of 16 to 18% dissolved solids are fermented. Above this level, production problems such as slow or stuck fermentations and poor yeast viability occur. Ethanol toxicity has been cited as the main cause, as brewers' yeasts are reported to tolerate only 7 to 9% (vol/vol) ethanol. The inhibitory effect of high osmotic pressure has also been implicated. In this report, it is demonstrated that the factor limiting the production of high levels of ethanol by brewing yeasts is actually a nutritional deficiency. When a nitrogen source, ergosterol, and oleic acid are added to worts up to 31% dissolved solids, it is possible to produce beers up to 16.2% (vol/vol) ethanol. Yeast viability remains high, and the yeasts can be repitched at least five times. Supplementation does not increase the fermentative tolerance of the yeasts to ethanol but increases the length and level of new yeast cell mass synthesis over that seen in unsupplemented wort (and therefore the period of more rapid wort attenuation). Glycogen, protein, and sterol levels in yeasts were examined, as was the importance of pitching rate, temperature, and degree of anaerobiosis. The ethanol tolerance of brewers' yeast is suggested to be no different than that of sake or distillers' yeast.