Experiences in the control of problems during the fermentation of high gravity worts at the South African Breweries are described. Many of these problems were at least partly caused by poor yeast vigour, so that the optimization of the use of a fermentation stimulant (sometimes referred to as "yeast food", although in the case described the product was not a nutrient but an additive of which the particles serve as nucleation sites to facilitate the release from solution of supersaturated carbon dioxide, which can be toxic to yeast) by adding it at pitching instead of in the whirlpool (from which much of it was lost in the trub) led to significant improvements in performance. Yeast vitality was also improved by cropping earlier (under 24 hours after final gravity was reached) and mixing to increase the efficiency of cooling (thus also producing a more homogeneous slurry and thereby facilitating accurate pitching rate control). High residual levels of unfermented fructose, causing excessive sweetness in the beer, were found to result from the inhibition of fructose uptake in yeast by glucose, maltose and ethanol. By changing from a sucrose adjunct, which had given rise to high glucose and fructose levels in the wort, to a high maltose syrup, the cause of the problem was removed. Performance problems with the fermentation of the first brew pitched each week were overcome by delaying pitching until a fresh yeast crop could be harvested and prepared, thus avoiding the need to use yeast stored over the weekend. Other problems still being attacked at the time of writing, including excessive production of sulphur dioxide and acetaldehyde, are briefly discussed.
Keywords : brewers' yeast fermentation high gravity brewing performance quality