The efficiency of the brewing fermentation as well as the flavor development and other quality related issues are directly related to the physiological condition of yeast. The following problems are associated with a poor yeast vitality and or viability (Back, 2005):Slow and stuck fermentationsHigh diacetyl values in the final beerInsufficient reduction of diacetyl and carbonylsFiltration problems and haze problemsSlow and insufficient pH dropIncreased risk of microbiological contaminationsPoor foam stability (due to protease release)Poor flavor stabilitySulfury and yeasty tastes and a broad harsh bitternessBesides these immediate problems, less serious consequences can also arise with varying yeast condition. There is a theoretically established link between yeast health and flavor components like esters and higher alcohols (Quain, 1988), but recent research has not confirmed this effect on the above named components (Thiele, 2006; Thiele and Back, 2007).
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