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Influence Of Yeast Propagation In High-Gravity Wort On Subsequent Fermentation Performance
Ch. Tenge, M. Nagel, U. Wellhoener and F. Jacob

The vitality of the yeast, e.g. the physiological condition and activity, has great influence on the quality of the manufactured beer. Therefore yeast propagation in brewing processes - like high gravity brewing - that put additional stress on the yeast needs special attention. A brewery practising high gravity brewing has only wort with a high original gravity at its disposal. The wort can be used diluted or undiluted for the cultivation of yeast. A series of experiments were conducted to examine the influence of propagation with high gravity wort on yeast and the resulting consequences for the fermentation, as well as the produced beer. The Yeast was cultivated, while varying the original gravity and composition of the propagation wort. The growth rate of the yeast decreased with increasing original gravity of propagation wort. At the same original gravity an increasing glucose content results in slower growth. Analysis of the activity of key enzymes leads to the conclusion that the growth rate is mainly influenced by a change in the yeast metabolism called the crabtree effect. To evaluate the fermentation performance of the harvested yeast, it was used to ferment wort with 12? Plato. The yeasts propagated in wort with different gravities showed no significant difference in the final gravity. Yeast propagated in wort with the addition of glucose syrup had an unsatisfactory fermentation performance. The concentration of volatiles and the foam stability in the produced beer were within a normal range. The bitterness and flavour of the beer produced with yeast propagated in wort with the addition of glucose syrup were not satisfying. The conducted experiments showed that a brewery, using all malt high gravity in the brewing process, can use their wort undiluted for yeast cultivation. The exception is the usage of glucose syrup as a wort additive. It can lead to a severely decreased fermentation performance of the yeast if the wort is used for propagation purposes.

Descriptors: Propagation, yeast, high-gravity, fermentation, enzyme

BrewingScience - Monatsschrift fr Brauwissenschaft, 59 (January/February 2006), pp. 41-44