Yeast Propgation in High-Gravity Worts

The viability, e.g. the physiological condition of the yeast respectively the activity of the yeast is of great influence on the quality of the produced beer. Therefore during brewing brewing processes that put additional strain on the yeast - like High-Gravity Brewing - yeast propagation is a special point of interest. In a brewery exclusive implementing High-Gravity Brewing only wort at a high original gravity is available. The wort can be used diluted or undiluted for yeast propagation.

Goal of this work was to determine the influence of propagation in High-Gravity wort on the yeast and the subsequent fermentation performance as-well-as the produced beer. For these purposes yeast propagations under different conditions were carried out. The obtained yeast was used to produce beer.

Growth rate of the yeast diminished with increased original gravity of the propagation wort. At the same level of gravity an increase in glucose content resulted in slower growth rate. Main reason for the change in growth rate is - according to the result of the enzyme activity tests - the Crabtree-Effect.The yeast wasn't negatively influenced by the High-Gravity during propagation; analysis of the cell cycle showed throughout good physiological condition. 

The at different conditions obtained showed differences in fermentation speed but not in regard to the final gravity. Yeast  enzymatically conditioned for anaerobic ambience fement faster. Slowest fermentation speed had yeast propagated in wort with glucose added.

Concetration of fermentation by-products and foam of the produced beer are within normal parameters. Beer from yeast propagated in wort with additional glucose shows sensory abnormalities regarding bitter and body.

A brewery using High-Gravity Brewing can use their wort undiluted for proapgation purposes. The addition von glucose to achieve a higher original gravity is not recommended; the sensorical quality could be affected.

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