FORMATION AND REMOVAL OF ACETOIN DURING YEAST FERMENTATION

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Abstract
The main yeast fermentation process can be characterized by two separate phases, the acetoin-producing phase (phase I) and the acetoin-reducing phase (phase II). The time at which yeast changes from production to removal is here designated as the ‘point of metabolic change’. The rise and fall in acetoin concentration was affected by composition of the wort, inoculum size, yeast strain, presence of traces of oxygen and addition of ergosterol, whereas temperature seemed to be of somewhat less importance. A high and positive correlation between the maximum level of acetoin and the concentration of ethanol measured at the ‘point of metabolic change’ has been demonstrated. The appearance of the ‘point of metabolic change’ is discussed in light of the level of acetoin in finished beer. The absolute threshold for acetoin was determined as 17 ppm.

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