||Diacetyl was responsible for a buttery flavor in beer. The threshold concentration was lower than 0.15 mg/L. The present study was engaged in the effect of fermentation and storage temperature on the formation of diacetyl during beer brewing. Another topic of this study was to culture the yeast cells to reach a quantity needed for pitching in the beer brewing. Effects of aeration and fed-batch fermentation on yeast cell growth were investigated. Intermitted aeration was used to grow the yeast cell to make a physiological condition adative for ethanol fermentation.
A button fermenting yeast from a famous beer brand, Heineken, and a top fermenting yeast from a Trappist beer brand, WP530, were used in this study. The fermentation temperatures for Heineken yeast were 10, 15, 20 ℃ respectively, while that for yeast WP530 were 15, 20, 27 ℃ respectively. The reduction rates of diacetyl were lower for both Heineken and WP530 yeast during fermentation at lower temperature, resulting in the accumulation of diacetyl in beer. On the contrary, the reduction rates of diacetyl were quite high during fermentation at higher temperature, resulting in a low diacetyl concentation in beer. When the fermented beer was stored in bottle for aging, again, the reduction rate of diacetyl was high at higher temperature. Therefor, it took shorter time for the diacetyl to be decreased to the threshold concentration.
The WP530 yeast was cultured in a 5-liter jar fermenter under the following conditions: temperature, 33 ℃; agitation, 250 rpm; aeration rate, 2 L/min; pH, automatically controlled at 5.0 (±0.1) by addition of 5N NaOH. When intermitted aeration was used, a dry cell weight of 2.62 g/L and an ethanol concentration of 28.9 g/L were achieved after 38 hrs of cultivation. Another fermentation was performed by feeding a 275 g/L glucose solution. Feeding of glucose was started at the end of the exponential phase of yeast growth. Intermitted aeration for 10 sec at every 3 min interval was used for the fed-batch fermentation. A dry cell weight of 8.22 g/L and an ethanol concentration of 73.7 g/L were achieved after 60 hrs of cultivation.