CONTINUOUS BEER FERMENTATION BY HIGH CELL-DENSITY CULTURE OF BOTTOM BREWERS-YEAST

Continuous beer production was investigated in a high cell-density cul ture system which consisted of two stages for the fermentation and sed imentation of yeast cells. The continuous culture was carried out for a fermentation time of 5,500 h without contamination, at varying dilut ion rates and fermentation temperatures in the ranges of 0.017-0.033 h -1 and 6.5-8.5-degrees-C, respectively. This process was found to be s uitable for continuous and stable beer brewing. Under these conditions , the cell concentration in the first stage was about 80 times as high as that in the exit of the second stage. Concentrations of viable cel ls, sugar and ethanol were maintained at 1.3 x 10(9) cells/ml, 25 and 36 g/l, respectively, and were hardly affected by fermentation tempera ture. Concentrations of ethyl acetate, isoamyl alcohol and isoamyl ace tate were similar in the fermentation temperature ranges of 6.5-8.5-de grees-C, and the amounts at a fermentation temperature of 7-degrees-C were comparable to those of lager-type beer. Diacetyl flavor, which is known to be an effluent component that causes deterioration in the se cond stage (young beer), was maintained at 1.2 ppm at a dilution rate and fermentation temperature of 0.022 h-1 and 7-degrees-C, respectivel y. The discetyl flavor was due to the accumulation of vicinal diketone , the precursor of which is acetohydroxy acid. The acetohydroxy acid w as converted to vicinal diketone by pretreatment at 60-degrees-C for 3 0 min. The vicinal diketone was then consumed by the yeast during afte r-fermentation at a fermentation temperature of 3-degrees-C. Using thi s method, total vicinal diketone decreased below 0.3 ppm for an after- fermentation time of 6.8 h, which was 225 times as fast as that of aft er-fermentation without the pretreatment. This process may make it pos sible to achieve continuous beer fermentation from the fermentation st age to after-fermentation for diacetyl removal.