Author : Vilai Santisopasri

Summary : The continuous-flow fermentation of sugarcane molasses into ethanol by the flocculent yeast Saccharomyces diastaticus NYCC 625 was studied at high yeast levels and microaerophilic conditions in the aeration tower and airlift fermenters. The effects of dilution rate, yeast cell concentration, cell recycle, and feed sugar concentration were determined in both fermenter types. Data were analyzed in terms of steady-state concentrations of ethanol and residual sugar, ethanol yield and fermentation efficiency, volumetric productivity, specific sugar uptake rate, and specific ethanol production rate. For the tower fermenter with cell recycle using 22 percent sugar feed at a dilution rate of 0.27 hr (superscript -1) and an initial concentration of 28 g dry yeast/L (5 x 10 (superscript 9) cells/ml), alcohol levels of 7.18 percent (w/v) and residual sugar at 3.6 percent were obtained at steady state. Ethanol productivity was 19.4 g/ one hour and fermentation efficiency was 83.6 percent. Under similar conditions, the airlift fermenter gave slightly higher ethanol concentration, ethanol productivity, and fermentation efficiency compared to the tower fermenter. Yeast cell recycle increased alcohol production rates and decreased residual sugar in the beer of both fermenter types. Higher feed concentration and dilution rate require higher specific maintenance rate of yeast. An apparatus which works based on the relationship of density, alcohol content, and residual sugar concentration in the beer was also tried for the automatic feedback control of the sugar input.