Effect of dissolved carbon dioxide on very-high-gravity fermentation

Abstract

The stoichiometric relationship between carbon dioxide (CO2) generated and glucose consumed during fermentation can be utilized to predict glucose consumption as well as yeast growth by measuring the CO2 concentration. Dissolved CO2 was chosen as opposed to off-gas CO2 due to the high solubility of CO2 in the fermentation broth as well as its ability to reflect on yeast growth more accurately than off-gas CO2. Typical very-high-gravity (VHG) ethanol fermentation is plagued by incomplete glucose utilization and longer durations. Aiming to improve substrate utilization and enhance VHG fermentation performance, characteristics of dissolved CO2 concentration in fermentation broths using Saccharomyces cerevisiae were studied under batch conditions. Based on this study a novel control methodology based on dissolved CO2 was developed and its effectiveness on enhancing VHG fermentation was evaluated by measuring the fermentation duration, glucose conversion efficiency and ethanol productivity. Four different initial concentrations 150, 200.05±0.21, 250.32±0.12, and 300.24±0.28 g glucose/L were used for batch ethanol fermentation without control. Zero substrate was indicated for 150 and 200.05±0.21 g glucose/L by a characteristic abrupt drop in dissolved CO2 concentration. On the other hand sluggish fermentation and incomplete substrate utilization were witnessed for 250.32±0.12, and 300.24±0.28 g glucose/L. A material balance equation was developed to compensate for the inability of the dissolved CO2 profiles to accurately predict the different growth phases of yeast. Dissolved CO2 was controlled at three distinct levels of 500, 750 and 1000 mg/L using aeration rates of 820 and 1300 mL/min for initial concentrations of 259.72±7.96 and 303.92±10.66 g glucose/L. Enhancement of VHG fermentation was achieved in the form of complete glucose utilization and higher ethanol productivities and shorter fermentation duration in comparison to batches without control. Complete glucose utilization was facilitated under ~250 and ~300 g glucose/L in 27.02±0.91 and 36.8±3.56 h respectively. Irrespective of the control set points and aeration rates, ethanol productivities of 3.98±0.28 g/L-h and 3.44±0.32 g/L-h were obtained for 259.72±7.96 and 303.92±10.66 g glucose/L respectively. The glucose conversion efficiencies for both 259.85±9.02 and 299.36±6.66 g glucose/L when dissolved CO2 was controlled were on par with that of batches without control.