| Aeration plays an important role in the production of glycerol by fermentation with yeast. Effective aeration depends on a number of factors, such as amount of air, fineness of air dispersion, rate of agitation and time of gas-liquid contact. This investigation deals with the effect of periodical variation of gas pressure on oxygen transfer measured by sulfite oxidation (SO) and glycerol fermentation in stirred tanks. The oxygen transfer rate (OTR) measured with the SO method was improved by 20--30% under the condition of pressure pulsation (PP) at 30°C. The yield and productivity of glycerol were increased by about 38 and 23%, respectively, in 72 h (26 and 6.8% in 48 h) by employing glucose concentration of 250 g/L with PP at 30°C. A mathematical model of OTR has been built and compared to experimental data for the effect of PP measured by SO. By manipulating the airflow rate and a control valve in a 4-L bioreactor, the peak pressure and frequency of PP were varied, respectively. OTR was also examined at constant gas pressure. The results indicate a good agreement between measurement and model prediction. OTR above 9 psig of PP has shown significant enhancement at 25°C. OTR plays an important role in the production of glycerol by fermentation with yeast. Metabolic flux analysis (MFA) and elemental balances of a simplified fermentation network have shown that PP reduced the formation of biomass and ethanol, but enhanced oxygen uptake rates 34% and 27%, and glycerol productivity 26% and 25% at 500 and 700 rpm, respectively, in 48 h, by initial glucose concentration of 250 g/L with 10 or 15 psig of PP at 30°C. Moreover, the assumption of metabolic pathway that back fluxed from G3P to DHAP, catalyzed by triose phosphate isomerase, was validated by MFA with PP at 700 rpm. |
