Effects of Glucose Releasing Rate on Cell Growth and Performance of Simultaneous Saccharification and Fermentation (SSF) in Sweet Potato Medium for Fuel Ethanol Production Using Saccharomyces cerevisiae

Sweet potato medium containing 230.0 g•kg-1 liquefied starch was used for simultaneous saccharification and fermentation (SSF) for fuel ethanol production. Glucose releasing rate was controlled by the initial addition of incremental glucoamylase. The increasing rates of glucose concentration display a positive relationship when glucoamylase is added in early stage (0 to 8 hours (h)). Serious cells growth inhibition occurred in the early stage when 1.0 g•kg-1 glucoamylase is added, whereas glucose providing limitation occurred in the batch with 0.2 g•kg-1 glucoamylase added in later stage (64 to 80 h). The optimum dosage of glucoamylase was 0.8 g•kg-1, where a final ethanol concentration of 118.2 g•kg-1 was attained within 72 h. The results of our study suggest cell growth inhibition and substrate providing limitation can be avoided simultaneously by adding a proper dosage of glucoamylase. It is indicated further that cells growth inhibition in early stage in the batch with 1.0 g•kg-1 glucoamylase added was due to the high increasing rate of initial glucose concentration, but not the high overall glucose concentration.