Improve biotransformation of crude glycerol to ethanol of Enterobacter aerogenes by two-stage redox potential fed-batch process under microaerobic environment

Abstract

Ethanol production from crude glycerol under microaerobic condition was optimized. The aeration rate in the reactor was controlled using redox potential (ORP) as to replace a conventional oxygen sensor that became insensitive at low oxygen concentration. The performance of Enterobacter aerogenes TISTR1468 in a batch reactor using a simple medium (crude glycerol and tuna condensate (GT) medium) was evaluated on various aeration strategies: continuous aeration at 0.5 vvm; controlled aeration at −350 mV and −400 mV ORP; and no aeration. The 72-h batch fermentation showed that a more reductive environment (lower aeration and ORP value) increased ethanol concentration and yield, but decreased the ethanol productivity. No-aeration fermentation with the lowest ORP value achieved the highest ethanol production (18.78 g L−1) and yield (0.94 g-ethanol.g-glycerol−1). A more oxidative environment (higher aeration and ORP value) fasten the specific growth rate (μ), but ethanol concentration and yield decreased. The optimum specific ethanol production rate (qp) and glycerol consumption rate occurred at ORP of −400 mV and −350 mV, respectively. When the ORP was very low, the μ decreased following substrate depletion, but this phenomenon was not observed at higher ORP value, an evident that adequate oxygen supply could maintain cell viability.Fed-batch with two-stage aeration strategy (−350 mV and −400 mV ORP) acquired higher ethanol production (30.31 g L−1) than fed-batch with single-stage aeration (25.95 g L−1) and batch process (12.33 g L−1).This study has revived the potential of crude glycerol biotransformation to ethanol and opened up opportunities for optimization of related microaerobic systems other than ethanol.