Ultrasound enhanced bioconversion of glycerol by Clostridium pasteurianum: A mechanistic investigation

With significant growth of biodiesel industry in past few years, very large quantities of crude glycerol are available as side product of transesterification process. Effective conversion of glycerol to value added products can boost economy of biodiesel industry. The present study attempts to investigate ultrasound assisted bioconversion of glycerol to 1,3-propanediol (1,3-PDO) and ethanol (EtOH) from mechanistic viewpoint. A dual approach of coupling experiments under ultrasound irradiation or mechanical shaking with simulations of cavitation bubble dynamics has been adopted. Effect of ultrasound on bioconversion has been assessed with the help of kinetic analysis of enzymatic reactions. The nature of convection generated by ultrasound irradiation at raised static or ambient pressure has dual character of oscillatory motion of liquid, accompanied by medium amplitude jerks by the acoustic or shock waves emitted by cavitation bubbles, which could contribute towards enhancement of the enzymatic reactions in metabolic pathway. In presence of ultrasound, the highest yield (0.06 mol/mol) of 1,3-PDO was obtained with lowest glycerol concentration (5 g/L) while highest ethanol yield (0.12 mol/mol) was observed at 10 g/L glycerol concentration. Yield of both products increases under ultrasound irradiation, but lesser enhancement is seen for 1,3-PDO than EtOH due to substrate inhibition. Maximum enhancement in yield (49%) of 1,3-PDO was seen for 10 g/L glycerol concentration. The role of ultrasound irradiation in enhancement of bioconversion of glycerol is found to be of physical nature, without any influence on biochemistry of glycerol conversion.

► Mechanistic investigation in ultrasound assisted bioconversion of glycerol.
► Approach of coupling experiments with simulations of cavitation bubble dynamics.
► Yield of both products, 1,3-PDO and ethanol, increases under ultrasound irradiation.
► Lesser enhancement for 1,3-PDO than EtOH due to substrate inhibition.
► The role of ultrasound irradiation in bioconversion is found to be of physical nature.