Intensification of Enzymatic Hydrolysis of Lignocellulose Using Ultrasound for Efficient Bioethanol Production: A Review

Improvements in enzymatic hydrolysis for production of bioethanol from sustainable biomass are necessary in order to reduce enzyme requirements and the overall processing times. Conventional techniques for pretreatment of lignocelluloses are quite costly, time-consuming, and also require substantial downstream processing. Ultrasound can be effectively used to improve the hydrolysis process by reducing the structural rigidity of lignocellulose and by eliminating the mass-transfer resistances, which can contribute to increase in the product yield with reduced processing time and enzyme consumption. The present work initially presents an overview of different pretreatment methods involved in the enzymatic hydrolysis of lignocellulosic biomass highlighting its limitations. Expected mechanism of intensification due to the use of ultrasound has been discussed, also giving recommendations for optimum operating conditions. An overview of the different studies related to enzymatic hydrolysis using ultrasound for bioethanol production has been presented, along with discussion related to types of sonochemical reactors and scale-up aspects. Possible combination of sonication by coupling with traditional pretreatment processes has also been discussed. Overall, it appears that ultrasonic irradiations can be effectively used for the intensification of the enzymatic hydrolysis process for efficient bioethanol production.