Microwave-assisted pretreatment with aqueous solutions of deep eutectic solvents for enhanced enzymatic saccharification of oat hull and brewer’s spent grain

The use of deep eutectic solvents (DES) in combination with microwave radiation is intriguing due to the potential for more comprehensive utilisation of lignocellulosic biomass in bioconversion processes. However, it is crucial to reduce material costs by employing aqueous solutions of DES or recirculating them. Our study aimed to assess the impact of microwave-assisted pretreatment with selected aqueous DES solutions on the effectiveness of biomass ingredient hydrolysis using cellulases and hemicellulases. The study also evaluated the possibility of recirculating an aqueous solution of choline chloride and imidazole as an agent facilitating effective biomass delignification under microwave conditions. Research demonstrated an increase in the susceptibility of various types of biomass to enzymatic hydrolysis following pretreatment with DES. The highest efficiency of cellulose hydrolysis (95.3 % and 97.5 %, respectively) was achieved for oat hull biomass pretreated with a mixture of choline chloride and ethylene glycol, and for brewer’s spent grain biomass pretreated with a mixture of choline chloride and glycerol. It was also demonstrated that a DES solution of choline chloride and imidazole can be regenerated and recirculated during effective microwave-assisted delignification of biomass over five cycles. The presented pretreatment method can be effectively applied to prepare oat hull biomass and brewer’s spent grain biomass for enzymatic degradation of cellulose. The demonstrated possibility of regenerating and recirculating DES solutions in microwave-assisted pretreatment of lignocellulosic biomass reduces the environmental impact of the process and lowers material costs.