Ball milling as an important pretreatment technique in lignocellulose biorefineries: a review

The conversion of lignocellulosic biomass into bioethanol remains a challenging process due to the recalcitrant structure of lignocellulose. The presence of the sturdy lignin protective sheath, complex structure, and partial crystallinity of cellulose often reduces the enzymatic susceptibility of lignocellulosic biomass. Therefore, pretreatment is aimed to increase accessibility by improving the physicochemical properties and composition of lignocellulosic biomass. It is the first and the most critical step that needs to be carefully selected and designed to overcome the constraints and improve the overall efficiency of bioethanol production. In recent years, ball milling has been applied as an emerging technique to produce bioethanol from lignocellulosic biomass efficiently and in an environment-friendly manner. Furthermore, ball milling technique coupled with chemical and physicochemical pretreatments has been shown to facilitate lignin removal, reduce cellulose crystallinity, and increase the specific surface area which ultimately improves the digestibility of lignocellulosic biomass. Over the last decade, several reports have been published on the application of ball milling to intensify the pretreatment process. However, a compiled report showing the progress of the technology in bioethanol processing is absent. In this review, a critical analysis and evaluation of published works on ball milling and ball milling-assisted chemical/physicochemical pretreatments are presented. It also addresses the synergistic effects of combining ball milling and chemical/physicochemical treatments to bring desirable characteristics of lignocellulosic biomass that will eventually improve hydrolysis yield and reduce chemical and energy consumption in bioethanol production.