Lignocellulose for biofuels production is considered as chief renewable biomass
source. Ethanol produced from lignocellulosic biomass is extensively recognized as
an additive or as a substitute to fossil fuel due to its trivial influence on the
environment. Second generation (2G) or cellulosic bio-ethanol is produced from the
biomass in three main steps that are pre-treatment, hydrolysis, and fermentation.
The pre-treatment method involves the use of different physical processes i.e., size
reduction, steaming or boiling, ultrasonication, and bursting, chemical methods i.e.,
solvents, acids, salts, bases, the physicochemical processes include liquid hot
water and AFEX (ammonium fibre explosion), biological methods involving whiterot/
brown-rot fungi and bacteria, and quite a few combinations thereof to
breakdown the lignocellulose into its components. Pre-treatment process changes
cellulose morphological, chemical and physical features, making it prone to
enzymatic attack for saccharification. In this review, we have discussed different
pre-treatment methods and their effect on recalcitrant structure of lignocelluloses
for making it readily available for enzymatic hydrolysis