Process for increasing the enzymatic accessibility of a thermally treated ligno-cellulosic biomass feedstock

A process for increasing the enzymatic accessibility of a steam exploded ligno-cellulosic biomass feedstock, wherein said steam exploded ligno-cellulosic biomass feedstock is free of an added base and comprised of xylans, glucans and lignin and is in physical forms of at least fibres, fines and fiber shives, wherein:

• a. the fibres each have a width of 75 μm or less, and a fibre length greater than or equal to 200 μm,
• b. the fines each have a width of 75 μm or less, and a fine length less than 200 μm,
• c. the fiber shives each have a shive width greater than 75 μm with a first portion of the fiber shives each having a shive length less than 737 μm and a second portion of the fiber shives each having a shive length greater than or equal to 737 μm,
• wherein the process comprises a fiber shives conversion step which is done by converting at least a portion of the fiber shives having a shive length greater than or equal to 737 μm to fines such that the percent area of fiber shives having a shive length greater than or equal to 737 μm relative to the total area of fiber shives, fibres and fines of the steam exploded ligno-cellulosic biomass after fiber shives conversion is less than the percent area of fiber shives having a shive length greater than or equal to 737 μm relative to the total area of the fiber shives, fibres and fines of the steam exploded ligno-cellulosic biomass before fiber shives conversion, wherein the percent area is measured by automated optical analysis, and wherein at least a portion of the fiber shives conversion step is done by applying a work in a form of mechanical forces to the steam exploded ligno-cellulosic biomass, and all the work done by all the forms of mechanical forces on the steam exploded ligno-cellulosic biomass is less than 500 WH/Kg per kg of the steam exploded ligno-cellulosic biomass on a dry basis.