The dominating factor for biomass cellulose accessibility to external enzymes:

To improve biomass cellulose accessibility to external enzymes, pretreatment is often required, which seems to plat the following roles depending on the processing conditions:

• lignin or hemicellulose removal
• destroy cellulose crystal structure,
• open channels/cell wall pores
• increase cellulose surface exposure.

Since enzymatic hydrolysis follows the tunneling mechanism: eroding glucose from the inside of cell wall to the outside. Therefore, the pores or channels in cell wall are the dominant factor for the efficiency of enzymatic hydrolysis, i.e. control the rate of diffusion/transport of enzymes and sugar dissolution.

Size reduction by mechanical milling before themo/chemical pretreatment is the worst scenario although the surface area are improved.