8-41: Why post treatment is required to increase the enzymatic hydrolysis of steam pretreated softwoods

Compared to agricultural and hardwood biomass feedstocks, softwoods typically present a much greater degree of recalcitrance such that pretreatment and enzymatic hydrolysis strategies have to be modified so that good overall sugar yields can be obtained with this substrate. Although steam pretreatment works very well for hardwood and agricultural feedstocks, the addition of an acid catalyst, such as SO₂ or H₂SO₄ is usually required for softwoods and the severity conditions have to be adapted so that high yields of the hemicelluose derived, hexose rich water soluble stream can be obtained while providing a water insoluble, cellulose rich stream that can be readily hydrolysed at low enzyme loadings. However, when low enzyme loadings (5 FPU/g cellulose) are used to hydrolyse steam pretreated softwoods, poor hydrolysis yields (>21 %) are obtained, even when the pretreatment severity is increased. As it was likely that substrate characteristics, particularly the condensed lignin resulting from steam treatment, were limiting the accessibility of the enzymes to the cellulose, we looked at several pretreatment and post-treatment strategies to try increase the efficiency of hydrolysis at low enzyme loadings. Earlier work had suggested that it might be possible to reduce the severity of pretreatment such that the overall sugar recovery could be increased without sacrificing hydrolysis yields by utilizing a post treatment step. In the presented work we will describe the effect of pretreatment severity on the ability of subsequent post-treatments to facilitate enzymatic hydrolysis and to enhance monomeric sugar yield. Steam pretreatment at a medium severity (200^oC 5 min 4.5% SO₂) followed by various post-treatments resulted in a four fold increase in enzymatic hydrolysis reaching 62% as compared to 16% without post-treatment.