The properties of lignocellulosic substrates obtained from different pretreatments have a strong influence on the downstream saccharification of ammonium sulfite fungal cellulase-based systems and cellulosome-based whole-cell biocatalytic systems. However, a comparative analysis of the corresponding effects of these two different glycation strategies has not yet been performed. In this work, three ammonium sulfite (AS)-based pretreatment combinations (i.e., AS + hydrothermal (HT) pretreatment, AS + xylanase (X) pretreatment and HT + AS pretreatment ) to process wheat straw. The obtained pretreated substrates with different properties were saccharified using fungal cellulase or engineered Clostridium thermocellum strains as whole-cell biocatalysts, and their ability to release sugars was comparatively evaluated. It was found that for whole-cell saccharification, total sugar digestibility was 10% higher in AS + HT/X pretreated wheat straw than in HT + AS pretreated wheat straw. However, for fungal cellulase-based saccharification, the enzymatic hydrolysis efficiency was not easily affected by the combined order of pretreatments. Thus, whole-cell biocatalytic systems are more sensitive to substrate accessibility than free enzymes. Furthermore, characterization and analysis showed that AS + HT/X pretreatment removed more lignin and produced a more accessible surface with a larger outer surface and lower surface lignin compared to HT + AS pretreatment coverage. Therefore, AS + HT/X pretreatment is more compatible with cellulose-based whole-cell saccharification.
Agricultural straw is one of the most abundant lignocelluloses, with an annual output of more than 600 million tons in China. 1 However, most of the straw was directly burned in the field before, causing serious environmental problems and waste of natural resources. Currently, the incineration of agricultural waste is strictly prohibited, while energy demand is also increasing. As a result, the Chinese government plans to use gasoline blended with 10% bioethanol for cars nationwide by 2020, which will require more than 10 million tons (about 3.3 billion gallons) of bioethanol. 2 In addition, the US Energy Independence and Security Act36 will produce 1 billion gallons of biofuels by 2022. However, currently in the US and China, more than 90% of bioethanol is produced from corn rather than lignocellulose. 3 In this context, better utilization of agricultural residues, such as wheat straw, has become a priority.