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Use of Carbohydrate Binding Modules To Elucidate the Relationship between Fibrillation, Hydrolyzability, and Accessibility of Cellulosic Substrates

Aïssa, Kevin, Novy, Vera, Nielsen, Fredrik, Saddler, Jack
ACS sustainable chemistry & engineering 2018 v.7 no.1 pp. 1113-1119
biomass, carbohydrate binding, cellulose, cellulose microfibrils, enzymatic hydrolysis, enzymes, hydrolysis, refining, scanning electron microscopy
It is widely acknowledged that the rate limiting step in the enzyme-mediated deconstruction of the biomass process is the restricted ability of the enzymes to access the cellulosic substrate. An ongoing challenge has been to find reproducible and quantifiable methods for measuring enzyme accessibility to cellulose. Type A (crystalline cellulose) and type B (paracrystalline) cellulose binding modules (CBMs) were used in parallel with microscopy, fiber analysis (aspect ratio), and water retention values (WRV) to determine if the observed and anticipated changes in differentially prepared microfibrillated cellulose (MFC) substrates were similar. It was apparent that with increasing refining there was a corresponding increase in fibrillation (SEM and WRV), as well as a decrease in aspect ratio. Although the initial degree and rate of enzymatic hydrolysis increased with prolonged refining, above 1000 kWh ton–¹ little improvement in either was observed. However, when cellulose accessibility was assessed by the CBM method, the observed trend followed the hydrolysis profile. Although the other methods (WRV, SEM, and aspect ratio) suggested increased refining should result in greater accessibility and a corresponding improvement in hydrolysis, the CBM method more accurately predicted enzyme accessibility, implying that refining did not significantly improve enzyme accessibility at the microfibril level of the cellulosic substrate.