The enzymatic hydrolysis of pretreated pulp fibers predominantly involves "peeling/erosion" modes of action

There is still considerable debate regarding the actual mechanisms by which a “cellulase mixture” deconstructs the cellulosic component of pretreated substrates. Previous work using “model/pure” cellulose substrates have suggested that the initial enzymatic attack takes place on the outer layer of the cellulose surface (fibers are peeled along their length, layer by layer, in an “onion peeling” fashion). Alternatively, it has been suggested that cellulose deconstruction is a two-step process where the cellulose-rich fibers are initially fragmented or disaggregated into shorter fibers. In this work, a library of pretreated pulp fibers with varying physical and chemical properties within the range of those likely to be found in the emerging Biorefinery industry sector was created. These substrates were used to determine the predominant mode of action, at the fiber level, of how a commercial cellulase mixture might hydrolyze industrially relevant feedstocks. It appears that regardless of the source of biomass, the type of pretreatment and the chemical composition, at the fiber level, pretreated pulp fibers are hydrolyzed through a two-step mode of action involving an initial rapid fragmentation followed by simultaneous swelling and peeling of the fragmented fibers. This latter mechanism is the predominant mode of action involved in effectively hydrolyzing the cellulose present in pretreated substrates. The very fast initial enzymatic fiber fragmentation was more apparent when the reaction was agitated, consequently enhancing the mechanically mediated fragmentation of the fibers. We will also discuss how this insight could be applied to enhance the effective hydrolysis of industrially relevant pretreated substrates.