4.5.3.3.1 Cellulase Enzyme Adsorption

The enzymatic hydrolysis of cellulose proceeds by adsorption of cellulase enzyme on the lignacious residue as well as the cellulose fraction. The adsorption on the lignacious residue is also interesting from the viewpoint of enzyme recovery after the reaction and recycling it for use on the fresh substrate. Obviously, the recovery efficiency is reduced by the adsorption of enzyme on lignacious residue, because a large fraction of the total operating cost is due to the production of enzyme. The capacity of lignacious residue to adsorb the enzyme is influenced by the pretreatment conditions, therefore the pretreatment should be evaluated, in part, by how much enzyme adsorbs on the lignacious residue at the end of hydrolysis as well as its effect on the rate and extent of the hydrolysis reaction.

The adsorption of cellulase on cellulose and lignacious residue has been investigated by Ooshima, Burns, and Converse [71] using cellulase from Trichoderma reesei and hardwood pretreated by dilute sulfuric acid with explosive decomposition. The cellulase was found to adsorb on the ligna — cious residue as well as on the cellulose during hydrolysis of the pretreated wood. A decrease in enzyme recovery in the liquid phase with an increase in the substrate concentration has been reported due to the adsorption on the lignacious residue. The enzyme adsorption capacity of the lignacious residue decreases as the pretreatment temperature is increased, whereas the capacity of the cellulose increases with higher temperature. The reduction of the enzyme adsorbed on the lignacious residue as the pretreatment tem­perature increases is essential for improving the ultimate recovery of the enzyme as well as enhancing the enzyme hydrolysis rate and extent. Lu et al. (2002) conducted an experimental investigation on cellulase adsorption and evaluated the enzyme recycle during the hydrolysis of SO2-catalyzed steam — exploded Douglas fir and posttreated steam-exploded Douglas fir substrates [72]. After hot alkali peroxide posttreatment, the rates and yield of hydrolysis attained from the posttreated Douglas fir were significantly higher, even at lower enzyme loadings, than those obtained with the corresponding steam — exploded Douglas fir. This work suggests that enzyme recovery and reuse during the hydrolysis of posttreated softwood substrates could result in less need for the addition of fresh enzyme during softwood-based bioconversion processes [72].

An enzymatic hydrolysis process involving solid lignocellulosic materi­als can be designed in many ways. The common denominators are that the substrates and the enzyme are fed into the process, and the product stream (sugar solution), along with a solid residue, leaves it at various points. The residue contains adsorbed enzymes that are lost when the residue is removed from the system.

In order to ensure that the enzymatic hydrolysis process is economically efficient, a certain degree of enzyme recovery is essential. Both the soluble enzymes and the enzyme adsorbed onto the substrate residue must be reuti­lized. It is expected that the loss of enzyme is influenced by the selection of the stages at which the enzymes in solution and adsorbed enzymes are recirculated and the point where the residue is removed from the system.

Vallander and Erikkson [46] defined an enzyme loss function L, assuming that no loss occurs through filtration:

L _ amount of enzyme lost through removal of residue amount of enzyme at the start of hydrolysis

They developed a number of theoretical models to conclude that an increased enzyme adsorption leads to an increased enzyme loss. The enzyme loss decreases if the solid residue is removed late in the process. Both the adsorbed and dissolved enzymes should be reintroduced at the starting point of the process. This is particularly important for the dissolved enzymes. Washing of the entire residue is likely to result in significantly lower recovery of adsorbed enzymes than if a major part (60% or more) of the residue with adsorbed enzymes is recirculated. An uninterrupted hydro­lysis over a given time period leads to a lower degree of saccharification than when hydrolyzate is withdrawn several times. Saccharification is also favored if the residue is removed at a late stage. Experimental investigations of the theoretical hydrolysis models have recovered more than 70% of the enzymes [46].