Strength of adsorption of polyethylene glycol on pretreated Pinus radiata wood and consequences for enzymatic saccharification

Softwood substrates are recalcitrant in enzymatic hydrolysis to reducing sugars, in part because of unproductive binding of enzymes on lignin. Interactions between polyethylene glycol (PEG) and a softwood substrate were characterised in order to estimate the PEG loadings required for effective blocking of unproductive enzyme binding. For the first time, the adsorption of Rhodamine labelled PEG (M_w 3400) on thermo-mechanically treated Pinus radiata fibres was characterised by fluorimetry, giving a best-fit binding constant of 68 L g⁻¹. This was considerably stronger than published binding constants of 6 L g⁻¹ for enzymes on lignin, accounting for the success of PEG as an additive. Glucose yields from enzymatic hydrolysis of the same substrate, when unlabelled PEG (M_w 4000) was used, were consistent with a PEG adsorption capacity of 0.01–0.02 g g⁻¹ substrate. Thus, PEG was shown to be effective at affordably low loadings and further mathematical modelling work predicts savings correspond to 6 kg PEG per tonne when pretreated Pinus radiata substrate was processed at higher substrate concentration. - Highlights: • Binding constant of Rhodamine labelled PEG on pretreated Pinus radiata is 68 L g⁻¹. • This value is ten-fold higher than binding constant for cellulase on lignin. • Effect of PEG molecular weight and substrate lignin content on saccharification. • PEG is affordable in saccharification of pretreated Pinus radiata. • Model predicts savings in PEG at high substrate consistency saccharification