Modeling and optimization of polyethylene glycol (PEG) addition for cost-efficient enzymatic hydrolysis of lignocellulose

Enzyme consumption is a key cost in the lignocellulosic sugar route for production of biofuels and chemicals, and polyethylene glycol (PEG) is a low-cost additive that improves hydrolysis efficiency. Despite many studies in this area, the relation of benefit over cost of PEG addition remains unclear. This article describes principles for expressing the effect of PEG as an equivalent enzyme amount, by merging PEG adsorption kinetics with a standardized enzyme dosage response. This model allows cost optimization of PEG addition by marginal analysis, as a function of enzyme dosage, solids concentration and price-ratio of enzyme and PEG. The model is based on the novel observations that the relative increase in apparent enzyme dosage by PEG addition is constant regardless of the absolute enzyme dosage, and that the increase correlates linearly with adsorption of PEG on hydrothermally pretreated wheat straw. The optimum ranged for most cases between 7 and 14 mg PEG per g substrate. The addition of PEG was attractive only above a threshold price-ratio, which decreased from 7.0–1.4 as enzyme dosage increased from 2 to 10 cost units per g substrate, showing that the incentive for introducing PEG to the process becomes stronger as the enzyme dosage increases.