Bridging industrial pretreatment and downstream bioconversion through low-enzyme high-solids ethanol production from unwashed wheat straw

High-solids enzymatic hydrolysis is essential for producing fuel ethanol from pretreated wheat straw at industrially relevant titers, but it is limited by high enzyme demand, poor slurry mobility, lignin-related inhibition, and soluble inhibitors retained in unwashed biomass. This study aimed to develop a low-water and low-enzyme strategy for ethanol production from industrial wheat straw pretreated by mechanical extrusion followed by steam explosion, with key hydrolysis and fed-batch operating parameters optimized at enzyme loadings of 2.7–10 FPU/g dry substrate and solids loadings up to 30% (w/w). Baseline hydrolysis showed that enzyme dosage was the main limitation under low-enzyme conditions, while increasing solids loading intensified mass-transfer resistance. Component-removal experiments demonstrated that lignin, rather than residual hemicellulose, was the dominant biomass-derived barrier, mainly through non-productive cellulase adsorption. Rheology and water-distribution analyses further showed that high-solids slurries were constrained by increased viscosity, yield stress, and restricted water mobility. For unwashed substrates, sodium hydroxide neutralization was more effective than other alkaline agents in maintaining hydrolysis performance. Combining sodium hydroxide neutralization, PVA-assisted enzyme protection, and fed-batch feeding improved hydrolysis yield by 18.4%. The maximum ethanol titer of 71.4 g/L was obtained with washed pretreated wheat straw at 30% solids and 10 FPU/g dry substrate. The optimized unwashed fed-batch process produced 35.4 g/L ethanol at 25% solids and 2.7 FPU/g dry substrate, with improved ethanol productivity per unit enzyme and a hydrolysis-stage consumables cost of 2988.8 CNY per metric tonne of ethanol under the tested conditions, excluding costs outside the hydrolysis stage. Overall, integrating direct use of unwashed industrial biomass with enzyme protection and staged feeding provides a practical low-water and low-enzyme route for high-solids lignocellulosic ethanol production, offering a process basis for reducing hydrolysis inputs while maintaining efficient biomass conversion.