Natural size fractionation of pretreated wheat straw governs solid-loading-dependent hydrolysis and reveals a PEG-induced leveling effect

Particle size exerts a critical influence on high-solids enzymatic hydrolysis of lignocellulosic biomass. In this study, industrial-scale dilute-acid pretreatment induced natural size fractionation of wheat straw, yielding coarse and fine particles with distinct tissue origins and pronounced structural heterogeneity. Coarse fractions originating from stem and bark tissues exhibited greater structural rigidity, whereas fine fractions derived from parenchyma and leaf tissues were more easily hydrolyzed but contained more ash. The four sieved fractions showed solid-loading-dependent differences in hydrolysis performance. At low solids, hydrolysis was primarily limited by pore accessibility and surface properties, while at high solids the dominant constraint shifted to particle-size-dependent rheology and restricted water availability. At 16% solids, the enzymatic hydrolysis yield gap between fine and coarse fractions increased, emphasizing the stronger role of particle size under high-solid conditions. PEG 4000 alleviated water limitation and reduced yield stress, selectively improving the hydrolysis of coarse fractions and generating a leveling effect across fractions. Under high-solid conditions, PEG increased the enzymatic hydrolysis yield of the coarse fraction by 21.89% and markedly reduced performance disparities. These results demonstrate that pretreatment produces a multiscale particle-size gradient, and that hydrolysis limitations transition from microstructural barriers to macroscopic rheological constraints as solid loading increases.