Fractal kinetics unveils the dual regulatory mechanism of sophorolipid in overcoming spatial heterogeneity during lignocellulose hydrolysis.

Fractal kinetics was employed to elucidate how sophorolipid enhances enzymatic hydrolysis of lignocellulosic substrates. Graded bagasse substrates with varying lignin and accessible surface areas were prepared by controlled alkaline hydrogen peroxide pretreatment. Non‑productive cellulase adsorption onto lignin was quantified by Langmuir isotherms, and accessible surface areas of cellulose and lignin were separately measured by dye adsorption. Sophorolipid reduced the Langmuir adsorption constant by 73.8-85.7 % and binding affinity by 77.8-88.3 %, with reductions positively correlated to lignin surface area. Fractal kinetic analysis of time‑course data (5-20 FPU/g) showed that, for pretreated substrates, sophorolipid increased the rate constant (k) by 15.4-52.0 % (0-10 h) and by 22.2-79.2 % (10-72 h), compared with 7.1 % and 50.0 % for the untreated control. The fractal dimension (h) exhibited substrate‑dependent responses: at 5 FPU/g, h increased for most substrates in both stages (up to + 189.4 % initially and + 39.0 % later), but decreased for some (down to -4.9 % in the later stage); at 20 FPU/g, h generally increased but decreased for highly accessible ones. Thus, sophorolipid substantially elevates k across all substrates, whereas h increases on recalcitrant substrates (due to sophorolipid‑lignin interfacial complexation) but decreases on less recalcitrant ones or at high enzyme loadings (due to barrier alleviation). The net efficiency gain is driven primarily by k enhancement, not by simple homogenization. Correlation analysis identified lignin content and cellulose accessible surface area as primary yield determinants. Sophorolipid was most effective on substrates with high lignin and low cellulose accessibility, providing a quantitative basis for cost‑effective strategies to reduce enzyme dosage in biorefineries.