Sequential Pyruvic Acid Pretreatment and Alkaline Extraction of Bamboo Leaf Phenolics: Two-Stage Mass Transfer Kinetics.

This study examined total phenolic release and mass transfer during sequential pyruvic acid (PA) pretreatment and sodium hydroxide (NaOH) extraction of bamboo leaves. A 4 × 4 design comprised PA concentrations of 7.0, 8.0, 9.0, and 10.0% and NaOH concentrations of 3.0, 4.0, 5.0, and 6.0%. Kinetic curves from 40 to 120 min showed rapid release followed by a gradual approach to equilibrium. Normalized conversion F(t) was used to evaluate release progression, and two consistent fitting windows were applied for parameter estimation. The LDF model described the 40, 60, and 80 min data and provided kf, while the Crank spherical diffusion model described the 80, 100, and 120 min data and provided Deff. These overlapping windows represent different portions of the same continuous extraction process rather than physically separate stages. Model comparison and predictive validation supported the framework. The characteristic time ratio τm/τd exceeded 2 under all tested conditions, indicating that external liquid film transfer contributed the greater relative resistance. The condition of 8.0% PA and 5.0% NaOH provided the most balanced performance, yielding 18.55 ± 0.26 mg GAE/g, equivalent to 94.4% of the maximum. These results provide a practical basis for rate control analysis and condition selection.