Ball Milling Controls Particle Descriptors and Diffusion-Limited Leaching in a Wet Particulate System

Ball milling can improve protein recovery from defatted rice bran, but the links among milling conditions, particle attributes, and extraction transport remain insufficiently defined. This study evaluated the effects of milling time (30–90 min) and rotational speed (30–120 rpm) on powder properties and alkaline protein extraction at pH 11 for 30–180 min at 24, 37, and 50 °C. Powders were characterized by laser diffraction, SEM image analysis, X-ray diffraction, and extraction-relevant indices describing the interfacial area and diffusion time scale. Extraction curves were fitted to first-order, pseudo-second-order, Peleg, and apparent Fick diffusion models. Milling reduced median particle size from 145 to 61 µm, increased fines (<45 µm) from 1.86% to 32.09%, and raised surface area proxies by about 30- to 40-fold. Compared with the control sample, milled samples generally showed faster extraction and higher protein recovery, with maximum endpoint recoveries of 89.91 mg g−1 at 24 °C, 90.06 mg g−1 at 37 °C, and 86.10 mg g−1 at 50 °C. Late-stage extraction data collapsed onto a Fickian master curve, indicating diffusion-limited behavior, and apparent effective diffusivity increased with temperature. At 37 °C, the radius–shape–circularity model explained nearly all the between-powder variation in lnDeR2=0.998;adjusted R2=0.996, and the shape factor remained significant after accounting for particle radius p0.0179. Overall, ball milling improved extraction primarily by reducing diffusion length and altering particle morphology, providing practical guidance for optimizing rice bran protein recovery.