Starch Extraction from Rejected Solanum tuberosum L. Optimized by Box–Behnken Design: Enzymatic and UV-Vis Spectrophotometric Quantification

Industrial potato processing generates substantial rejected-tuber streams that are rarely valorized. This study optimized wet-process starch extraction from rejected Solanum tuberosum L. (Pastusa variety) sourced from a Colombian snack processor (Comestibles Ricos S.A., Super Ricas, Bogotá), using a Box–Behnken design with three factors: temperature (20–40 °C), solid-to-liquid ratio (1:2–1:4, w/v), and citric acid concentration (0–6%, w/v; n = 26 per material). The reduced model (F = 9.236, p = 0.0003) identified temperature (β1 = +0.729) and solid-to-liquid ratio (β2 = +0.695) as the dominant yield drivers, with the identified optimum located at the boundary of the tested domain, indicating that an expanded range may yield further gains. Across the full experimental domain, rejected-grade potato produced significantly higher starch yields than standard-grade material (3.06 ± 1.20% vs. 1.85 ± 0.53%; Welch’s t = 4.70, p < 0.0001, Cohen’s d = 1.30). Under the identified optimal conditions (40 °C, 1:4 w/v, 3% citric acid), rejected-grade tubers yielded 5.41 ± 1.30%, 3.6-fold higher than standard-grade material processed under the same conditions (1.49 ± 0.14%), consistent with pre-existing intracellular starch release from mechanically damaged tubers. Total starch content reached 83.4 ± 2.6% (dry basis) as determined by the Megazyme K-TSTA-100A enzymatic kit (AOAC 996.11), with CV < 1.5% between independent extractions. UV-Vis spectrophotometry of the iodine–starch complex (600 nm; R2 = 0.9830) confirmed native amylose-rich starch identity. These results establish rejected Pastusa potato as a statistically robust circular-economy feedstock for native starch recovery.