RESPONSE SURFACE METHODOLOGY (RSM) OPTIMIZATION OF MICROWAVE DRYING FOR NUTRIENT RETENTION IN BLACK BEAN (Vigna Unguiculata) SEEDS

Abstract

Black bean (Akidi) seeds are nutritionally valuable but suffer post-harvest losses due to inadequate drying practices. Microwave drying was optimized using response surface methodology across five power levels (140, 280, 420, 560 and 700 W), with drying times decreasing from 255 min at 140 W to 66 min at 700 W. Microwave power significantly affected all nutritional composition – moisture, ash, protein, fat, fibre, and carbohydrate (p < 0.0001), with drying rate constant increasing 452% from 0.0225 to 0.1243 min⁻¹. Drying time and power exhibited significant interaction (p = 0.0012), confirming cumulative energy input drives moisture removal. Drying exhibited a falling-rate period (Page exponent n decreasing from 0.8213 to 0.7698), with equilibrium moisture content of 0.024–0.089 g water/g dry matter. Effective moisture diffusivity increased 6.9-fold from 1.82 × 10⁻⁹ to 1.25 × 10⁻⁸ m²/s, with activation energy of 15.68 kJ/mol. Multi-response optimization identified optimal conditions of 560 W for 78 minutes, yielding moisture 6.80%, protein 22.61% (93.8% retention), fat 1.97%, fibre 5.38%, and carbohydrate 60.42%; meeting international standards (moisture ≤10%; protein ≥90% retention). Validated RSM models (R² = 0.9988–0.9994) confirm microwave drying at 560 W offers a commercially effective preservation strategy for the underutilized Vigna specie seeds.