Optimization of papain-assisted hydrolysis for salmon by-product protein powder production using response surface methodology.

This study optimized papain-assisted enzymatic hydrolysis of salmon processing by-products using response surface methodology. A central composite design with three factorspapain concentration (1,591-18,409 U/g substrate protein), reaction time (99.5-200.5 min), and temperature (43.2-76.8 °C)was employed to optimize degree of hydrolysis (DH), protein recovery (PR), peptide yield (PY), and protein solubility (PS). Quadratic models demonstrated excellent predictive capability (R² > 0.98). Papain concentration exhibited the most significant positive effect on all responses (P < 0.001). Optimal conditions were: papain concentration 15,000 U/g substrate protein, reaction time 165 min, and temperature 55 °C, yielding predicted DH 15.42%, PR 86.85%, PY 68.92%, and PS 77.64%, which were subsequently validated by triplicate confirmation experiments (relative errors 0.9-1.6%). The spray-dried powder (with 15% w/w maltodextrin as carrier) contained 75.30% protein, 3.57% fat, 3.51% ash, 2.24% moisture, and 15.38% carbohydrate (by difference). Essential amino acids accounted for 46.71 g/100 g protein (45.26% of total amino acids), exceeding the FAO (2013) reference pattern. Lysine (8.95 g/100 g protein) and leucine (8.24 g/100 g protein) were the predominant essential amino acids. The powder exhibited excellent solubility (> 85% at pH 3-9) and favorable emulsifying properties (EAI 45.8 m²/g protein, ESI 42.5 min). These findings demonstrate that papain-assisted hydrolysis effectively converts salmon by-products into functional protein ingredients for food applications.