Valorizing the Limit Proteins of Soy Sauce Residue Using Halotolerant Alkaline Proteases from Endogenous Bacillus velezensis J2

• A novel endogenous strategy for SSR valorization is established. • Protease J2 from B. velezensis J2 targets recalcitrant limit proteins. • The protease retains high activity in broad pH (6-11) and high salt (4 M NaCl). • Outperforms commercial proteases in hydrolyzing SSR. Soy sauce residue (SSR), a high-salt industrial waste, contains a recalcitrant protein system characterized by low solubility (50% activity at 4 M NaCl). Protease J2 significantly outperformed commercial alternatives in hydrolyzing SSR’s "limit proteins". Scanning electron microscopy (SEM) confirmed microstructure modification of SSR from dense aggregates to loosened fibrous matrices. Collectively, the B. velezensis J2 proteases provide a sustainable, chemical-free route for SSR valorization, enabling high-value protein recovery without secondary pollution.