Production of Protein Hydrolysates from Grass Carp (Ctenopharyngodon idella) Scales and Their Antibacterial Activity

Fish scales, an abundant yet underutilized by-product of fish processing, are rich in proteins and thus hold significant potential for value-added applications. This study aimed to develop a sustainable method for converting grass carp (Ctenopharyngodon idella) scales into bioactive protein hydrolysates and evaluate their potential as natural antimicrobial agents. Fish scale protein hydrolysates (FSPHs) were prepared through citric acid extraction followed by pepsin enzymatic hydrolysis. Antimicrobial activity and stability were systematically assessed against Escherichia coli and Staphylococcus aureus, alongside mechanistic investigations. Results demonstrated the potent inhibitory effects of FSPHs against both pathogens, with minimum inhibitory concentrations (MICs) of 4.2 μg∙mL−1 and minimum bactericidal concentrations (MBCs) of 67.5 μg∙mL−1 for E. coli and 33.7 μg∙mL−1 for S. aureus. FSPHs exhibited exceptional thermal stability (<100 °C) and retained functionality over 10 freeze–thaw cycles. Mechanistic studies have revealed enhanced bacterial membrane permeability upon FSPH treatment, with microscopic evidence of cell aggregation and lysis after 16 h of exposure. This work validates grass carp scales as a viable source of antimicrobial peptides through optimized extraction protocols, offering a circular economy solution for fishery waste. The findings provide actionable insights for policymakers to promote eco-friendly alternatives to synthetic antibiotics while advancing methodologies for bioactive peptide research.