Gamma Irradiation-Assisted Recovery of Keratin Hydrolysates from Chicken Feather Waste: Process Optimization, Peptide Characterization, and Antimicrobial Activity

Chicken feathers are a poultry by-product rich in keratin, a recalcitrant protein whose valorization requires efficient solubilization. This study evaluated gamma irradiation as a pretreatment for enhancing the enzymatic susceptibility and extraction of keratin from chicken feather powder (CFP). In vitro pepsin digestibility increased significantly from approximately 31% in non-irradiated CFP to 48% after irradiation at 25 kGy (p < 0.05), a 54% relative increase. Four solubilization methods were compared, and an alkaline–bisulfite system containing 1.78% NaOH and 0.5% NaHSO3 was selected based on protein recovery, reproducibility, and responsiveness to irradiation. Response surface methodology identified extraction conditions of 75 °C for 2 h at a liquid-to-solid ratio of 30:1. Irradiation increased the soluble protein recovery from approximately 72% in the control to over 90% at 20 kGy, although no significant improvement occurred beyond 15 kGy. The hydrolysates consisted predominantly of low-molecular-weight material (<15 kDa). Reversed-phase high-performance liquid chromatography (RP-HPLC) revealed irradiation-dependent variations in the relative abundance of chromatographic components, whereas the profiles of ten selected free amino acids remained broadly comparable among treatments. In preliminary antibacterial screening at a single concentration, fractions below 10 kDa produced the greatest growth reductions in Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa, particularly after irradiation at 15 and 25 kGy, whereas Serratia marcescens responded more strongly to the 10–30 and >30 kDa fractions. Overall, gamma irradiation increased the enzymatic susceptibility and extractability of feather keratin and influenced the hydrolysate characteristics, supporting further evaluation of these fractions for antimicrobial and other value-added applications.