Biovalorization of Poultry Feather Waste via Indigenous Keratinolytic Strains (Pseudomonas sp., Serratia marcescens, and Actinomyces howellii): Enzyme Kinetics and High-Cysteine Functional Protein Recovery

Abstract Global poultry processing generates over 8.5 million metric tons of feather waste each year. Most of this biomass ends up in landfills or undergoes high-pressure thermal rendering that destroys heat-sensitive essential amino acids. In this study, we developed an enzymatic alternative using four native Egyptian bacteria: Pseudomonas putida CR1, Pseudomonas fluorescens CR2, Serratia marcescens CR3, and Actinomyces howellii CR4. Molecular screening confirmed the catalytic kerA keratinase gene (~ 1.1 kb) exclusively in P. putida CR1. The other strains lacked kerA , showing that consortium-mediated feather breakdown relies on diverse, non-redundant proteolytic families. When grown together as a four-strain consortium, the bacteria eliminated the initial lag phase observed in single cultures, degraded whole feathers within 72 h, and achieved a peak keratinase titer of 3.45 U/mL a 30.2% increase over the top monoculture. Comparative tests against mild alkaline (0.5 M NaOH at 40°C) and concentrated acid (2.0 M HCl) hydrolyses demonstrated the clear biological advantage of the bacterial route. Enzymatic processing operated under physiological conditions (37°C, pH 7.0), preserving 80% to 95% of native cysteine and over 70% of lysine and tryptophan. Consequently, the bacterially hydrolyzed feather protein achieved an in vitro protein digestibility of 88.5 ± 2.1%, a PDCAAS of 85, and an estimated biological value of 80, compared to only 45 and 35 for alkaline and acid isolates. Wastewater chemical oxygen demand was just 30 ± 4 g/L COD, representing an 82.8% to 86.0% reduction in organic discharge relative to chemical treatments. Containing 40.53% cysteine, this hydrolysate serves as a bioavailable, high-cysteine protein concentrate to balance animal feed rations and support the regional circular bioeconomy.