In vitro pepsin digestibility and amino acid composition in soluble and residual fractions of hydrolyzed chicken feathers

Beta-keratin in poultry feathers is a structural protein that is resistant to degradation due to disulfide and hydrogen bonds. Feather meal can be a valuable feed compound if the digestibility can be increased. The objective of the present study was to analyze the effects of chemical, enzymatic, and pressure-thermic treatments for chicken feathers on solubility, in vitro protein digestibility (IVPD), and amino acid composition of solubilized and residual fractions. Two experiments were conducted. In experiment 1, models for solubility and IVPD were developed including the above factors applying a central composite face-centered design. Addition of sodium hydroxide (NaOH) and sodium sulfite (Na₂SO₃) and autoclaving time affected solubility and IVPD of the feather hydrolysates, but not addition of keratinolytic enzyme. In experiment 2, 7 combinations of the hydrolysis factors NaOH, Na₂SO₃, and autoclaving time with a predicted IVPD of 900 g/kg of DM, calculated for the sum of solubilized and residual feather fractions, were included to measure effects on IVPD and amino acid composition in each fraction. The IVPD values were higher for solubilized than residual fractions when treated with NaOH and autoclaving, but no differences were found when treated with Na₂SO₃ and autoclaving. Losses of cystine were substantial for all treatments, but lower for Na₂SO₃ than for NaOH. Furthermore, use of lower Na₂SO₃ concentration and longer autoclaving time reduced losses of cystine. Compared with NaOH treatments, Na₂SO₃ gave lower losses of threonine, arginine, serine, and tyrosine. With reference to the ideal protein profile for Atlantic salmon (Salmo salar L.), the treatments with 60 or 90 min autoclaving and 0.36 or 0.21% Na₂SO₃ had the highest chemical scores. The scores were generally higher for amino acids in residual than solubilized fractions, but with 90 min autoclaving and 0.21% Na₂SO₃ differences were small. In conclusion, hydrolysis of chicken feathers with low concentrations of Na₂SO₃ combined with autoclaving results in feather meal with high nutritional value for Atlantic salmon; separation of solubilized and residual fractions is not necessary.