Acid hydrolysis of gelatin extracted from cow skin: properties and potential for use as a source of small peptides and free amino acids for broiler chickens

Abstract

Context Acid hydrolysis of animal and plant protein ingredients can generate beneficial and cost-effective peptides and free amino acids for use in livestock feed. Aims The aim was to determine whether a cow-skin gelatin (CSG) hydrolysate, rich in low-molecular-weight peptides and free amino acid and produced through acid (4 M HCl) hydrolysis, improved the digestibility of CSG and performance of broilers. Methods Day-old Ross 308 chicks (n = 320) were allocated to four treatments (control, maize–soy basal diet; CSG at 92 g/kg basal diet; acid-hydrolysed CSG (AHCSG) at 45 and 92 g/kg basal diet) in a completely randomised design with eight replicate pens per treatment and 10 chicks in each pen. Molecular weights of CSG and AHCSG were characterised by SDS–PAGE, and protein concentration and degree of hydrolysis of AHCSG determined. Broilers were assessed for performance measures and intestinal and serum characteristics. Key results The AHCSG had a protein concentration of 84.3%, with a degree of hydrolysis of 66.5%. Respective molecular weights of CSG and AHCSG were 20–128 kDa and 3.5–10 kDa. Bodyweight gain and feed intake were dramatically (P ≤ 0.001) reduced and feed conversion ratio increased with inclusion of CSG and especially AHCSG in the diet. Inclusion of AHCSG reduced (P < 0.05) ileal digesta viscosity compared with CSG, and reduced (P < 0.05) plasma uric acid concentration, villi height and crypt depth compared with the CSG and control diets. AHCSG inclusion in the diet reduced protein digestibility by ~25% (at 45 g/kg) and 50% (at 92 g/kg) compared with the control; CSG also reduced protein digestibility by ~50%. The Salmonella population of ceca was reduced (P = 0.05) with the diet containing 92 g AHCSG/kg compared with the CSG and control diets. Trypsin activity was not affected by diet, but total alkaline protease activity was reduced (P < 0.01) at days 35 of age by inclusion of AHCSG in the diet. Carcass and breast relative weight, and breast fillet crude protein were lower (P < 0.05) for broilers fed diets containing CSG or AHCSG than the control. Conclusions Acid hydrolysis of CSG produced large quantities of free amino acids and small peptides, but this did not improve the performance of broiler chickens compared with CSG. Performance was similarly poor with dietary inclusion of AHCSG and CSG relative to the control diet without CSG products. Implications Although large quantities of free amino acids and small peptides were produced by acid hydrolysis of CSG, further research is needed to understand the possible problems with acid hydrolysis and why it negatively affects broiler performance.