The nutritional values of the protein are related to its amino acid content, and its physicochemical properties. We aimed to determine the relationship between the variation of protein digestibility or the releasing coefficient of amino acids with the physicochemical properties of different feedstuffs. We analysed the secondary protein structure and solubility of 10 protein-rich feed ingredients, including soybean, soybean meal, dehulled soybean meal, fermented soybean meal, distiller’s dried grains with solubles (DDGS), feather meal, fish...