Trans-gauche-trans disulphide conformers measured by means of FT-Raman may be predictors of apparent digestibility of crude protein in feather meal fed to rainbow trout (Oncorhynchus mykiss)

•FT-Raman spectroscopy is a suitable method to predict digestibility of protein in feather meal.•Trans-gauche-trans conformers are predictors of digestibility of crude protein in feather meal.•The total of disulphide bonds is a weak indicator of digestibility of protein in feather meal.