Pink colour formation in hydrolysed casein

Summary Pink discolouration in a casein hydrolysate prompted an investigation into its formation. When an aqueous solution of the hydrolysate was held at 110 °C, an intense red colour was generated, as was a strong sulphide odour. In a study of colour and amino acids in a hydrolysate solution held at 110 °C for 0–11 h, a substantial cystine loss occurred, with which colour formation was correlated. The red material exhibited affinity for an anion exchange resin, and its analysis by HPLC found ≥7 apparent pigment compounds. These characteristics, along with the free amino acids in the hydrolysate, seem consistent with the formation of thiosulphinate‐derived, conjugated, carboxylated and pyrrole‐based pigment compounds proposed to explain pinking in processed onions. The data raise the possibility that pathways responsible for pink discolouration in the hydrolysate (and possibly to some instances of pink discolouration observed in cheese) are related to those associated with onion pinking. Upper left: casein protein hydrolysate (CPH) pink colour was inversely correlated with cystine concentration; Upper right: intense pink colour was generated in 20% (w/w) aqueous solutions of CPH A and in CPH B held at 110 °C; Lower left: Pink colour generation in 20% aqueous CPH solutions increased with time at 110 °C; Lower right: Pink colour in 20% aqueous CPH solutions increased with cystine loss