Pepsin-induced hydrolysis and coagulation of proteins in goat, sheep and cow milk

The kinetics of pepsin-induced κ-casein hydrolysis and coagulation in cow, goat, and sheep milk were investigated at 37°C, pH 6.3 and 6.0. At a 0.1 U/mg pepsin-to-κ-casein ratio, sheep milk showed the fastest κ-casein hydrolysis, followed by cow and goat milk as assessed by quantifying the release of para-κ-casein using RP-HPLC. Sheep milk coagulated most rapidly, with κ-casein hydrolysis of 64% and 59% at pH 6.3 and 6.0. Goat milk required higher levels of κ-casein hydrolysis (90% and 86% at pH 6.3 and 6.0), before coagulation could occur. Sheep milk formed denser curds, whereas goat milk curd had a more porous structure. Additionally, small-angle neutron scattering showed differing rates of aggregate size growth among species. These findings indicate κ-casein hydrolysis and coagulation differences are not just due to casein content but also physicochemical characteristics such as casein micelle size. This study deepens our understanding of sheep and goat milk coagulation mechanisms compared to cow milk during the early stages of gastric digestion.