Formation of a structured clot during the gastric digestion of milk: Impact on the rate of protein hydrolysis

The digestion behaviour of unheated milk and heated milk (90 degrees C for 20 min) was investigated in a human gastric simulator (HGS). Unheated milks formed a clot after 10 min digestion, but the structure of the clot formed from unheated milk was different form that formed from heated milk. The clot obtained from unheated milk showed a closely knitted network with numerous small pores interspersed throughout the matrix, while a network structure with larger voids was observed in the clot of the heated milk. With increasing digestion time, as the pH decreased further, the structures of the clots tightened and became less permeable to serum and solutes. These changes apparently affected the hydrolysis of casein by pepsin in the gastric system. In unheated milk, casein hydrolysis was much slower than that in the heated milk. Whey proteins, beta-lg and alpha-la, remained largely intact during the whole digestion period in unheated milk but in heated milk whey proteins were readily hydrolysed by pepsin. In heated milk, both casein and whey protein are involved in the formation of the clot. The present results indicate the clot formation is initially driven by the action of the milk-clotting enzyme pepsin on K-casein and suggest that the formation of clots with different structures due to the different previous treatments affects the rate of protein proteolysis in the stomach. (C) 2015 Elsevier Ltd. All rights reserved.