Explanation

Porennin is secreted in the young ones of mammals i.e., it is prominent in infants. $HCl$ secreted by parietal cell activates the inactive form of enzymes. Rennin enzyme is responsible for the digestion of milk protein in infants Porennin $\overset{\ \ HCl\ \ }{\rightarrow}$ Rennin Casein $\overset{\ \ Rennin\ \ }{\rightarrow}$ Paracaesin $Ca \rightarrow Ca$ Paracaseinate