Introduction:
Curdling of milk in the stomach of adults is a natural process that occurs during digestion. This process is facilitated by an enzyme called rennin, also known as chymosin. Rennin is primarily found in the stomach lining of young mammals, including humans, but its production gradually decreases as they grow older. However, some adults still produce small amounts of rennin, which allows them to digest milk.

Function of Rennin:
Rennin plays a vital role in the digestion of milk proteins, particularly casein. Casein is the primary protein present in milk, constituting about 80% of its total protein content. It is a complex protein made up of several smaller protein molecules.

Process of Curdling:
When rennin is present in the stomach, it acts on casein and causes it to undergo a process called curdling. Curdling is the thickening and coagulation of milk, resulting in the formation of a solid mass known as curds. The curds are then separated from the liquid portion of the milk, known as whey.

Mechanism of Curdling:
The process of curdling involves several steps:

1. Activation: Rennin is secreted in an inactive form called prorennin. In the presence of hydrochloric acid (HCl) in the stomach, prorennin is converted into its active form, rennin.

2. Coagulation: Once activated, rennin acts on casein molecules by cleaving a specific peptide bond. This results in the formation of smaller casein fragments called paracasein. Paracasein molecules then aggregate together to form a network or mesh-like structure, entrapping fat globules and other substances present in milk.

3. Curd Formation: The aggregated paracasein molecules further bind together, forming larger curds. These curds are solid and can be easily separated from the liquid whey.

4. Digestion: After the formation of curds, other digestive enzymes, such as pepsin, act on the curds to break them down into smaller peptides and amino acids. These smaller molecules are then absorbed through the intestinal lining for further utilization by the body.

Conclusion:
In summary, the curdling of milk in the stomach of adults is facilitated by the enzyme rennin. Rennin acts on casein, the primary protein in milk, causing it to undergo curdling and resulting in the formation of curds. This process is an essential step in the digestion of milk proteins and enables the breakdown and absorption of nutrients present in milk.

Rennin