The initial step in the digestion of milk in humans is carried out by pepsin. Here's a detailed explanation:

Digestion of Milk
Milk is a nutrient-rich liquid that contains proteins, carbohydrates, fats, and various vitamins and minerals. The digestion of milk begins in the stomach, where it is broken down into smaller molecules that can be absorbed by the body.

Role of Pepsin
Pepsin is a digestive enzyme that is secreted by the stomach lining. It breaks down proteins into smaller peptides and amino acids. In the case of milk, pepsin plays a crucial role in breaking down the protein casein into smaller peptides. Casein is the primary protein found in milk and is responsible for its white color and creamy texture.

Action of Pepsin
Pepsin is activated by the acidic environment of the stomach. When milk enters the stomach, it mixes with gastric juices that contain hydrochloric acid. The acid denatures the proteins in milk, making them more susceptible to pepsin digestion. Pepsin then cleaves the peptide bonds in casein, producing smaller peptides that are more easily absorbed by the body.

Other Digestive Enzymes
While pepsin is the primary enzyme involved in the digestion of milk, other enzymes also play a role. Lipase breaks down the fat in milk into its constituent fatty acids and glycerol, while lactase breaks down lactose, the primary carbohydrate in milk, into glucose and galactose.

Conclusion
In summary, the initial step in the digestion of milk in humans is carried out by pepsin. This enzyme breaks down the protein casein into smaller peptides that can be absorbed by the body. Other enzymes, such as lipase and lactase, also play a role in the digestion of milk.