Coagulation of milk is:

Options

A. hydrolysis of lactose
B. denaturation of proteins
C. breaking of peptide bonds
D. breaking of proteins into amino acid

Explanation

To solve the question "Coagulation of milk is:", we need to analyze the options provided and understand the process of coagulation in milk.

### Step-by-Step Solution:

1. **Understanding Coagulation**: Coagulation refers to the process where liquid transforms into a semi-solid or solid state. In the context of milk, this involves the proteins in milk aggregating together.

2. **Identifying the Role of pH**: When curd (which contains Lactobacillus bacteria) is added to milk, these bacteria produce lactic acid. The production of lactic acid lowers the pH of the milk.

3. **Effect of pH on Milk Proteins**: The primary proteins in milk, such as casein, are sensitive to changes in pH. When the pH decreases, the proteins begin to aggregate instead of remaining dispersed in the liquid.

4. **Denaturation of Proteins**: As the pH changes, the structure of the proteins is altered. This process is known as denaturation, where the proteins lose their natural structure (secondary and tertiary structures) but do not break down into amino acids.

5. **Final Conclusion**: The coagulation of milk is primarily due to the denaturation of proteins caused by the change in pH from the lactic acid produced by the bacteria in curd. Therefore, the correct answer is that the coagulation of milk is **denaturation of proteins**.

### Answer:
**Coagulation of milk is denaturation of proteins.**