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

**Step-by-Step Solution:**

1. **Understanding Coagulation**: Coagulation of milk refers to the process where milk proteins, primarily casein, aggregate and form a gel-like structure. This process is crucial in the formation of products like cheese and curd.

2. **Role of pH**: The coagulation of milk is significantly influenced by the pH level. When the pH of milk is altered (usually decreased), it causes the proteins to change their structure.

3. **Casein Protein**: Milk contains casein proteins which exist in a stable form at a specific pH. When the pH changes, the secondary and tertiary structures of these proteins begin to denature.

4. **Denaturation of Proteins**: Denaturation is the process where proteins lose their natural structure due to external factors such as pH changes or heat. In the case of milk, when the pH decreases (for example, by adding lactic acid from bacteria like Lactobacillus), the casein proteins unfold and aggregate.

5. **Formation of Curd**: As the casein proteins aggregate, they form lumps or curds. This is the coagulated state of milk. The process is similar to what happens when making curd from milk by adding a small amount of existing curd, which introduces lactic acid bacteria that lower the pH.

6. **Conclusion**: Therefore, the coagulation of milk is primarily due to the denaturation of casein proteins caused by a decrease in pH, leading to the aggregation of these proteins and the formation of curd.

**Final Answer**: The coagulation of milk is the denaturation of proteins.

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