Why does milk turn into curd when the lactose sugar present in it changes to lactic acid?

Options

A. Due to the release of antibodies
B. Due to coagulation of the casein protein
C. Due to the formation of carbon dioxide gas
D. Due to dehydration through the process of osmosis

Answer

To understand why milk turns into curd when lactose sugar present in it changes to lactic acid, we can break down the process into several steps:

### Step-by-Step Solution:

1. **Understanding Milk Composition**:
- Milk contains various components, including water, fats, proteins, lactose (milk sugar), and minerals.
- The primary protein in milk is called casein.

2. **Introduction of Bacteria**:
- To turn milk into curd, we introduce a specific type of bacteria known as Lactobacillus into the milk.
- These bacteria act as an inoculum that starts the fermentation process.

3. **Fermentation Process**:
- The Lactobacillus bacteria thrive in the milk at an appropriate temperature and begin to multiply.
- During this process, they convert lactose (the sugar in milk) into lactic acid.

4. **Change in pH**:
- As lactic acid is produced, the pH of the milk decreases, making it more acidic.
- This drop in pH is crucial because it triggers the next step in the curdling process.

5. **Coagulation of Casein Protein**:
- The acidity caused by the lactic acid leads to the coagulation of casein proteins in the milk.
- Coagulation means that the proteins begin to clump together, forming lumps or curdles.

6. **Formation of Curd**:
- These lumps of coagulated casein proteins float on the surface of the liquid, which is now referred to as whey.
- The solid mass that forms is known as curd.

7. **Conclusion**:
- Therefore, milk turns into curd due to the conversion of lactose into lactic acid by Lactobacillus bacteria, which causes the casein proteins to coagulate and form curd.

### Summary:
Milk turns into curd because the lactose sugar is converted into lactic acid by Lactobacillus bacteria, leading to a decrease in pH that causes the casein proteins to coagulate and form curd.