Milk tastes sour when kept in the open for sometime due to formation of

Options

A. carbonic acid
B. citric acid
C. lactic acid
D. malic acid

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks why milk tastes sour when left open for some time. This indicates a change in the chemical composition of the milk.

2. **Identifying the Process**: When milk is exposed to air, it undergoes fermentation. This process is primarily due to the action of bacteria present in the environment.

3. **Role of Bacteria**: The specific bacteria responsible for this souring of milk is called *Lactobacillus*. These bacteria are naturally present in milk and can thrive when the milk is left out in the open.

4. **Lactose Conversion**: The *Lactobacillus* bacteria act on lactose, which is the sugar present in milk. They ferment lactose and convert it into lactic acid.

5. **Formation of Lactic Acid**: The accumulation of lactic acid in the milk leads to a decrease in pH, which causes the milk to taste sour. This sour taste is a characteristic of lactic acid.

6. **Conclusion**: Therefore, the sour taste of milk when kept open is due to the formation of lactic acid as a result of the fermentation of lactose by *Lactobacillus* bacteria.

**Final Answer**: Milk tastes sour when kept in the open for some time due to the formation of lactic acid.

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