Curd is sour in taste due to the presence of an acidic substance.

Answer

**Step-by-Step Solution:**

1. **Understanding Curd Formation**: Curd is made from milk through a process of fermentation. When milk is left to ferment, it undergoes a transformation due to the action of certain bacteria.

2. **Role of Lactobacillus Bacteria**: The primary bacteria involved in the fermentation of milk to form curd is called Lactobacillus. This bacterium plays a crucial role in converting lactose (the sugar present in milk) into lactic acid.

3. **Lactose Fermentation**: When Lactobacillus acts on lactose, it breaks it down and produces lactic acid as a byproduct. This process is known as fermentation.

4. **Acidic Nature of Lactic Acid**: Lactic acid is an organic acid that contributes to the sour taste of curd. The presence of this acid is what makes curd taste sour.

5. **Conclusion**: Therefore, the statement that curd is sour in taste due to the presence of an acidic substance is true. The acidic substance in question is lactic acid, produced by the fermentation of lactose by Lactobacillus bacteria.

**Final Answer**: True, curd is sour in taste due to the presence of lactic acid.

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