Write the process of making curd from milk. Explain the changes that take place in the milk.

Curd, also known as dahi, is a common dairy product made by fermenting milk. The process of making curd is simple and involves both physical and chemical changes. Here’s a step-by-step explanation suitable for Class 10 CBSE students.

Process of Making Curd from Milk

1. Boil the Milk: First, fresh milk is boiled to kill any harmful bacteria and to make it safe for fermentation. Boiling also helps to thicken the milk, giving the curd a better texture.
2. Cool the Milk: After boiling, allow the milk to cool until it becomes lukewarm. The temperature should be around 30–40°C. If the milk is too hot, it may kill the bacteria needed for curd formation. If it’s too cold, the bacteria won’t grow.
3. Add a Starter (Curd): A small quantity of already prepared curd (about one tablespoon for half a litre of milk) is added to the lukewarm milk. This starter curd contains Lactobacillus bacteria, which help in the fermentation process.
4. Let It Set: The milk with the starter is kept undisturbed in a warm place for 6–8 hours. During this time, the bacteria multiply and convert lactose (milk sugar) into lactic acid.
5. Curd is Ready: After a few hours, the milk sets into a thick, white curd. It should have a slightly sour taste and a smooth texture.

Changes That Occur in Milk During Curd Formation

The major change in the milk during curd formation is a chemical change. The Lactobacillus bacteria convert lactose (a sugar) in the milk into lactic acid. This acid causes the proteins (mainly casein) in milk to curdle or coagulate, turning it into a semi-solid form.

Once formed, curd cannot be changed back into milk, which is a clear sign of a chemical change. However, the physical appearance of the milk also changes, from a liquid to a semi-solid.

In conclusion, making curd from milk is a natural fermentation process involving beneficial bacteria. It results in a nutritious and easily digestible dairy product commonly used in Indian households.

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