How to curdle milk without lemon and vinegar?

I use citric acid crystals (called nimbu satva in India). Just a pinch in hot milk, and it curdles beautifully. This is also safer because no extra flavor is added.

If you don’t have any acidic item, you can simply boil milk longer. After a while, the proteins separate naturally, but it won’t be as clean as with lemon/vinegar.

Another method is souring with buttermilk. Heat the milk, then add buttermilk slowly while stirring. Works especially well for making paneer.

You can use curd (yogurt) as a natural curdling agent. Just heat the milk and add 1-2 tablespoons of curd. Stir and keep it warm. It will curdle nicely in 5–10 minutes.