Why does curd set in warm milk and not hot or cold milk?

The setting of curd is a process of coagulation or thickening of milk proteins, primarily casein. The optimal temperature range for curd formation is between 30-37 degrees Celsius (86-99 degrees Fahrenheit). This temperature range allows for the proper activity of the enzyme rennet, which is responsible for breaking down casein into smaller, soluble proteins.

In warm milk, the rennet reacts with the casein proteins, causing them to form a gel-like network that traps the fat and other solids in the milk. This network of proteins gives curd its characteristic texture and consistency.

However, if the milk is too hot (above 37 degrees Celsius), the rennet becomes denatured and loses its ability to coagulate the casein. As a result, the curd will not set properly. Similarly, if the milk is too cold (below 30 degrees Celsius), the rennet activity is slowed down, and the curd formation process is delayed or may not occur at all.

It's worth noting that different types of milk may vary in their ideal setting temperatures due to variations in protein composition and acidity levels. Therefore, the specific temperature range for setting curd may need to be adjusted slightly depending on the type of milk being used.