Question

What is curdling of milk? Is it a desirable change or a undesirable change?

Answer 1

Curdling. ... This is what happens when milk curdles, as the pH drops and becomes more acidic, the protein (casein and others) molecules attract one another and become "curdles" floating in a solution of translucent whey. This clumping reaction happens more swiftly at warmer temperatures than it does at cold temperatures.

When things curdle, they turn from liquid gradually to solid, forming clumps along the way. If you leave milkout of the refrigerator long enough, it will curdle. When a liquid curdles, it forms curds, or lumpy solid masses. In some cases this is deliberate, as when you make cheese or tofu.

If you want to curd the milk then it will adesirable change, and if want to make a tea but the milk is curd at that time you can not able to make a tea, hence, in this case, this will be an undesirable change.