This is what happens when milk curdles. When pH levels drop in milk, it turns acidic and milk protein (casein and others) molecules attract one another to form "curdles" or lumps. These lumps then float on the surface of the solution. The lumps are formed faster at warmer temperatures.

At what pH curdling occurs?

All of the casein molecules have at least one ester bound phosphate. None of the whey proteins have this. The isoelectric point of casein is 4.6. Since milk's pH is 6.6, casein has a negative charge in milk.

What can cause curdling?

Curdling occurs when the proteins in a sauce denature and bind together, separating from the water and tightening up into curds. Dairy or egg-y sauces can curdle for several reasons: There might not be enough fat in the sauce; skim milk will curdle much more easily than other, fattier dairy products.