Milk curdled so as to become thick.

raw milk that has soured and thickened

turn into curds; "curdled milk"

Milk which has soured to the point where it is thick and curdy but not separated.

Clabber is a food produced by allowing unpasteurized milk to turn sour at the proper humidity and temperature. Over time the milk thickens or curdles into a yoghurt-like substance with a strong, sour flavor. In rural Virginia it is commonly eaten for breakfast with brown sugar, nutmeg, cinnamon, or molasses added.