Curdle (verb)

1. To cause a liquid, typically milk, to separate into solid and liquid parts, typically as a result of souring or the addition of a substance such as rennet.
2. To cause a liquid to become thick and lumpy, often as a result of being spoiled or sour.
3. To cause something to become thick and lumpy.

Origin:

Middle english: from old english cyrdelian "cause to curdle", from cyrdel "curd, cheese".

Examples:

1. The milk curdled when it was left out of the fridge.
2. The milk was curdled and had to be thrown away.
3. The mixture curdled and became difficult to work with.
4. The batter curdled and ruined the cake.
5. The cream curdled when it was added to the hot soup.