Curdle/ˈkɜːrdl/ Definition 1 of 3Pro Tip 1/3Heat Sensitivity Curdling typically occurs when heat is applied to milk or cream, changing its texture. If you heat the sauce too quickly, the cream may curdle. Pro Tip 2/3Acid Reaction Acidic ingredients can cause dairy to curdle, important in cooking and food science. Adding lemon juice to warm milk will make it curdle. Pro Tip 3/3Spoilage Indicator Curdled milk is often a sign of spoilage and may indicate it's unsafe to consume. I knew the milk was bad when it curdled in my coffee.