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How to curdle milk
Gently warm the milk.
Pour the milk into a saucepan and place the saucepan on the stove. Gradually warm the milk up over medium heat until steam just begins to form.
While the acid you will use in this method may curdle the milk on its own when used in large enough quantities, heat dramatically accelerates the process, making the milk curdle faster and more noticeably. This is especially important if you plan to separate the solid curds from the liquid whey, as you would do when making cheese.