What makes custard curdle?

Warm custard and sauces on a low heat to avoid lumps – unless you’re making cheese, tofu or lemon curd.

10th September 2015
What makes custard curdle? (iStock)

Asked by: BBC Focus - The Great British Bake Off Q&A special

Milk, one of the main ingredients of custard, contains proteins. These proteins have a specific structure that is sensitive to changes in temperature and acidity.

At room temperature, the proteins float around by themselves. But adding an acid like lemon juice, or increasing the temperature, damages or ‘denatures’ the specific structure, exposing charged parts of the molecule. Charged proteins are attracted to each other in the milk and form clumps, resulting in lumpy custard.

This question was part of The Great British Bake Off Q&A special, which appeared in the September 2015 issue of BBC Focus magazine. Read more of our Bake Off questions.


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