Asked by: Anonymous
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.