Custard Theory and Practice

In the West, custards are almost always made with milk or cream, but just about any liquid will do as long as it contains some dissolved minerals. Mix an egg with a cup of plain water and you get curdled egg floating in water; include a pinch of salt and you get a coherent gel. Without minerals, the negatively charged, mutually repelling protein molecules avoid each other as they unfold in the heat, and each forms only a few bonds with a few others. With minerals, positively charged ions cluster around the negatively charged proteins and provide a neutralizing shield, which makes it possible for the proteins to unfold near each other and bond extensively into a fine network. Meats are rich in minerals, and the Japanese make savory custards, chawanmushi (soft) and tamago dofu (firm), from both bonito and chicken broths. Vegetable stocks also work.