Purple, or red, cabbage is sweeter and milder in flavor than other types of cabbage. The substance that provides its red color can turn blue when cooked in an alkaline liquid. This is remedied by cooking purple cabbage with an acidic ingredient such as wine or vinegar.
Color Chemistry
The color of purple cabbage comes from pigments called anthocyanins. Heat from cooking causes these molecules to burst, releasing pigments that turn blue in an alkaline environment. Tap water typically has a pH of 6.5 to 8.5. A pH over 7.0 is alkaline.
Cooking Purple Cabbage
Add an acidic substance, such as vinegar, wine or lemon juice, to the pot to retain the color of purple cabbage. Just 1/2 tablespoon per 1 cup of water should be enough. Many common recipes, such as for German red cabbage, include ingredients such as vinegar, so you won't need to add more.