Why Does Red Cabbage Change Color When Cooked?
Red Anthocyanin Pigments in Cabbage Turn Color with Changes in pH
Feb 26, 2009
Red cabbage (Brassica oleracea var. capitata f. rubra), also referred to as purple cabbage, is one of many fruits and vegetables to contain a class of reddish purple pigments called anthocyanins. Anthocyanins are a type of flavonoid pigment responsible for the red, purple and blue colors of most plants, including leaves, flowers and fruits.
Red Cabbage Contains High Concentrations of Anthocyanin Pigment Molecules
Red cabbage contains at least thirty-six of the over 300 different anthocyanins responsible for the range of red and blue colors in plants, according to researchers Charron et. al. in their recent article published in the Journal of Agriculture and Food Science (Vol 55 No 3).
These pigment molecules are stored in the cells of the red cabbage leaf. When exposed to heat during cooking, anthocyanin containing cells burst open, causing the water-soluble color pigments to bleed into the surrounding liquid. This explains the immediate color change in the cooking water as red cabbage boils, eventually resulting in a colored liquid referred to as cabbage juice.
Anthocyanin Pigments in Red Cabbage are Sensitive to Changes in pH
Like other flavanoid molecules, anthocyanins consist of several carbon rings onto which hydrogen or hydroxyl groups are attached. This particular chemical formation allows anthocyanin molecules to take on two forms - one where a hydrogen atom attached is attached to the exterior and one where it is not. Acidic ingredients are characterized by having more hydrogen atoms (H+) than hydroxyl groups (OH-) so when exposed to an acid, anthocyanin grabs a hydrogen atom and turns red. In alkaline conditions where there are not excess hydrogen atoms, the molecule appears blue or green.
Color Changes in Red Cabbage Can be Minimized or Enhanced with Kitchen Ingredients
Anthocyanin turns red in acidic conditions when the pH is less than 7. It is not uncommon for recipes for braised red cabbage to include apples or lemon juice, as this maintains the cabbage's red color rather than having the cabbage juice turn an unappetizing blue or purple. Common acidic ingredients used in cooking include:
- vinegar
- lemon juice
- cream of tartar
- citric acid
- fruits and fruit juices
Anthrocyanin pigment turns blue or green in alkaline environments or when the pH is greater than 7. Baked goods, which frequently use baking soda or baking powder as a leavening agent, can discolor fruits and vegetables, like red cabbage, which contain anthocyanins. Common basic ingredients include:
- baking soda
- baking powder
- calcium carbonate
- hard tap water
This reaction is also the reason that eggs dyed with red cabbage juice turn blue, as the alkaline calcium carbonate shells of the egg react with anthocyanin molecules.
Red cabbage juice is a known acid/base indicator as the anthocyanins change color in the presence of acids and bases. An understanding of simple cabbage chemistry allows an individual to adjust the pH of a recipe to prevent undesirable discoloration.
For more information on related red and purple food pigments, see Beets and Betanins.
References on Red Cabbage Chemistry
Brown, A. 2004. Understanding Food: Preparation and Principles. Wadsworth/Thomson Learning.
Charron, C et al. 2007. "Effect of Dose Size on Bioavailability of Acylated and Non-acylated Anthocyanins from Red Cabbage (Brassica oleracea L. Var capitata)" J. Agric. Food Chem. 55 (13) pp 5354-5362
McGee, H. 2004. On Food and Cooking. 2nd Ed. Scribner.
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