Quick Answer:Red cabbage contains anthocyanin pigments whose chemical forms change with pH. Acidic conditions favor red colors; changing the acidity can shift the mixture toward purple, blue or green. The color reflects the pigments' environment, so a blue piece of cabbage is not automatically less nutritious than a red one.
What makes red cabbage red?
Anthocyanins are colored plant compounds. Red cabbage contains several of them, largely based on cyanidin with different attached groups. The cabbage leaf is a mixture, rather than a bottle containing one pure dye. Pigment amount, acidity and interactions with other compounds all influence its appearance.
How does red cabbage turn blue?
pH describes acidity. In an acidic solution, the red flavylium form of an anthocyanin is favored. Changing pH changes the balance between protonated, deprotonated and hydrated forms. Those forms differ in their electronic structures and in the wavelengths of visible light they absorb. The resulting color change is a chemical equilibrium, not a claim that tiny particles simply vibrate faster.
A cabbage extract can demonstrate the effect: acidic vinegar and a baking-soda solution shift it toward different colors. The mixture is a qualitative indicator, not a precise pH measurement.
Why can cooking change the color?
Cooking puts the pigments in contact with water and other ingredients, changes their concentration as they leak from the leaves, and can cause chemical degradation. The water's pH and added acidic or alkaline ingredients affect the shade. Areview of anthocyanin chemistry by Dangles and Fengerdescribes both pH-dependent transformations and instability during heating and storage. A color change during cooking therefore does not establish that acid has evaporated or that the food has become healthier.
What color can and cannot tell you
The same pigment mixture can look red or blue when its environment changes. Comparing those colors does not measure the amount of anthocyanin remaining, antioxidant activity in a person, or protection against disease. Laboratory radical-scavenging tests and human health outcomes are different questions.
Red cabbage's color is a visible demonstration of chemistry: molecules interact with their surroundings, their forms change, and the mixture absorbs light differently. That is enough to explain the blue without attaching an unsupported nutritional ranking to it.
Sources & further reading
That’s the mechanism. The wonder is still allowed.
