Why does red cabbage extract change it's colour when cooled?

Answer 1

Red cabbage contains water soluble pigments called "Anthocyanins." These pigments will change color depending on the pH, or acidity of the solution, and to a lesser extent, temperature. (try adding vinegar or baking soda to the solution....watch it change color.....)

This is an example of "Chemical Equilibrium."

To completely understand why this happens, you need to know quite a bit about chemistry. I'll try to summarize as much as possible.....

Basically, the pigment in cabbage exists in two forms, an ionized version (reddish-purple), and a non-ionized version (blue-green.) In technical terms:

[An]OOH ↔ [An]O(+) + OH(-)

This reaction can go both ways, however, an acidic solution tends to favor almost exclusively, the production of the ionized version. Basic solutions tend to neutralize nearly all of the reddish ionized form, converting most of the pigment into the blue green.

If the pH of the solution is near the so-called "critical point"; where both forms exist in roughly equal proportions, then the tendency of pigment to be converted into one form or another, can be strongly affected by temperature.

The reason for this is somewhat more complicated. In order to become ionized, it requires a certain amount of energy, usually in the form of heat. If the solution is cold, the production of the ionized version happens much more slowly, since less heat is available, which means there will tend to be more of the blue-green pigment. However, if the solution is already mostly acidic, then the color will be unaffected by temperature, since most of the pigment is already ionized. If the solution is basic, then any of the pigment that *does* get ionized will quickly be converted back into the non ionized form.

Hope than makes sense,
~W.O.M.B.A.T.