We found some beautiful purple cauliflower at a farm shop today. I have boiled this stuff in the past and had trouble keeping the intense colour when cooked. This time I tried steaming it, and the colour stayed put. Having studied physical chemistry (some years ago now), I asked myself why this should be the case.
|Cauliflower steaming water (left), and with two drops of vinegar (right)|
I noticed that the cooking water in the pan had turned a rather delightful shade of emerald green, which at first seems odd. Recalling that organic colouring molecules often have pH dependence, I added two drops of vinegar to the liquid to acidify it - within ten seconds the colour had switched to pink-purple.
Our kettle here tends to coat with limescale pretty quickly here - there is certainly calcium carbonate dissolved in our water supply, which is basic (alkali) in character. So, the dye is green in alkali conditions, and pink/purple when acidic.
So, if one wants to keep the nice purple colour, we need to keep the cauliflower acidic whilst it is cooking,. Steaming it, or addition of vinegar or lemon juice to the boiling water would seem to be the way to go. Although the latter would make it taste of vinegar.
This may well apply to other vegetables!