Why does red cabbage sometimes turn purplish-blue when cut with a knife?

If a non stainless metal knife is used to cut a red cabbage, the anthocyanin pigment in the cabbage will react with the metal ions in iron, tin, or aluminium and create off-colors. Stainless steel and glass are consequently the best choices for the preperation and storage of foods rich in anthocyanins. Most of red, purple, and blue color seen in vegetable and fruits derive the color from anthocyanin.

There are four nitrogenous bases that make up dna. the two pyramidines are thymine and cytosine. the two purines are adenine and guanine. these bases pair in a very specific way. adenine - thymineguanine - cytosine i remember it like this: a tall, good cat.