Why does bicarbonate of soda alter the colour of cabbage?
Why does the addition of bicarbonate of soda to the cooking of cabbage make it much softer and the colour stay much brighter?
The reason that cabbage goes a nasty brown colour when you cook it for too long is because it changes the chemical nature of the green stuff in the leaves, or the chlorophyll. If you put bicarb in there, it changes the conditions by making it more alkaline. This means that the magnesium ion that would pop out of the leaves in normal water, stays in there.