I bought a red cabbage and when I was cooking, it turned the water blue.. which reminded me of an experiment we did at school.. the juice from the cabbage has an interesting property – it turns red in the presence of an acid and blue in the presence of an alkali.
I’ve taken some pictures but the lighting isn’t great – turns out fluorescent kitchen lights aren’t best for showcasing colours
1/4 cup Sliced red cabbage, 1/4 cup boiling water, a few teaspoons of vinegar and some bicarbonate of soda.
Put the cabbage in the boiling water and stir it a bit. The water will go a bluish purple. (Our water is obviously slightly alkali, as it is really blue)
Here is the cabbage juice, vinegar and bicarb:
I’m adding the vinegar (an acid) to the cabbage juice..
You can see it starting to go red.. (well, reddish purple)
and then when you add the bicarb (an alkali)..
..It goes blueish again (and fizzes beautifully)
It occurred to me that since alcohol is acid, you should be able to use cabbage juice to figure out how much alcohol is in homebrew.. (By comparing it with various dilutions of vodka). If you really wanted to.