As sauerkraut ferments it becomes more acidic, and since purple cabbage is a pH indicator, the sauerkraut should change color. I shredded a head of purple cabbage, mixed it with salt and thyme (yum!) and packed it into a plastic container. I took a picture every evening when I got home from work to see how it had changed.
I had hoped to post the whole series of photos here, but the sauerkraut didn't act as I had expected. The change was dramatic, but it happened too quickly for the series of photos to be of any use. The appearance of the sauerkraut did not change at all, until day 3 when it changed drastically. After that its appearance did not change again.
I will try this experiment again, but will have to change how I photograph the sauerkraut. I plan to set up a camera on a tripod taking pictures several times a minute so I can stitch them together into a time lapse video. Hopefully that will catch the dramatic change in detail, as well as some of the more subtle action like formation of bubbles.
And since I'm already pulling stuff off my camera, here's some dude rocking out on an old busted spinet piano at the MacArthur BART station.