Have you ever kept a batch of kraut around long enough to see it discolor at the top? This happened to me recently. It’s been so long since this happened – because we usually eat our ferments so fast – that I had kind of forgotten about the phenomenon.
But I remembered back when I used to keep ferments in a refrigerator in half-gallon jars. Eventually I had some of the top of the kraut go from green to brownish. Or, in this case, a jar of kimchi ended in two completely distinct coloring patterns – the red of the chilies at the bottom and the blah brown cabbage at the top.
It turns out there is a chemical compound that causes this to occur and, thinking back on the occurrences, I think I may have figured out why it happens. Or at least I have a guess.