Funny thing happened to me tonight. I was buying some falafel and noticed that they did not have any "purple cabbage" (a.k.a. red cabbage) as a side dish option. I asked if they had any in back, and was told they are not allowed to have it because they are a mehadrin eatery.
Now, the Purple Cabbage in the store carries a mehadrin Hechsher, and when I asked about it further he said something about the pickles, or vinegar, I'm not sure which.
What Kashrut issue could there be with purple cabbage that would not exist with regular "sauerkraut", that the Mehadrin Rabanut would not allow this person to carry it in their Falafel-only stand?
Edit: I guess the context wasn't very clear, but this is in Israel. The Rabanut of the city, in order to give a "Mehadrin" Hashgagcha requires that they don't use any "purple cabbage", (Which is Red cabbage mixed with mayonaise), but if they just wanted a regular "Kosher" status, then they could have used it. They do allow the use of Regular white cabbage though, which has no mayonnaise and is pickled.
To further clarify