At work, we use wedged then cooked baby candy stripped beets in several of our dishes, and the day after we cook them, they start getting black spots that get bigger and uglier. I have used the following methods: Sous Vide 137 deg F for ~40 min (they still get spots whille under sous vide ); simmering in escabiche liguid (sherry and sherry vinegar) for ~15, and cooking in a pan with EVO and minimal liquid with a parchment cover. In all these cases, they have been cooked untill they still have a little bite to them. Only the candy stripes get these spots, baby golds and red seem immune. Is there any way I can prevent this?