golden beets...turn brown after cooking? ok to eat?

[melitoto]

hello,

i cooked some golden beets in the oven last night in a foil packet, let them cool, peeled them, and sliced them into rounds. they were on my kitchen counter (i was waiting for them to cool completely before i stored them in the fridge) and they started turning brown...like apples do. is this common? is this just oxidation?

thanks in advance!
melissa