Over the last 2-3 months I have been saving the shells of lobsters I have eaten, so that I can make bisque, which I am planning to do next week. I have them in a freezer bag. I noticed today that some of the shells are turning black, to about the colour they were before being cooked. Does this mean they have gone bad, or is this normal? If they are bad, do I have to toss out the whole lot, or just those that have changed colour? The former, I think. Opinions?