Something new after decades of cooking: A dark gray substance on the walls of the plastic bottle above the level of the milk or cream. I first noticed this in a half-empty plastic bottle of cream. It was not old, and it had been kep refrigerated. It happened again to a similar bottle of cream, and now, for the first time, to a bottle of milk. The only similarity among the three is that I bought the cream from our local Amish market. It is excellent cream. The milk bottle is from Whole Foods, but I had added some leftover milk from the Amish market to consolidate them. I will try to attach a photo. Has anyone ever seen this and/or know what it is? Mold maybe?? I was afraid to use the cream and milk so I tossed them. Googled but found nothing so I'm turning to my Food52 peeps!