Milk/cream mystery?

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!

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Maedl October 28, 2015
It looks like it is mold--and since you added milk to the bottle, perhaps the first milk contained the spores and you are spreading it by mixing.
jilhil October 27, 2015
I'd contact the store where you bought the Amish products if it only happens with them. Their dairy product manager may know about this already (and if they don't they should want to know about it.).
cookbookchick October 27, 2015
Yes, I will do that, thanks!
ktr October 27, 2015
I would think there must have been bacteria on the inside of the bottle. It could have gotten there when you washed out the bottle before filling it. In the future, I'd use a glass bottle and sterilize it prior to filling.
cookbookchick October 27, 2015
I guess I wasn't clear. These were not bottles I reused. The cream bottles were new, just partially empty be caused I'd used some of the cream and kept it refrigerated. The milk was 2/3 full of Whole Foods mill to which I added a bit of Amish market milk to consolidate. The Amisj brand is Krieger's.
ktr October 27, 2015
Oh ok. I have no idea then.
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