I read that if you combine milk from two pumping sessions and the older milk is already cold, to make sure the new milk is cold before combining. I didn’t know that and some of my freezer stash has milk where I immediately combined cold and freshly expressed milk. I have no idea which bags.

What’s the reason for this rule? What’s the risk of doing it? I’d probably have to throw out all the milk I expressed in October and the end of September because I may have combined sessions so I’d like to know why.

Thanks!