As a long time nurse and a fan of Sherlock Holmes, I would be sure to
eliminate other contamination sources from unseen germs on pump or
containers, hands and refrigerator. I'd also try a test of cow's milk in
the same pump or container and see if it acquires the sour taste. Are we
sure the milk is bad or just tastes sour on cooling? In Special Care
Nurserys we often culture milk for the presence of too many or unwanted
bacteria and virus.
Liz Spannraft IBCLC