Marilee, How is the milk being warmed? I have heard that some centers use a crock pot with warm water to set the bottle in to warm the milk. Is it possible the water is contaminated and is flowing through those thin bags to the milk? Has mom tried a hard side bottle to see if the same thing happens. I have seen a couple of moms too, who do not remove the orange valve and the white valve membrane completely for cleaning and introduce bacteria from the pump kit. Please post the solution to this problem if you find it. Good luck. Linda Pohl, IBCLC Phoenix, Arizona USA [log in to unmask]