When Yvie was about a month old, I started pumping milk so I could build up a frozen supply. The books all say that breastmilk stays fresh for about 4-6 days in the fridge, but it seemed that mine went sour much faster, usually in less than 24 hours. By the time I made this discovery, two months of diligent pumping had gone by, and I ended up having to trash my whole frozen supply of milk, as it had all spoiled before being frozen.
I figured the problem was:
a) the containers the milk was stored in
b) the sterilization of the pump
c) the conditions of the fridge
d) my bad habit of storing milk in the pump (not airtight) overnight
"Maybe it's you," my husband joked, after I established an elaborate scientific process to test each of the possibilities. None of the conditions I listed seemed to explain the problem.
Turns out my husband was right. Some women produce an excess of the enzyme lipase in their breastmilk. Lipase is responsible for breaking down fats during digestion - we all have it in our bodies and all breast milk has some to make the fats easier for baby to digest. My milk had so much of it, the fat in the milk was being digested in the first six hours after it was pumped, before it got to the baby or to the freezer.
The solution, fortunately, is fairly simple: scald the milk before storing it, and the lipase is inactivated. The drawback to this is that some of the immunological properties of the milk are also killed off during scalding. This would be more of a problem if I were relying on pumped milk for full-time feeding, but since Yvie only gets a bottle once at day at the most, I am not concerned about it. Since I started scalding milk, I notice it separates less and reconstitutes much more easily than it did before. And it no longer tastes sour, soapy, or metallic, even when it's a day or two old.
Yvie has been complaining about my reintroducing bottle feeding after almost three weeks off, but I am bigger than she is and I have more stubborn in me, so gradually she is getting used to having a bottle once a day. I breath a sigh of relief knowing that I will be able to leave her with Papa or with a sitter for more than two hours at a time.