I've been pumping milk for the past several weeks, here and there, to build up a back stock for when I go back to work. The midwife at my six-week appointment pointed out that all I really need is enough for the first day, but it was very nice last time being able to send frozen packets along to the mothers' or aunt's with Chloë at will without having to pump extra, so I pumped enough for a decent bagful in the freezer and was quite pleased.
Then, when we realized we needed to reintroduce bottles to Maia and figure out what size meal she needed, I pumped a test bottle in the wee hours and we tried it that night. Maia refused it, howling and shrieking. After a couple of attempts, Eric noticed the milk smelled a little off. I pumped a completely fresh bottle and she still refused to take it, so that didn't explain the bottle troubles, but it did present a concern of its own. Milk usually stays good around forty-eight hours, at least twenty-four, and this had been twelve. We tried it again the next day, and again the early morning milk smelled and tasted off.
Eric did some research and found out that some women produce milk with too much lipase in it. Lipase breaks down fats and causes rancidity, and an excess makes that happen much more quickly than normal. It doesn't technically spoil the milk, but it makes it taste nasty enough that no baby will drink it anyway. He also read that scalding the milk, on the stove or in a bottle warmer, will deactivate the lipase and save the milk, without causing quite as much destruction as microwaving it would do.
I was somewhat skeptical that this was what was going on, since I didn't have this problem last time and felt vaguely that I was being criticized. But it made most sense to proceed as if this were true, so we bought a bottle warmer and I started scalding all my pumped milk. Then I thawed one of the stashed aliquots to verify, since lipase will continue to work when frozen, if more slowly. It smelled and tasted awful.
So, we're throwing out the entire back stock. Luckily we--Eric--caught this in enough time that I'll have just about enough time to pump enough for my first day back at work without being crazy about it. I'm also going to have to bring the bottle warmer to work with me, which will be a pain (and too bad for Eric, since it would be very useful for him--I don't know why we didn't think of buying one last time; they're pretty inexpensive and very fast). But it's better than the alternative. I do wonder what's causing it, though. One possible cause is diet. It better not be the chocolate.