What caused my breastmilk to curdle/separate ... but not actually spoil?

June 21, 2017 2:13 PM

Twice in the last week, despite following recommended safety handling guidelines AND the same procedures I've been using w/o incident for the last 7 months, my freshly-expressed breastmilk has seemed to "curdle" in the bottle, but not actually go bad. What happened? More specifics and pix inside.

Two days in a row last week, my breastmilk appeared to curdle in the bottle. Here's a pic -- it doesn't look like normal milkfat separation, where you just get a cream layer on top and thinner liquid below. After vigorously shaking the bottle, the curds sort of ... whipped, like very lightly beaten cream.

The milk did not smell rancid or off (and both liquid and curd tasted fine to me). I threw away the first batch, but the second time it happened, I heated the bottle and the fat curds re-homogenized and the milk got smooth again.

Baby ate the second batch happily so this is more a "WTF?" question than a "safe to eat" question. I'm guessing this was some sort of fat separation, but does anybody have a guess as to why? I've followed the same routine for 4 months and this had never happened before (and hasn't happened since). Could it be something I ate?

Routine:
I pump 2x daily at work, at 11 and 3:30, with a double pump (Medela Lactina Select). After my 11 pumping, milk from both bottles is decanted into 1 bottle, and said bottle + all used pieces that touch milk (shields + other now-empty bottle) go into the fridge. For my afternoon session, I pump into the same set of bottles (so in one bottle, body-temp milk goes into chilled milk). Pump parts are washed following the second session, and bottles are fridged until I leave for the day. My commute home is about 45 min, which is well within allowable time for milk to be unrefrigerated.

Both times, the "curds" had appeared when I looked at the bottles around 6 pm, but I didn't see any curdling in between pumping sessions. I did not use the same bottle set between the two days, and the bottle sets were not washed at the same time (so I don't think it was, like, a reaction to soap residue or something).

Any clues?

Thanks! And the MetaFilter Election Baby thanks you too.

posted by alleycat01 to Food & Drink (5 answers total)

This is a pretty common occurrence and is most likely a result of temperature differences between the two volumes of milk when you combine them. Do you live where it's hot right now? If so, you might need to separately cool each additional pump volume of milk before you combine it with another already-chilled volume.
posted by late afternoon dreaming hotel at 2:51 PM on June 21, 2017

I'd pump into empty bottles at each session. A good rule of thumb is, "cold to cold when new to old" - I.e., avoid mixing milk at different temps. It's not going to ruin the milk but like any mammal milk can affect taste and fat separation - what you're seeing.
posted by pecanpies at 4:18 PM on June 21, 2017

Another strategy: breast milk is just fine at cool room temps (68ish degrees) for around 6 hours (sources vary, but that's a safe general timeframe). If you're pumping in a place that stays around that temperature, you could leave everything from the 11 am pump at room temp. After your 3:30 pump, put everything into the fridge. If leaving milk at room temp makes you leery, consider a lunch box with a cold pack instead of the fridge.
posted by pecanpies at 4:21 PM on June 21, 2017

I hear what y'all are saying about temperature difference, but I'm confused as to why it happened only those 2 times when I've been following the same procedures for months. I'm in NYC and it is hot(-ish), but I'm in a temperature-controlled office the whole day (and so is the milk).

Thanks also for the strategic advice, but tbh I'm not really worried about the safety, I'm just curious about why it happened only those times (or I guess, why it's not happening all the time if it's the expected result of my usual process). OK done threadsitting!
posted by alleycat01 at 7:26 PM on June 21, 2017

I think I've seen this before. What I think is happening: milkfat becomes more viscous when it's chilled. Normally the chilling doesn't drastically affect the speed of separation, but when you pump warm milk over cold the fat globules in the warm milk may accumulate on the surface of the cold fat globules before the warm globules cool down. This will only happen when the warm milk mixes vigorously with the cold, allowing the globules of different temperatures to touch, which is why shaking the bottle made it worse. Other times, the cold milk was probably kept more still as the warm was added, and the two batches reached the same temperature more gradually.
posted by Joe in Australia at 9:47 PM on June 21, 2017