Speaking of rage… an email from a reader the other day reminded me of the time that two close friends nearly split up when one member of the couple left the freezer door open all night, defrosting thousands of ounces of stored breast milk. I am not sure the other partner has completely forgiven this, even five or six years later.

And I can see why. If you are pumping milk and storing it, or even just using it in the moment, it is precious. Anyone who pumps at work can recall the time they spilled a bottle and broke down in tears (this cannot be just me).

Because the milk is so precious, it’s galling not only to spill it by accident but to have to throw it out on purpose. There are guidelines for safe breastmilk handling. They suggest milk must not be left out for more than 4 hours, and if you start using a bottle it must be used within 2 hours. The CDC says: “NEVER refreeze human milk
after it has been thawed.” Caps are theirs. It’s very important!

But…why? Is it important? What would happen if my friend had closed the freezer and refrozen her milk? If your kid eats half the bottle and you pop it in the fridge and use it again 6 hours later…is that okay? What might happen?

Like with any food storage, the big worry here is pathogens. If you leave chicken soup out all day at room temperature, it can start to grow nasty stuff. If you leave milk out, it eventually sours. So one question is simply how robust is breastmilk to pathogens. A second question is nutrient decay. As food sits out (or is frozen, unfrozen, refrozen) it could lose nutrients. Both could happen in principle; the question is whether they happen in practice with milk.

It is not especially easy to study this in situ. For various reasons, we do not have a lot of studies in which researchers go into people’s homes, wait for their baby to drink a half a container of milk and then see how long it takes to go bad.

But this is something you can study using donor milk, in carefully controlled conditions. Which is precisely what this 2006 paper does. The authors look at both bacterial load (a measure of pathogens) and vitamin and fatty acid content in milk treated in various ways. This includes being left out for 8 hours at room temperature, 24 hours in the fridge, 4 hours in the fridge, 4 hours at room temperature and, most interestingly to me, milk that was frozen, thawed and then refrozen.

The researchers find that none of the samples shows an appreciable bacterial load. They are all well under the FDA limits for contamination (well under, not just a little bit under but orders of magnitude). Vitamin A is unaffected and free fatty acids are minimally different. There is a small amount of degradation in Vitamin C from being left out or freezing and thawing, although the authors note that the concentrations are still at reasonable levels for a healthy infant.

A 2016 review article draws broadly similar conclusions, although notes that deep freezing for lengthy periods may impact nutritional content (calories and fat). Data from Nigeria showed storage for 9 hours in tropical conditions (i.e. left out) did not generate bacterial loads which exceeded acceptable levels.

This literature is not very large (there are a lot of things like this in women’s health where I feel like it wouldn’t be that hard to learn more than we know). But what we do know here suggests that breast milk is…robust. It seems to be pretty resistant to bacteria, and the freeze-thaw-refreeze doesn’t seem to promote pathogens.

Obviously, if you leave milk out for three days, it’s not going to taste or smell good, and it may well start to grow bacteria. There is a common sense element here. But if your kid drinks half the bottle and the other half sits out for four hours (or even 8), the available data suggests reusing it is likely safe.

Oh, and if your partner leaves the freezer open, and everything defrosts, it may be fine to just close it, refreeze and move on. They probably do owe you dinner, though.