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There's a lot of information available about how long pumped breast milk can be kept for, generally citing different temperature ranges.

From the CDC (though the same reference is also cited by the Canadian Pediatric Society):

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It's cited that milk thawed from the freezer should be used within 24 hours in the fridge, or 1 hour at room temperature.

The Mayo Clinic has an article saying "Other studies have shown that refrigeration beyond two days might reduce the bacteria-killing properties of breast milk", but no references to where this comes from or any further information.

My wife is exclusively pumping, and so there is a lot of milk cycling in and out of the fridge. She is always concerned about using milk taken out of the fridge (generally +/- 1 day old) within an hour or two. We often talk milk out (and let it warm up to room temperature), only to have baby fall back asleep. Is there a basis to how long fridge-temperature milk (stored at 3 degrees C for a day or two) can be out of the fridge for before it has to be used/tossed?

  • Cooling the newly pumped milk in an ice bath will increase it's longevity. Instead of letting it warm to room temperature, if you warm it in warm water (replace as it loses heat) while swirling, you can warm up a bottle in a few minutes. If you don't use it, reverse the process in an ice bath. That way, it only spends a maybe 20 minutes in the red zone at most. If rewarmed and not used, I wouldn't re-cool or use it. – anongoodnurse Jan 11 '15 at 9:26
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    Hello gregmac & welcome! Thank you for such a good question! It's great to see that you have "done your homework" before asking and that you are asking a precise question. – Stephie Jan 11 '15 at 16:07
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    One important caveat to the very good answers here, is that for some mothers, breast milk will not last very long either in the fridge or out of it. For mothers who express overly high proportions of lipase (one of a group of fat digesting enzymes), the milk will taste soapy or sour within a day or two, regardless of treatment (fridge, freezer, you name it). The milk isn't dangerous to consume at this point, but tastes like soap. In our case the milk lasted at most 48 hours before the children didn't like it and refused it. – Joe Jan 13 '15 at 18:52
  • Pour the amount of ounces your baby is eating and then heat that up keeping the non eaten ounces in the refrigerator – Nicole A Aug 1 '17 at 19:33
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The Short Answer

Breast milk can be safely consumed if it has been at room temperature for no more than 6 to 8 hours total, which includes all of the time the milk spends above refrigerated temperature. So, if you heat it up to room temperature, then put it back in the fridge after half a period of time, you to account for:

  • The time it takes to heat up the milk
  • The time the milk spends outside of the fridge
  • The time it takes for the milk to cool back down, once back inside the fridge. Depending on the initial temp, the bottle size, and the fridge temp, this could take another 30+ minutes.

The longer the breast milk stays above refrigerated temperatures, the less time it will be viable in the fridge. Guidelines say that milk in the fridge can last 5-8 days, but that time will be considerably less if the milk isn't properly placed in the refrigerator immediately after pumping. Conversely, the longer the milk stays in the fridge, the less time it will be viable at room temperature. Continue reading for more explanation and details.

The Long Answer

In food service, temperatures that are ideal for breeding micro-organisms are called the Danger Zone or something similar. Different foods have different amounts of time they can spend in the danger zone based on the types of bacteria/organisms that grow on/in them.

Once a food has been taken into the danger zone, you also shorten its refrigerated expiration date. While fresh breast milk that's immediately put in the fridge may last 5-8 days, a bottle that's spent 2 hours at room temp may only last 3-4 days instead.

I don't think you'll find any hard data about how many more days a bottle will last in the fridge based on how long it spent at room temperature. That would have to be some very controlled experiments that, in the end, wouldn't serve much usefulness.

To answer your question, we must make some inferences with the data we do have.

For instance, we also know that if you freeze the expressed milk at all, then after you thaw it out it must be consumed within 24 hours or thrown out.

So, we know:

  • Keeping milk at higher temperatures decreases how long it will last at lower temperatures
  • Keeping milk at lower temperatures does not necessarily preserve the initial maximum safe time at higher temperatures

Putting that together, and some general assumptions:
The longer the milk has been in the fridge, the sooner it will go bad. If it's only been refrigerator for a day, then it should still be okay for about 6 hours. If it's been in there for 5 days, I'd only give it 2 hours.

Your wife shouldn't have to worry about using day-old milk within two hours. I'd be surprised if you didn't get at least four hours. However, if the room/milk temperature is greater than 77 F/25 C, then it will last significantly less than the noted 6 hours.

Other factors to keep in mind that will affect how long milk last:

  • The cleanliness of the nipple area before/during pumping
  • How well sanitized the storage containers are if they're reusable
  • How well sanitized and maintained the pieces of the pump that contact the milk are
  • Whether or not you store it in the back of the fridge (because the front meets warmer air every time the fridge door is open)
  • How much you warm up the milk, if "serving" it above room temperature

Personal Experience

We never had problems with refrigerated milk spoiling within 1-2 hours. When my son was under 1 year old we'd often leave a bottle in his crib with him at night. The ones he didn't finish would still be okay 4-6 hours later. We usually didn't give it to him again, but if he woke up and decided to finish the bottle before we had a chance to take it there were never any problems. Rarely, if ever, did the milk stay in the fridge for longer than 2 days.

However, we did have times when milk stored in an insulated cooler bag with ice packs would spoil much faster than we expected, which made some of our longer car trips surprisingly more difficult.

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    @Joe I made some revisions. Let me know if you have any other suggestions. – user11394 Jan 13 '15 at 1:08
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My wife and I went through this with our first child. Digging through the internet searching for the perfect "hard-and-fast" answer. We took a different approach with our son (now 7 months). Just take a common sense approach. Smell it. It's milk after all. If it smells bad... it is.

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    We did this as well. I thought I'd put it in my answer, but I guess I didn't. After having a our first road trip siderailed by spoiled milk, we starting smelling every bottle as part of our routine. – user11394 Jan 12 '15 at 15:58

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