Hi,
I am working on the re-write for the ABM human milk storage protocol, so I will share my thought process on this, based on the literature I've reviewed.
First of all, literature on human milk storage is lacking. We don't even have consistent standards to say when milk is spoiled or should be discarded.

At room temperature, the maximum amount of  bacteria in the milk is killed by the antibacterial factors such as white cells and antibodies within the first 4 hours. This means that at 4 hours, the milk has it's lowest bacterial count.  At cooler temps, the killing of bacteria is slower, so it might take 6 hours to reach the lowest bacterial count. The bacteria are killed faster in warmer temps. Any left-over bacteria multiply faster at warmer temps than at cooler temps.

In the refrigerator, the process of killing bacteria is much slower because it is cooler, and bacterial counts reach their lowest levels at around 72 hours. (bacterial killing by anti-infective factors does not occur in the freezer, its too cold)

So, best case scenario,  if the milk was pretty clean, not very contaminated, it might be that the milk was at its lowest # of bacteria at 6 hours at 68 degrees F. This would mean that refrigeration for at least 3 days would be fine, because the milk is going into the frig with a low amount of bacteria, and in the cooler temp of the refrigerator, it would take at least 3 days to build up to an unacceptable amount of bacteria.

Worst case scenario- the bacterial load reached its lowest level at 4 hours, and started to rise at 6 hours. If the milk was not very clean when collected (ie shields, bottles, hands not cleaned well)  then the relatively large amount of bacteria left over, which had a chance to grow for at least 2 hours at room temp, would continue to climb when stored in the frig. In that case, it might be best used by 24 hours.

So, it is of question, but my educated guess is that it will be fine for 24-48 hours in the refrigerator.

Anne


Anne Eglash MD, IBCLC, FABM
Clinical Professor, Department of Family and Community Medicine
University of Wisconsin School of Medicine and Public Health
Medical Director, UW Lactation Services
Medical Director, Mothers Milk Bank of the Western Great Lakes
Founder and President of The Milk Mob, dedicated to building Breastfeeding Friendly Medical Systems and Communities

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