“In a landmark study, mature human milk was expressed into clean, not sterile, containers, some stored at room temperature (19-22o C or 66 to 72o F) and some refrigerated for ten hours. The milk was then cultured to evaluate bacterial formation. No statistically significant difference was found between levels of bacteria in the milk that had been refrigerated and the milk stored at room temperature (Barger and Bull 1987).”

So if it’s not bacterial growth (that is, “spoiling”) then why did Jen’s milk taste bad so quickly? I did this research because the same thing happens to my milk, which becomes rancid after about 24 hours in the refrigerator, instead of the 8 days the experts say is acceptable:

“Some moms find that their milk seems to spoil rather quickly despite careful attention to proper storage and handling. This is thought to be a result of the milk having an excess of lipase, an enzyme which helps break down the fats in human milk. One way to keep milk from spoiling so quickly is to halt the breakdown of fats by scalding the milk just prior to storing it. This is done as soon after expression as possible and over a stove eye. Scalding the milk involves allowing small bubbles to appear on its surface but removing it from the heat before an actual boil occurs. The milk should then be stored immediately either in the refrigerator or freezer.”

As an experiment I took four ounces from the same pumping session and scalded half of it. Then I put both bottles in the fridge for a day or so. The raw milk smelled terrible by the next day, while the scalded milk was just fine!

http://www.kellymom.com/bf/pumping/milkstorage.html
http://www.kellymom.com/bf/pumping/lipase-expressedmilk.html