Wondering if you can use the milk your baby didn’t finish last time? Well you’re not the only one, it’s a very common question!
While reading this over it’s important to know that with babies who are premature or health compromised, the standards of feeding pumped milk are less flexible and different guidelines need to be followed. This article is for healthy term babies only.
While some mothers and caregivers reheat expressed milk that was leftover and refrigerated after a previous feeding, there is no current research on the safety of this practice. In fact, there is no research at all, old or new on the topic of oral bacterial contamination of stored/freshly expressed breastmilk.
There is also no current consensus about whether freshly expressed milk left unfinished at room temperature should be discarded, or whether it can be saved for a short time (up to one hour as reported by some mothers and caregivers) to finish the feeding if the baby wakens from having fallen asleep, for example. Some experts suggest that it is reasonable to discard previously used breastmilk 1-2 hours after a feeding.
Research indicates that human milk has previously unrecognized properties that protect it from bacterial contamination. One study, Pardou, A. et al (1994), found that after 8 days of refrigeration some of the milk actually had lower bacterial levels than it did on the day it was expressed.
In a recent study by D. Silverstre, titled Bactericidal Activity of human milk: stability during storage it was concluded that “Bactericidal activity was detected in all milk samples…This activity persisted after refrigeration for 48 hours and after freezing for 10 days”. What this is saying is that the levels of bacteria drop over the first four days when storing of fresh milk in the refrigerator. The study continues to discuss their finding that fresh milk was still declining in bacterial numbers at 72 hours, but the rate was slowing down.
It makes sense that the oral bacterial of a baby would be less likely to grow in the actively bactericidal milk of a mother who had expressed her milk within the previous four days. We could also assume from this last study, that milk that had been previously frozen would recuperate from the inoculation of bacteria much slower, and become unsafe faster.
*Depending on the microbial diversity of the child receiving the stored breastmilk, bacteria present may vary. Lactobacillus, Bifidobacterium, Escherichia coli, and Enterococcus are commonly colonized in the baby’s mouth after delivery, from passing through the vagina and then from breastfeeding (these bacteria are normal and critical to the health of the baby). The bacteria then work down into the whole GI tract. Various bacterial can divide and reproduce rapidly, increasing the total amount of bacteria present in the milk very quickly.
I have yet to find data that supplies the actual quantity of bacteria delivered through saliva or mouth contact when using a bottle or cup. It seems highly probable that the milk left over would be viable for several hours particularly if placed back into the refrigerator to slow bacterial growth.
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