Guidelines for milk storage can be confusing. Many common recommendations are not evidenced based or practical, differ for term and preterm infants, and can lead to families wasting their precious milk.
- Containers. Mothers should aim for cleanliness but realize milk is a biologic substance and full of probiotic and commensal bacteria. Hand washing, using clean or new containers, minimizing milk transfers (which waste fat and calories) and pumping directly into storage containers are recommended. Plastic or glass containers are fine, no evidence supports one over the other. Mothers can mix warm milk and cold, or even consider pooling milk from 24 hours together, which may help even out variability in nutrients due to pumping time or breast emptying (which influences fat content of the milk).
- Cold storage. Milk has much biology that leads it to maintain its nutrient value and discourage bacterial growth when kept at room temperature or in the refrigerator. Raw milk likely has better biology than frozen with intact milk fat globules and maternal cells. After 4 hours at room temp or 4 days in the refrigerator it may be best to freeze milk for long term storage. Milk is generally considered safe for feeding for up to 12 months of freezer storage; the colder the better, and less exposure to high temperatures the better- so a deep freezer is better than the door of a standard freezer.
- Free fatty acid build up. Human milk contains lipases that continue to break down triglycerides even in frozen milk. These free fatty acids (FFA) can give the milk an unpalatable taste, but cause no GI upset, no additional bacteria, or altered nutrition. Some babies however will reject the taste of the milk with high lipase content, others do not seem to care. Keeping the milk colder may help slow down the process, and mothers can consider a mini pasteurization or scald step after expression but before freezing expressed milk. Lipase is very heat sensitive and will be eliminated thus preventing the FFA build up and tears over lost milk. While it is true that this heating may alter some of the biology of the milk, it is surely better than formula if the baby rejects the untreated milk.
For more information see the Healthy Children Article Tips for Freezing and Refrigerating Breastmilk,
And the AMB Clinical Protocol: Human Milk Storage Information for Home Use For Full-Term Infants
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