Many years ago, breast milk was thought to be sterile. While this is far from being the case (it actually contains all manner of germs1), the role it plays in helping keep babies free from harmful disease means it does display some pretty impressive bug-busting capacities.
The immunological components of breast milk help to protect both a mother’s breast and her baby from infection during feeding, as well as aiding the development of the baby’s immune system2. They also have another useful consequence, however: protecting breast milk from disease for some time after it has been expressed, enabling it to be stored. Several studies have examined whether it is safe to keep expressed milk for short periods, and there is general agreement that it can be stored for 8 hours at room temperature (25 degrees C), for three days in the fridge (4 degrees C) and for up to a year in the freezer (-20 degrees C) without any increase in the levels of pathogens (harmful bacteria) it contains3.
Not only does breast milk inhibit the growth of pathogens, however – it actively reduces them. This was convincingly demonstrated in a piece of research examining what happened to milk during short term storage4. Milk was collected from 9 mothers and divided into three samples: the first was analyzed the same day; the second was refrigerated (at 4 to 6 degrees C) for 48 hours; and the third was refrigerated for 72 hours. Each sample was then contaminated with an E.coli solution (the kind of nasty bacteria that dwells in toilets) and left for two hours. When the samples were tested, levels of E.coli had reduced by 80% in both the milk that was fresh and the milk that was 48 hours old. Levels had also diminished in the 3 day-old milk, but only by around 10%, indicating that the antibacterial properties, whilst still present, had started to degrade by this point.
If your baby needs to feed from a bottle or cup, a considerable body of research indicates that it’s safe to give him breast milk that has been stored in the fridge for up to three days, or in the freezer for several months. There is also evidence that if the milk you express does come into contact with germs (keeping pumping equipment sterile in your bag at work isn’t always easy), then the bactericidal components of breast milk should be able to take care of them, providing the milk is under two days old. There may still be potential issues associated with feeding stored, rather than fresh breast milk to your baby: various chemical changes occur in milk once it has left the body, and it’s possible some of these may affect its nutritional value3,5,6. Nevertheless, expressed breast milk remains a healthy alternative to formula, and as a result of its antibacterial qualities, you can rest assured that if your baby can’t feed from you directly, he still has a safe source of food and drink.
The Truth About Breastfeeding 