Freezing or refrigerating expressed breast milk for longer than 48 hours substantially lowers its antioxidant content, reveals a small study in the Fetal and Neonatal Edition of the Archives of Disease in Childhood.
This may be of particular importance for premature babies, who both have reduced antioxidant capacity as well as high levels of oxygen free radical activity as a result of infection and blood transfusions, say the authors.
The researchers assessed fresh breast milk samples taken from eight mothers who had delivered premature babies and from eight mothers who had delivered their babies at term.
The antioxidant capacity of these samples was tested fresh, after refrigeration at 4 degrees Centigrade for 48 hours and for seven days, and after freezing at minus 20 degrees Centigrade for 48 hours and for seven days.
The antioxidant capacity of five different brands of formula milk was also tested under the same conditions.
The results showed that fresh human milk, irrespective of whether it came from a mother who had given birth to a premature or a term baby, had the highest antioxidant capacity of any of the samples, and significantly more than formula milk.
The authors point out that differences between human and formula milk are not only due to its source, but also to the way in which it is processed to prepare the formulas.
But while antioxidant levels of formula milk remained stable whether refrigerated or frozen, levels in fresh human milk fell the longer it was stored and the colder the temperature at which it was stored.
Compared with fresh milk, human milk frozen for seven days had the lowest antioxidant levels. And refrigeration for seven days was equivalent to freezing for 48 hours in terms of the effects on antioxidant levels.
They conclude that in order to preserve its antioxidant content, expressed breast milk should be stored no longer than 48 hours at refrigerator temperature, and that it should not be frozen.