Freezing or refrigerating expressed breast milk for longer than 48 hours substantially lowers its antioxidant content, according to US researchers.
Expressed breast milk should not be frozen
Antioxidants help beat infections, making the findings particularly important for premature babies who have low natural defences, they say.
Formula milk remained stable whether refrigerated or frozen, the Robert Wood Johnson Medical School team found.
The small study appears in Archives of Disease in Childhood (Fetal Edition).
Babies who are breastfed are reported to be at less risk of stomach upsets and infections of the ear, respiratory and urinary tracts than those who are bottle-fed formula milk.
Part of the benefits of breast milk may be related to its antioxidant content, as well as other essential nutrients.
Antioxidants are thought to prevent or slow damage caused by certain reactive oxygen compounds, called free radicals, that are produced under stressful circumstances, such as infection.
Professor Thomas Hegyi and his team assessed fresh breast milk samples from eight mothers who had delivered premature babies and from eight 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.
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.
However, after 48 hours of freezing or refrigerating antioxidant levels in the fresh human milk fell significantly.
The longer it was stored and the colder the temperature at which it was stored, the more the antioxidant levels fell.
"To preserve antioxidant capacity, [breast] milk should only be stored for a short time at refrigerator temperature and not frozen," say the authors.
Rosie Dodds of the National Childbirth Trust said: "It's useful to know that the antioxidant levels go down...and babies may need extra antioxidants.
"But levels tested in the breast milk were always higher than in formula milk.
"You should not say as a result of this study that mothers cannot freeze breast milk.
"There are many situations where you do need to store milk. For example, for very premature babies who need to be tube fed or women who are going back to work.
"It's still going to be far, far better for their babies than formula milk."
She said breast milk contained many other beneficial ingredients other than antioxidants.