Breast milk that is refrigerated or frozen loses its antioxidant capacity over time, new study results suggest.

“To preserve the antioxidant activity of human milk, storage time should be limited to 48 hours” at refrigerator temperature, advise Dr. Thomas Hegyi and colleagues at the Robert Wood Johnson Medical School, New Brunswick, New Jersey.

Hegyi’s group obtained breast milk samples within 24 hours of delivery from 16 women. Antioxidant capacity was measured immediately and after being stored at -20 degrees or -4 degrees Centigrade for 2 days and 7 days.

Results were expressed in units called “Trolox equivalent antioxidant capacity,” the researchers explain in the Archives of Disease in Childhood.

Fresh milk exhibited 1.66 Trolox equivalent antioxidant capacity, which declined to 1.58 units after refrigeration for 48 hours and 1.48 units after 7 days. Antioxidant capacity dropped even more after freezing and thawing, to 1.34 units at 7 days.

The effects of storage were similar for samples obtained from mothers who delivered at term and those who delivered prematurely.

The researchers also measured antioxidant capacity in five samples of formula and found that it did not decline from its initial capacity of 1.07 units after refrigeration or freezing.

“Preterm infants are born relatively deficient in antioxidant defenses, and ingesting human milk rapidly increases antioxidant concentrations,” Hegyi and his associates note. Increased antioxidant protection is believed to protect these susceptible babies from a number of disorders, they add.

SOURCE: Archives of Disease in Childhood, Fetal and Neonatal Edition; November 2004.