Background. Women who breastfeed have to store expressed milk while at work for later feeding to their infants; however, storage conditions are often not optimal.

Objective. To assess microbial growth and stability of milk protein and lipid at 15°C to 38°C for up to 24 hours.

Methods. Sixteen healthy women who breastfed exclusively, either at home (n = 11) or who expressed milk for their infants (n = 5), were studied during early (1 month) or late (5 to 6 months) lactation. Expressed milk was stored at 15°C, 25°C, and 38°C for 1 to 24 hours for quantitation of pH, proteolysis, and lipolysis; bacterial growth was quantified at 0, 4, 8, and 24 hours of storage.

Results. Milk pH decreased 2 units by 24 hours of storage at all temperatures tested. Proteolysis was minimal during milk storage at 15°C or 25°C for 24 hours and was apparent only after 24 hours of storage at 38°C. Lipolysis was rapid, starting in the first hours of storage and progressing to 8% at 24 hours. Thus, while the greatest increment in proteolysis products was a 40% increase above baseline after 24 hours of storage at 38°C, free fatty acid concentration at this storage time was 440% to 710% higher than in freshly expressed milk. Bacterial growth was restricted mainly to nonpathogens, was minimal at 15°C throughout the 24 hours of storage, was low at 25°C for the first 4 to 8 hours, and was considerably higher at 38°C even during the relatively short period of 4 hours.

Conclusions. Storage of human milk is safe at 15°C for 24 hours, whereas at 25°C it is safe for 4 hours. Milk should not be stored at 38°C. Minimal proteolysis during storage suggests that milk proteins probably maintain their structure and function during short-term storage, while the marked lipolysis might slow bacterial growth during this time.

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