BREAST-FEEDING AND THE WORKING MOTHER - EFFECT OF TIME AND TEMPERATURE OF SHORT-TERM STORAGE ON PROTEOLYSIS, LIPOLYSIS, AND BACTERIAL-GROWTH IN MILK

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. Objectives. To assess microbial growth and stab ility of lipid at 15 degrees C to 38 degrees C for up to 24 hours. Met hods. 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 m ilk was stored at 15 degrees C, 25 degrees C, and 38 degrees C for 1 t o 24 hours for quantitation of pH, proteolysis, and lipolysis; bacteri al 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 degrees C or 25 degrees C for 24 hours and was apparent only after 24 hours of sto rage at 38 degrees C. Lipolysis was rapid, starting in the first hours of storage and progressing to 8% at 24 hours. Thus, while the greates t increment in proteolysis products was a 40% increase above baseline after 24 hours of storage at 38 degrees C, free fatty acid concentrati on at this storage time was 440% to 710% higher than in freshly expres sed milk. Bacterial growth was restricted mainly to nonpathogens, was minimal at 15 degrees C throughout the 24 hours of storage, was low at 25 degrees C for the first 4 to 8 hours, and was considerably higher at 38 degrees C even during the relatively short period of 4 hours. Co nclusions. Storage of human milk is safe at 15 degrees C for 24 hours, whereas al 25 degrees C it is safe for 4 hours. Milk should not be st ored at 38 degrees C. Minimal proteolysis during storage suggests that milk proteins probably maintain their structure and function during s hort-term storage, while the marked lipolysis might slow bacterial gro wth during this time.