Methods: Mothers (n=40) in the NICU donated 100 mL of milk. Milk aliquots (maintained in a -80 C freezer) were obtained at 0 time and after 7 days of freezer (-20 C) storage. Subsequently, aliquots were removed and stored at -80 C after steps in the simulated NICU human milk handling processes: thawing, refrigeration x 24 h, warming, and maintenance in room temperature (RT) x 4 h. Two thawing-warming processes (tepid water and waterless) were compared. Milk integrity was described as the effect on pH, total protein content, total bacterial colony counts (TBCC), Gram positive (GPCC) and Gram negative (GNCC) bacterial colony counts. Data were analyzed by repeated measures ANOVA.
Results: There were no differences in milk integrity between waterless and tepid water thawing and warming processes. There was a significant decline in total protein from 28 to 24 g/L (mean) from 0 time to after thawing, p < 0.001. There were no further changes in total protein when thawed milk was warmed immediately, thawed refrigerated milk was warmed, or when warmed milk was maintained at RT. There was a significant change in pH from 0 time (median pH 7.14) with thawing (7.07), refrigeration and warming (6.66), and RT (6.95), p < 0.001. There were significant changes in TBCC with processing (p < 0.001). TBCC declined significantly from 0 time (median 5.6 x 104 cfu/mL) to after thawing (3.8) and warming after refrigeration (3.0), p < 0.05. TBCC did not change between thawing (3.8) and warming (4.0) processes but increased after warmed milk was maintained at RT (8.8), p = 0.016. There were significant changes in GPCC which declined from 0 time (3.8 x 104) to thawing (2.3) and to warming after refrigeration (1.25), p< 0.001, but increased to values similar to 0 time after maintenance at RT (4.1). Although 72% samples had no Gram negative bacteria, there were no changes with processing.
Conclusion: These preliminary data suggest that thawing and warming do not change the integrity of previously frozen human milk. The integrity of frozen and subsequently thawed and warmed human milk does not differ using two available methods of processing. There may be concern with maintaining warmed milk at RT but this needs further investigation.