Do thawing and warming affect the integrity of human milk?

To evaluate the integrity of the human milk (pH, bacterial counts, host defense factors and nutrients) subjected to thawing, warming, refrigeration and maintenance at room temperature.

Mothers in the neonatal intensive care unit donated freshly expressed milk. A baseline sample was stored at -80 °C and the remainder of the milk was divided and stored for 7 days at -20 °C. The milk was then subjected to two methods of thawing and warming: tepid water and waterless warmer. Thawed milk also was refrigerated for 24 h prior to warming. Lastly, warmed milk was maintained at room temperature for 4 h to simulate a feeding session. Samples were analyzed for pH, bacterial colony counts, total fat and free fatty acids, and the content of protein, secretory IgA and lactoferrin. Data were analyzed by repeated-measures analysis of variance and paired t test.

There were no differences between processing methods and no changes in fat, protein, lactoferrin and secretory immunoglobulin A with processing steps. Milk pH and bacterial colony counts declined while free fatty acids rose with processing. Refrigeration of thawed milk resulted in greater declines in pH and bacteria and increases in free fatty acids. Bacterial colony counts and free fatty acids increased with maintenance at room temperature.

The integrity of the milk was affected similarly by the two thawing and warming methods. Thawing and warming change the integrity of previously frozen human milk, but not adversely. Concerns about maintaining warmed milk at room temperature need to be explored.