Effect of Frozen Storage on Macronutrient, Mineral, and Immune Component Content of Pasteurized Donor Human Milk

Background: Pasteurized donor human milk (PDHM) is widely recommended for preterm infants when their mother’s own milk is unavailable. Storage duration of frozen PDHM varies across countries. In Japan, PDHM is stored at −30 °C; however, there is no information on whether long-term storage at −30 °C affects PDHM quality. Research Aims: This study aimed to determine the effects of freezing at −30 °C for 6, 9, and 12 months after expression on bacterial growth and nutrient content of PDHM. Methods: This was an experimental laboratory study of nutrient and bacterial stability over time. A total of 20 milk samples were pasteurized within 3 months after expression and stored at −30 °C. Bacterial counts and levels of macronutrients, minerals, secretory Immunoglobulin A (sIgA), and lactoferrin in PDHM were analyzed after pasteurization and at 6, 9 and 12 months after expression. A Friedman test followed by a Bonferroni-adjusted Wilcoxon signed-rank test was performed. Results: No bacteria were detected in the 20 samples after pasteurization or at any time point. When the concentrations of nutrients were compared to the values immediately after pasteurization, carbohydrates at 9 and 12 months and zinc at 12 months showed slight increases, but the respective increases were minimal (carbohydrate: +3.2% at 9 months and +3.8% at 12 months; zinc: +7.8% at 12 months). The concentrations of protein, fat, calcium, inorganic phosphorus, sIgA, and lactoferrin remained stable at 6, 9, and 12 months. Conclusions: These results suggest that PDHM can be safely stored at −30 °C for 12 months after expression without compromising bacterial safety and nutritional quality.