Effects of storage on the physicochemical and antibacterial properties of human milk

Storage of human milk for limited periods of time is unavoidable in neonatal units and also in the home where increasing numbers of mothers go back to work soon after delivery. Many mothers, convinced of the importance of prolonged exclusive or complementary breast-feeding, often express and store human milk for use during the period of separation. This study examines the effects of different storage methods on the pH and some antibacterial activities of human milk. Portions of milk and colostrum samples from healthy lactating mothers were stored at 4 degrees C and -20 degrees C for periods ranging from one day to 12 weeks. The stored milk samples were analysed for pH, bactericidal and bacteria sequestration activities against a serum-sensitive Escherichia coli, and compared with freshly collected samples, with and without EDTA. Milk became progressively more acidic during storage. While the bactericidal activities of refrigerated samples diminished rapidly, up to two-thirds of the original activity level was maintained by freezing for up to three months. The ability of milk fat globule membrane to adhere to suspended bacteria was gradually lost in frozen milk samples, while it was greatly enhanced during the first few days in refrigerated samples, before declining sharply. This study shows that loss of bactericidal activity in refrigerated milk is well compensated for by enhanced bacteria sequestration activity, and allays any fears that might arise concerning the suitability of stored human milk for infant consumption.