Effect of storage processes on the bacterial growth-inhibiting activity of human breast milk

Human breast milk samples were collected from lactating mothers, and aliquots were maintained at room temperature, frozen, and pasteurized. Samples were inoculated with 10 to 50 colony-forming units per milliliter (cfu/ml) of Escherichia coli or group B streptococcus, and incubated at 37 C. Quantitative growth was measured at eight and 24 hours. No inhibitory activity was demonstrated by control broth, commercial formula, and pasteurized breast milk, which had a rapid logarithmic growth to a maximum of 10(8) to 10(9) cfu/ml at 24 hours. Compared with these controls, fresh breast milk, fresh frozen breast milk, and breast milk frozen for 21 days demonstrated a significant inhibition of bacteria growth. A trend toward gradual loss of inhibiting activity was noted with prolonged freezing of breast milk. Although freezing may quantitativly decrease the amount of some breast milk host-defense factors, it cannot be assumed that comparable functional reductions will necessarily result.