In intensive care nurseries it has become common practice to use microwave thawing of frozen human milk for more rapid accessibility. Twenty-two freshly frozen human milk samples were tested for lysozyme activity, total IgA, and specific secretory IgA to Escherichia coli serotypes 01, 04, and 06. The samples were heated by microwave for 30 seconds at a low- or high-power setting and then reanalyzed. One-mL aliquots of 10 additional human milk samples were micro-waved at low (20°C to 25°C), medium (60°C to 70°C), and high (≥98°C) setting before the addition to each of 1 mL of diluted E coli suspension. E coli growth was determined after 3½ hours of incubation at 37°C. Microwaving at high temperatures (72°C to 98°C) caused a marked decrease in activity of all the tested anti-infective factors. E coli growth at ≥98°C was 18 times that of control human milk. Microwaving at low temperatures(20°C to 53°C) had no significant effect on total IgA, specific IgA to E coli serotypes 01 and 04, but did significantly decrease lysozyme and specific IgA to E coli serotype 06. Even at 20°C to 25°C, E coli growth was five times that of control human milk. Microwaving appears to be contraindicated at high temperatures, and questions regarding its safety exist even at low temperatures.

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