Noting that the use of microwave ovens to thaw out frozen human breast milk had become commonplace in US intensive day nurseries, researchers tested the effect of heating freshly frozen human breast milk samples in microwave ovens for 30 seconds to different temperatures.

• Gently heating human breast milk (to 20-25oC) reduced both the milk’s lysozyme activity* and its immunoglobulin A** (IgA) protection against contamination by E coli serotype 06. Left to stand in a room with a room temperature of 37oC for 3½ hours, the microwaved milk proved to be a friendlier home for bacteria than the unheated breast milk. A sample of E coli 06 added to the breast milk spread five times more quickly than an identical E coli sample placed in unheated human milk left to stand in the same conditions
• Using a microwave oven to heat breast milk to moderate temperatures (25-53oC) reduced lysozyme activity but reduced its IgA protection against E coli 06
• Using a microwave oven to heat breast milk to high temperatures (70-98oC) reduced both lysozyme activity and IgA protection against E coli serotypes 01, 04 and 06. Left to stand (as above), the microwaved milk developed E coli population (all serotypes) eighteen times greater than the unheated sample

Ed.- (i) Whilst agreeing that further research was needed to determine whether it was the microwave radiation itself, or the uneven way microwave ovens heated foods (they produce cold and hot spots) which caused the damage, the researchers strongly rejected the use of microwave ovens for warming breast milk.

(ii) In 1991 Young Families, an extension service of the University of Minnesota (US), warned against heating breast milk or formula in microwaves for both practical safety and nutritional reasons. "Heating the bottle in a microwave can cause slight changes in the milk. In infant formulas there may be a loss of some vitamins. In expressed breast milk some protective properties may be destroyed."

*   lysozyme - an antibacterial enzyme
** immunoglobulin A - a key antibody in the human immune system