In 1991 Young Families, an extension service of the University of Minnesota, warned against heating babies' milk bottles 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."

In 1992 Richard Quan and colleagues published the results of tests they had carried out on the effect of heating freshly frozen human milk samples in microwave ovens for 30 seconds at different temperatures. They found that:

• the milk heated at high (98°C) temperature had lost nearly all its resistance to contamination. A sample of E coli added to the breast milk spread 18 times faster than in unheated breast milk

• in the milk heated at low (20-25°C) temperature the E coli sample spread five times faster than in unheated breast milk

Whilst agreeing that further research was needed to determine whether it was the microwave radiation itself or the uneven way microwave ovens heat foods (i.e. they have hot spots) that caused the damage, the researchers strongly rejected the use of microwave ovens for warming breast milk.

Ed.- (i) Another study carried out in Vienna, found that microwave cooking induced high rates of change in food proteins that were not observed after conventional cooking. D-proline* and cis-D-hydroxyproline were found in significant quantities in microwave-heated infant milk formulas, whereas only L-proline* is normally found in biological material. Lubec and his colleagues warned that "the conversion of trans to cis forms could be hazardous because when cis-amino acids are incorporated into peptides and proteins instead of their transisomers, this can lead to structural, functional, and immunological changes" (Lubec et al. Lancet 1989;9:1392-93).

Another study also found that microwaving infant formula can produce molecular changes to the amino acids in milk proteins, causing toxicity or affecting the nutritional value of the milk formula. Nevertheless, the quantity of proteins changed was very small (Journal of the American College of Nutrition 1994;13:209-10).

* (L stands for laevo-rotary, D for dextro-rotary, referring to the direction electrons rotate in their plane of optical polarisation).

(ii) Blood for transfusions is also routinely warmed. According to a 1991 law suit in the US, when a nurse warmed a sample in a microwave oven (not normal practice) it killed the patient.