Heating or thawing human milk by microwave causes a decrease in the level of anti infective factors in the milk, even when low temperatures (20-53!C) are used (Pediatrics, 1992; 89: 667-9).
In one study, conducted at Stanford University in California, microwaving at higher than 72!C was found to cause a considerable decrease in all the tested anti infective factors. The Stanford researchers strongly rejected the use of microwaving, even at low temperatures, of human milk in hospitals.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.
(L stands for laevo-rotary, D for dextro-rotary, referring to the direction electrons rotate in their plane of optical polarisation.)
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 trans isomers, this can lead to structural, functional, and immunological changes" (Lancet, 1989; 9: 1392-3).
Other research has also found that microwaving infant formula can produce molecular changes in 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 (J Am Coll Nutr, 1994; 13: 209-10).