‘Cold spots’ detected in microwaves

Press, 7 September 1989, Page 28

‘Cold spots’ detected in microwaves

PA Wellington D.S.I.R. scientists have called for more extensive testing of microwave ovens after discovering they have “cold spots.” The Chemistry Division’s research group leader, Dr Dave Bibby, said a series of simple chemical tests showed uneven heating. It was worth more research work and a variety of models should be examined, said Dr Bibby. The D.S.I.R. chemists used a chemical which is pink when wet but blue when dry. Plates of the chemical were layered within the oven up to the ceiling and taken out after a minute. Several “cold spots” were discovered which proved the oven was not heating consistently. At the top of the oven the samples stayed pink as they did scm above the bottom. The tests verify overseas studies which show uneven cooking of food. An independent food safety group in Britain said recently that microwave ovens can leave intact harmful bacteria which are destroyed by conventional stoves. It quoted United States research which found eight out of nine samples of salmonella-infected chicken cooked in microwave ovens to a temperature of 85 to 91 deg. remained contaminated.

No bacteria survived in samples cooked in ordinary ovens. A Leeds University team in Britain discovered similar problems with cooked food infected with listeria bacteria. The compiler of the report, Ms Sue Dibb, spelt - out the importance of standing time — the period which must elapse before eating to complete the cooking process. She said because microwaves penetrate only the outside few centimetres of food, the standing time allowed heat to be distributed through the food by conduction. Dr Bibby also warned of the need to completely defrost frozen food because ice does not absorb microwaves as well as water. He said people should realise the microwave oven was only a tool with certain capabilities. The D.S.I.R.’s microwaves expert, Dr John Holden, said consumers were caught between the marketing pitch of microwave oven manufacturers and technical realities. He said the more expensive the oven the fewer problems that could be expected. This was because of additional features like rotating plates and ceiling reflectors which forced the microwaves more evenly inside the oven.