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16 Concise Monograph SeriesBOX 3Nutritive values of proteins in microwaved foodsNumerous studies using different methodologies, have considered how heating protein foods by microwaves affects thenutritional value of the proteins. No detrimental effects have been shown. A study compares the effects of heating beefpot roast and beans with frankfurters on the availability of lysine and methionine, the two most important essentialamino acids in food proteins. The study also looked at instant mashed potatoes, peas with onions, and breaded fish. Forall foods it was found that neither conventional nor microwave heating seriously decreased the nutritive value of theprotein as measured by chemical analysis and by human and animal trials.In another study, the nutritive value of a casein solution heated by microwave or conventional methods was determinedin a feeding study of rats. The net protein utilisation, digestibility and biological values of unheated casein were 61.1%,100%, and 61%, respectively. Comparable values were obtained for casein heated by microwave or conventionally. Theauthors concluded that there are no negative effects on protein nutritive value by either microwave or conventionalheating under conditions likely to be encountered in household situations.It has also been shown in well-designed studies carried out by a variety of laboratories that there are no adverse effectson milk or infant formulas when heated in microwave ovens. This is true whether normal domestic conditions wereused, or higher but controlled temperatures and longer duration.NutrientsProteinsWhen proteins are heated they are denatured; that is,their molecular structure is modified. This limits theirphysical properties such as solubility and theirbiological properties including their enzymic andimmunologic activities.There are slight differences in denaturation rates infoods heated in the microwave compared withconventional heating, because of differences in heatingtime and temperature. In general, however, the nutritivevalue of proteins in foods is comparable, whether thecooking is done by microwaves or conventional means.Studies on thawing of human plasma and breast milkshow that when the microwave treatment is wellc o n t rolled to avoid overheating, such biologicalproperties as the clotting activity of plasma and theantibacterial activities of human milk are reasonablymaintained. For further information, refer to Box 3.LipidsThe effect of microwaves on lipid stability has also beeninvestigated. The hydrolysis of triglycerides has beenstudied in soya, egg yolk and meats. Fatty acid stabilityhas been studied in meat, chicken fat, beef tallow, baconfat, rainbow trout and peanut oil. The peroxidation ofpolyunsaturated fatty acids, which generates rancidity,has been studied in meat, egg yolk and chicken. The