Saturday, April 20, 2013

Safe microwaving

My wife, who has been married to a food microbiologist for 43 years, mentioned something she read in a Q & A section of a local newspaper, in which the writer discussed whether microwave heating killed Salmonella.

The main effect of putting food into a microwave oven is to cause the water molecules in the food to absorb energy.  The molecules move faster and we register this as the food getting hotter.  (It can be shown, by careful experiments, that there is a small effect of microwaves per se on microorganisms, but the main effect is the result of the heating).

When salmonellae and most other vegetative cells reach 75C, some critical enzymes or other cell components, such as the cell membrane, are damaged and the cells die.  So the writer was correct in saying that microwave heating makes food safe.

However, there are a couple of provisos.  First, it is essential that the food reaches this temperature throughout.  Some microwave ovens have hot spots, so the food may appear to be boiling, but there may be areas where the temperature is much lower.  This can be alleviated by stirring the food a couple of times during heating.  Secondly, spores of pathogenic bacteria in the food will probably not be destroyed and may actually be activated by the heating.  If the food is consumed immediately, there is no hazard, but if it is then cooled and stored, the germinating spores can multiply and may cause food poisoning.  (In this regard, microwave heating is no different from stove top cooking).

Finally, when we think of food safety, we tend to think of food poisoning.  However, the microwave oven can superheat liquids above boiling point.  When the liquid is disturbed, perhaps by bumping the container, the superheated water can suddenly flash into steam, causing a painful burn.  A couple of my students have discovered this when using a microwave oven to heat microbiological culture media.

However, in my opinion, using a microwave oven to cook or to reheat food is just as safe as a conventional stove.