For the vast majority of us who never paid any attention in chemistry or physics classes, microwaves are basically magic. In what seems like something straight out of a scene from the as yet unpublished “Harry Potter and the Three Minute Dinner”, anything can go from stone cold to piping hot with a few prods of a button. Over the years, microwaves have saved us all millions of hours. There’s not a lot that they can’t do.
However, as anyone who has ever bothered to read an instruction manual will tell you, there are a few caveats to nuking your food. Everyone knows, for instance, that metal is a big no-no, unless you plan on exploding your kitchen. So, apparently, are paper bags. But perhaps most surprising of all the microwave contraband is the humble and decidedly innocent looking grape.
Discovering that you’re not allowed to microwave a grape is a lot like being told you can’t touch an antique. You almost certainly weren’t going to do it before, but now you know you can’t there is an irresistible urge to find out what would happen if you reached out for a cheeky stroke. However, unlike an antique, grapes can actually do way more damage than you’d ever thought possible.
If you make the mistake of slicing a grape in half, so that only the thinnest sliver of skin connects the two pieces, and blasting it on full power, you can expect to see literal fireworks. Within a few seconds, spouts of white hot flame will erupt from the fruit, making your microwave look more like Mount Vesuvius on a particularly inauspicious day. It is a spectacular, if scary reminder that familiar foods can still hold a mystery or two.
As mystical as a spontaneously combusting grape might seem, there is a perfectly rational scientific explanation for the strange phenomenon. According to a new study from a crack team of Canadian scientists, the explosions are caused by a highly combustible mixture of charged molecules containing sodium and potassium.
The team, led by Dr Aaron Slepkov from Trent University in Ontario, revealed that, “Emission spectra from grape plasma suggest that potassium and sodium species, abundant in the grape skin, are field-ionised by a strong concentration of electric field near the point of contact. The ions themselves are resonant with the driving microwave radiation and can evolve a cascade of ionisation in the air, forming a microwave-heated plasma that grows and becomes independent from the dimer (pair of identical source objects).” In English, this means that the ability of water to absorb microwaves can, in specific circumstances, cause some ingredients to set on fire. Dinners everywhere could well become more explosive than ever.
If you are feeling irritated and confused by the complex physics behind exploding grapes, you are not alone. Suffice to say, most of us will never properly grasp why microwaving a bunch is a bad idea. At least now we can all say that we were at least warned that there would be side effects.