Research
Kitchen Calamity
By New Scientist 11/6/08
Jun 13, 2008 - 8:50:48 AM

While reducing red wine and olive oil in a flat frying pan on the stove, the mixture exploded with an audible pop, spraying the wine - but not the oil - up to 2 metres away from the pan. Significantly, the wine was not hot enough to scald me. I hadn't stirred the pan for at least a minute, and the wine and oil had separated. What happened?

Several ideas about what went on here. If anyone feels the urge to find out for themselves which scenario is correct, please exercise extreme caution

I don't know if chefs have their own term for this phenomenon, but chemists call it "bumping". It occurs when a liquid is heated above its boiling point but does not boil due to a lack of nucleation sites - scratches, sharp corners, or solid particles where bubbles form easily.

You can observe nucleation sites in a flute of champagne. They are the spots on the inside of the flute where bubbles are continuously forming before streaming upwards. They usually mark the location of a tiny scratch or a speck of dust: a very clean flute, with few or no nucleation sites, will keep your champagne fizzy much longer.

When there are no good nucleation sites in the frying pan, the temperature can rise well above the liquid's normal boiling point, until it is so high that bubbles can form even without a nucleation site. Once a bubble forms, however, it acts as a nucleation site for the adjacent liquid. When such superheated liquid suddenly finds itself adjacent to a nucleation site, it boils explosively. This often happens in the chemistry lab because chemical glassware is so clean and defect-free that it often has no nucleation sites at all.

“When a superheated liquid is suddenly next to a nucleation site, it boils explosively”

To prevent bumping, a chemist might deliberately scratch the inside of a flask to create nucleation sites, or may add chemically inert "boiling chips" to a solution. In other cases, bumping is a necessary evil and the chemist tries to catch or deflect the resulting spray using a "bump trap" - usually in the form of a protective glass screen.

Nucleation is an important concept in many disciplines. In meteorology, for example, seeding clouds involves providing nucleation sites for the condensation of water vapour. And in metallurgy, the way that atoms crystallise into grains around nucleation sites greatly affects the strength and other properties of a metal or an alloy.

In the case mentioned above, my guess is that the wine in the frying pan bumped. It may be that oil stuck to the surface of the pan, creating a completely smooth surface, free of nucleation sites, or it may be that the pan was simply very clean and smooth. The wine was presumably hot enough to scald when it left the pan, but tiny droplets flying through the air cool rapidly. To prevent bumping in your kitchen, you might provide some nucleation sites, perhaps by dropping a sprig of rosemary into your oil and wine before heating it