Combustible Combination

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By Robert L. Wolke
Wednesday, July 5, 2006

Perhaps you can explain whether the explosion in my oven was due to an inherently hazardous recipe or a faulty oven. I have this wonderful cookbook and had successfully tried numerous recipes from it. Then I tried a pot roast recipe, which called for 4 cups of dry white wine and various seasonings (no other liquid).

The recipe directed that I brown the meat, then add the 4 cups of wine and bring to a simmer, then cover and bake in a 400-degree oven for two hours. Well, after about 15 minutes of roasting, I went to check on the roast. The second I opened the oven door, there was a bright flash of light followed by a loud bang. I shut the door and turned off the oven. Later, I noticed my arm was covered in ashes; the hair on one arm had been singed off!

I called the fire department to get their opinion. They insisted on coming over and said that it must have been the amount of alcohol in the roasting pan.

The firefighters were correct.

By following this recipe, you filled your oven with alcohol vapor, which has a penchant for either igniting or exploding when mixed with oxygen in the air.

Ethyl alcohol is more volatile than water. When a mixture of alcohol and water boils, the alcohol vaporizes preferentially, so the proportion of alcohol in the vapors is greater than it was in the liquid. Because your oven temperature was so high (and who braises at 400 degrees, anyway?) the liquid was boiling furiously. With so much alcohol in the pot -- four cups of wine contain about half a cup of pure ethyl alcohol -- your oven quickly became filled with hazardous vapors, and there was a handy-dandy gas flame or red-hot electric element in there to set them off.

Whether a mixture of a combustible vapor and air can simply catch fire or actually explode depends on the proportion of that vapor in the mixture. The vapors above a mixture of alcohol and water will be higher as the percentage of alcohol in the liquid increases. Moreover, as a given mixture of alcohol and water (let's say wine) is heated, the proportion of alcohol in the vapors goes up. Thus, the vapors can be ignited when either the concentration of alcohol in the liquid or the temperature is high enough.

The lowest temperature (the so-called flash point) at which wine can form enough alcohol vapor to ignite is only about 125 degrees. So hot wine can easily produce enough alcohol vapor to catch fire.

You may have noticed that when you deglaze a hot frying pan by pouring wine into it, the wine boils to produce not only a cloud of steam, but also enough ignitable alcohol vapor to produce a brief flame. It's great fun to watch a chef do that, but when doing it at home, be prepared for the flare-up and stand back.

Distilled beverages, containing 40 to 50 percent alcohol, have flash points of only about 80 degrees. It is therefore quite easy to ignite the warm vapors of a rum or brandy flambe with only a match, or even by simply tilting the pan over a flame. Being heavier than air, the alcohol vapor will pour over the edge of the pan and down into the flame, where it ignites. The flame then follows the vapor trail back up into the pan.

A fact that surprises many people is that after the flame has died out, as much as 80 percent of the alcohol may be left in the liquid in the flambe pan. The flame burns only until the alcohol is reduced below its flash point and then goes out.

So far, we have explained the flame in your oven. But what about the bang that followed? That has all the earmarks (or should I say arm marks) of a true explosion.

In addition to a flash point, flammable vapors have what are called explosion limits: the proportions of vapor in air that are capable of actually exploding. For ethyl alcohol vapor, the lower explosion limit is 3.3 percent by volume and the upper limit is 19 percent. Below 3.3 percent, there can be no explosion; above 19 percent, the vapors may burn but they won't explode.

It's hard to know the exact sequence of events in that fraction of a second after you opened the oven door. But it's quite probable that the incoming air made a vapor mixture somewhere within the alcohol's explosion limits, and the flame was already there to set it off as an explosion.

Moral: Any recipe that tells you to add a lot of wine and put the pan directly into a hot oven is hazardous. The mixture must be simmered (not just brought to a simmer) on top of the stove to expel most of the alcohol before it is put into a closed chamber like an oven. Small amounts of wine, say half a cup in a quart or two of liquid, are safe, however, because at that dilution very little alcohol vapor is released when the liquid simmers or boils.

But a whole quart of wine? Ye gods!

Robert L. Wolke (http://www.robertwolke.com) is professor emeritus of chemistry at the University of Pittsburgh. His latest book is "What Einstein Told His Cook 2, the Sequel: Further Adventures in Kitchen Science" (W.W. Norton, 2005). He can be reached atwolke@pitt.edu.


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