There, before it goes away!
By Robert L. Wolke
Wednesday, July 24, 2002; Page F01
I recently completed training as a chef and have asked everyone at the school the following question, without getting a satisfactory answer. Maybe you can help. We were told in our bread class that in order to get the best browning and crisping effect on French bread, we should bake it in an oven that has steam in it, either from spritzing water into the oven or by putting a pan of hot water in the oven during baking. Why would moist heat result in a crispier product?
It's counterintuitive, isn't it? You'd think that moisture would make the crust soggy, not crisp. Nevertheless, it does work.
According to Prosper Montagne, in his encyclopedic Larousse Gastronomique, "Bread is a food which is used, and abused, more than any other, especially in France." (He never bought a loaf of white bread at an American gas station.)
But despite Montagne's disloyal dig at his countrymen, many people -- even outside of France -- consider a golden brown, crisp-crusted, light-but-chewy baguette fresh from the boulangerie to be the best bread in the world. When I lived in France, I relished carrying home each morning before breakfast a long, slender baguette; or else a ficelle ("string," an extra-slender baguette) or an épée ("sword," a half-dozen oblong rolls strung together like leaves on a stalk), the last two intended to maximize the crust-to-crumb ratio. (The soft interior of a bread is called its crumb.)
Nevertheless, when I lived in Puerto Rico I found the Puerto Rican crisp-crusted pan de agua ("water bread") to be every bit as good as the French version, though unfortunately not as famous because of a comparative lack of chauvinism. But read on; you won't have to run over to Paris or San Juan to pick up your daily bread. There is great "French bread" to be had right here at home.
As many of us have discovered to our disappointment, it is difficult to duplicate good French bread in our own kitchens. The major reason is that the professional bread ovens have built-in steam injectors, and it's the steam that does the crust-crisping trick. Many recipes for home-baked French-style bread substitute either placing a shallow pan of hot water on the oven floor or spritzing a mist of water into the oven from a spray bottle. I've even seen recipes that advise tossing a few ice cubes into the oven. But a sauna is not the same as a steam bath.
Steam not only makes the crust crisp but helps to produce a well-risen, well-shaped loaf. The science behind these phenomena is quite interesting. Here's the scoop.
What Steam Does
If a long lump of dough were to be put into a hot, dry oven instead of a steamy one, its large surface area would quickly dry out, producing a tough, inflexible skin that resists expansion. The dough then wouldn't be able to rise to its maximum volume or optimum shape, and the loaf would turn out to be puny and misshapen. At the same time, the interior crumb would be unable to expand to its sought-after open structure and would turn out to be denser than desired. If steam is present during this early phase, however, it will soften the surface and allow the loaf to expand its best structure in what bakers call oven-spring.
In the later stages of baking, steam plays another role: It helps the starch in the dough's surface to gelatinize, and gelatinization is necessary for a crisp crust. This is how gelatinization works.
When cold water is added to a starch-containing flour, as when making the dough, the starch granules absorb water and swell. Then, when heated in the presence of enough water (emphasis on enough; see below), the starch granules burst, releasing their gooey, gelatinous contents. That's gelatinization. The gelatinized starch is viscous, with a texture more like molten glass than a liquid, paste or powder. When baked in the oven, it dries down into a crackling, crispy crust similar to cooled glass. But note that gelatinization won't happen unless there is enough water present. That's where the steam comes in. If the dough's surface were to be allowed to dry out before gelatinization takes place, the crust wouldn't end up glassy and crackly. And not incidentally, that glassy surface gives a beautiful luster to the crust, which lesser bakeries attempt to achieve by brushing the loaf with an egg wash.
There's one more important hurdle for our beleaguered loaf to surmount: gelatinization occurs only above a certain temperature (it differs for different starches), and the surface of the dough must be permitted to reach that temperature.
As the dough bakes, the oven's hot air, of course, raises the temperature of its surface. But at the same time, water in the dough begins to rise to the surface and vaporize, and vaporization is a cooling process; that is, water absorbs heat as it evaporates. (That's why you feel cold when you come out of a swimming pool.) Thus, while the surface of the dough is absorbing heat from the oven's air, it is losing heat by evaporation. The more its temperature rises from the oven's heat, the faster its water will evaporate, increasing the rate of cooling.
Eventually, the heating and cooling rates become equal, and the temperature of the surface levels off at a constant value known as the "wet bulb temperature." The problem for our bread is that this temperature is often not high enough to gelatinize the surface starch and the crust will therefore not be crisp.
Steam (again) to the rescue! Steam in the oven produces a high humidity environment that inhibits the evaporation of water from the surface (your perspiration doesn't evaporate when the humidity is high), hence it squelches the cooling effect and allows the air's heat to prevail.
The resulting hot surface is beneficial in yet another way. The chemical reactions (the so-called Maillard reactions) that produce browning and "browned flavor" take place only at high temperatures, so while helping the loaf to rise and its crust to become crisp, the steam also helps it brown to a dark golden color.
Bread at Home
Unfortunately for us home bakers, an occasional spritz of water into the oven or a shallow pan of boiling water on the bottom shelf can't compete with a commercial steam-injected oven. But don't despair. Many of the artisan bread bakeries operating right here under the red, white and blue make baguettes that are as good as those made under the blue, white and red. These American bakeries, many of whom are members of the Bread Baking Guild of America (www.bbga.org
), are equipped with steam-injecting ovens and know how to use them.
In fact, in the 1999 Coupe du Monde de la Boulangerie, an international artisan bread baking competition held in Paris, the specialty breads category (which includes baguettes) was won by a baking team from the United States. In Paris, of all places! Many French faces turned various hues of the tricolor.
So run to the phone and call all the bread bakeries within a 100-mile radius of where you live and ask them if they bake baguettes in steam ovens. It will be worth the trip. Don't forget to buy an extra one to tear into on the way home. You won't be able to resist.
Robert L. Wolke (www.professorscience.com
) is professor emeritus of chemistry at the University of Pittsburgh and the author, most recently, of "What Einstein Told His Cook: Kitchen Science Explained" (W. W. Norton, hardcover, $25.95). Send your questions to