Flour, Eggs, Sugar, Chocolate . . . Just Add Chemistry
| December 28, 2004
By KENNETH CHANG
ATLANTA - With two bad knees, Shirley O. Corriher is not
quite as nimble as she once was when she performs her "protein hop"
- an interpretive dance of sorts to
demonstrate the molecular transformations that turn flour,
eggs and sugar into a cake.
Her arms wave vigorously, mimicking how proteins in wheat
flour interlock to provide the structure that holds a baked
"When you add water to flour and stir," she says, "these
two little proteins - glutenin and gliadin - grab water
first, and each other, to make these springy elastic sheets
Ms. Corriher, who trained as a biochemist and once cooked for 140 boys in a boarding school she started with a former husband, has found a comfortable niche dispensing folksy, scientific wisdom about what really happens during cooking. Her home phone number is the 911 for chefs and cookbook authors with kitchen crises. Susan G. Purdy, author of "The Perfect Cake" who is working on a book about baking at high altitudes, called Ms. Corriher in a panic when a chocolate cake recipe that had previously worked perfectly ended up as a chocolate puddle at 9,000 feet. Ms. Corriher found that Ms. Purdy was using so-called Dutch-processed cocoa, treated with an alkaline substance like baking soda to produce what many consider a better, less bitter taste. Before the glutenin and gliadin proteins can clump together as gluten, they must first partly unwind and unfold, and acidity in the batter induces that unwinding. The Dutch-processed cocoa neutralized the acidity; the glutenin and gliadin did not unwind; and Ms. Purdy's cake never solidified. Ms. Corriher had a simple solution: throw out the Dutch-processed cocoa. "It's a disaster for you," she told Ms. Purdy. Ms. Purdy substituted natural cocoa. The cake worked.
On the other hand, too much gluten produces a chewiness that, while welcome in bread, is usually not a quality sought in a cake. That is why cake flour contains 7.5 to 8.5 percent protein, while bread flour is more protein rich, 11.5 to 12.5 percent. (All-purpose flour is somewhere in between and thus not ideal for either use.) Cooking is often taught as a step-by-step progression of instructions or an art learned through some nebulous communion with the ingredients, but it is also chemistry, predictable, repeatable molecular reactions brought about by mixing ingredients - which are, after all, chemicals - and applying heat to them. Ms. Corriher's 1997 book, "CookWise: The Hows and Whys of Successful Cooking, " brought such scientific facts to home cooks, and although the first 150 pages of the book discuss baking, her publisher wanted a sequel devoted entirely to baking. The publisher gave her three years to deliver the manuscript. Today, four years after her deadline, Ms. Corriher is still working on the book, to be titled "BakeWise." For the last two years, she has been stuck on the cake chapter.
"She's - how should we say it? - a very thorough writer," said her friend Harold McGee, author of "On Food and Cooking."
For example, Ms. Corriher performed an experiment on sifting. Many recipes call for sifting to mix flour with the other dry ingredients, and it is especially important to disperse the leavener - typically, baking powder - so that a cake rises evenly. So Ms. Corriher took a black powder with roughly the same characteristics as baking powder and sifted it with flour. Sifting, it turns out, is not particularly effective."You just didn't get a good mix, even after three or four times," Ms. Corriher said. "You can see little dark streaks." Simply stirring the dry ingredients together with a wooden spoon is better, she said.
The other historical need for sifting was to fluff up the flour after it had been compacted during shipping, but that is also no longer necessary, because flour, sifted at the factory, is now shipped in individual packages instead of large barrels and is not crushed as much. Contributing to the delay, Ms. Corriher has also not been able to stop herself from making yet another variation of a familiar cake recipe, in search of yet one more refinement."Oh, I've been making cakes and making cakes and making cakes and making cakes," she said. "I end up making five or six or seven cakes testing out things that I think up."
Last year, she and her husband, Arch Corriher, ate more pound cakes than they could remember. The traditional pound cake got its name because it contained a pound of flour, a pound of sugar, a pound of eggs and a pound of butter. That was also a large cake, so modern day recipes usually cut those quantities in half. It is also a fairly dense, dry cake. Bakers over the years have improved the traditional pound cake by adding other ingredients: buttermilk or sour cream to make it moister, baking powder to make it airier. Ms. Corriher has her own wrinkles to the recipe, substituting cooking oil for part of the butter - that greases the wheat proteins, producing less gluten and leaving more moisture for the cake - and potato starch for some of the flour. Potato starch granules are considerably longer than the starch granules found in wheat flour. "It really absorbs water like crazy," Ms. Corriher said. "And it makes things really moist." (Potato starch is not a panacea for all cakes. When she tried potato starch in a basic white cake, the result was grainy. "It was awful, awful, awful," Ms. Corriher said, recoiling at the memory. "Life goes on.")
While her pound cake was delicious, Ms. Corriher was not satisfied, because it sagged when baked in a loaf pan. She tried more variations, like additional flour, to produce an attractive, domed top, but those turned out drier. "Of course, the one with too much butter and sugar is going to taste better," she said. Finally, she decided the solution was to bake the pound cake in a Bundt or tube pan, so that the shape of the pan held the cake together. "The moral of the story is, Here is a cake that is technically wrong," Ms. Corriher said, "but the taste is great."
Taking the destabilizing effects of sugar to an extreme is the tunnel-of-fudge cake. "It is so deadly, you can't believe how deadly it is," she said. "We are deliberately doing a cake with way too much sugar." Sugar, she explained, binds to the flour proteins, preventing them from forming the structural lattice. " Glutenin runs off with sugar," Ms. Corriher said. "Gliadin runs off with sugar, and you don't get much gluten." When baked, the outer portions reach a high enough temperature that they harden, but the cooler inner part remains soft and gooey. " We've eaten a whole cake, the two of us," Ms. Corriher said.
Her husband interjected, "Over a period of time."
"It was a very brief period of time," Ms. Corriher said. "It's very addicting."
"It's really rich," Mr. Corriher said.
"And wonderful," Ms. Corriher added.
The original tunnel-of-fudge cake won second place in the 1966 Pillsbury Bake-Off Contest for Ella Rita Helfrich of Houston. That version used a fudge icing mix to create a gooey chocolate center. But Pillsbury discontinued the icing mix, and the resulting clamor of home bakers led Pillsbury to release a recipe for making tunnel-of-fudge cake from scratch. Ms. Corriher again applied her standard tricks, substituting two yolks for one of the eggs, substituting oil for part of the butter. In addition, she substituted dark brown sugar for some of the white sugar. "The reasoning there is dark brown sugar brings out a fudgy taste in chocolate," she said. She also roasted the nuts. "You get much more flavor," she said. "You know how much better a roasted nut tastes than a plain old raw nut."
Dutch-processed cocoa in a tunnel-of-fudge cake would probably spell disaster, preventing even the outside of the cake from setting, she said. Ms. Corriher said she had learned much about cakes by making one that leaves out one of usual ingredients. "You really learn what flour does and what eggs do," she said.
A chocolate cake called crazy cake leaves out the eggs. Ms. Corriher suspects it dates to World War II when staples like eggs were rationed. Without the egg proteins, which also help a cake hold together, gluten is essential for the cake to hold its shape. Thus, cake flour, with its lower protein levels, will not work for crazy cake. Ms. Corriher improved the recipe by cutting down on the vinegar - needed to make the batter acidic - and the leavener, but she hit a snag when she tried adding buttermilk to make it moister. The resulting cake remained pudding. That led to a debate with Harold McGee about why the cake did not set. She thought that the buttermilk was affecting the acidity. Mr. McGee thought the milk proteins were preventing the flour proteins from holding together. Mr. McGee was correct. When she added more flour, the cake again set. Milk proteins differ from egg and flour proteins in that they do not coagulate, Mr. McGee said. "You can boil milk for hours and it doesn't set," he said, "whereas if you boil an egg for 20 seconds, it turns into a solid." Milk solidifies - into yogurt or cheese, for instance - only if it has been treated with an enzyme that knocks off part of the proteins, making them more like other proteins.Meanwhile, Ms. Corriher said she wanted to finish " BakeWise" within a few months. She thinks that maybe it will even be in bookstores by next Christmas.
"I really think I'm on the homestretch on the cake chapter," she said.