The champagne cork wasn't the only thing popping at our house on New Year's Eve. Our low-key, stay-at-home festivities included The Great Popover Experiment. Armed with eggs, milk, flour, butter, and dashes of salt and thyme, I conducted my own kitchen chemistry research -- much to the Time Lord's gastronomic delight.
Back in November, while giving a talk at Skepticon V in Missouri, we had a lovely dinner at a local steakhouse, which served freshly baked popovers instead of the usual sourdough rolls. A popover is a puffed-up, eggy type of roll that is hollow, crispy on the outside and deliciously moist and airy on the inside. They make terrific breakfast fare with butter and jam, as well as an excellent dinner accompaniment, especially with roasted meats and gravies. The popovers at the steakhouse were sooooo good! I resolved to make some myself; our waiter assured me they were incredibly simple and he made them at home all the time. (We take turns manning the kitchen in our house as our crazed schedules permit. The Time Lord makes a mean osso buco. Me? I love to bake!)
So bright and early on the morning of New Year's Eve, I whipped out my implements of construction:
Cooking is basically kitchen chemistry, and baking requires particular attention to mixing the ingredients in the correct proportions. The batter for popovers is pretty similar to crepe batter, except the latter produces ultra-thin flat pancakes. How do you get a high-domed puffy roll out of the same basic ingredients, and not a hint of yeast (the usual leavening agent used for bready things)? According to Busy Cooks at About.com, "You need three things: good gluten formation, high heat, and a narrow, high container."
It's simple science: the liquid in the batter evaporates rapidly (thanks to the high heat) as it bakes and produces steam, which is trapped into a giant bubble by the web of glutens, starches, and proteins also contained in the batter. Because that web forms so quickly, it prevents the steam from escaping, so the roll literally "pops" over (hence the name) without bursting, guided by the shape of the deep container cups. Let's break the recipe ingredients down for a bit more detail:
1. 2 cups all-purpose flour. It's important to use a high-protein flour for popovers, not a cake or pastry flour, because the low-protein varieties don't produce as rigid a gluten network. There are two key proteins in flour, glutenin and gliadin, and when you add liquid (see milk, below), these two proteins combine into a new protein, gluten. Flour also contains starch (carbohydrates comprised of strings of glucose) that sets in response to heat, providing more support for the batter's structure during baking.
Kelly Stewart, editor of Roast magazine, delved more deeply into this issue after a failed attempt at baking cheesy gougeres, normally puffy except this one time, when they turned out "as flat as cookies." It turns out she unwittingly used a low-protein flour. Apparently the labeling on various flours isn't especially precise; manufacturers round up or down quite a bit when it comes to assessing protein content.
Stewart consulted with Vanderbilt University biochemistry Shirley Corriher about this, who said the only way to find out for sure is to test your flour by putting 2 cups of flour in a bowl and stir while adding a cup of water. "If you have a high-protein flour, it's going to suck in water like crazy," Corriher told her, thereby forming a dough. Adds Stewart: "Less protein-rich flour won't come together unless you add more flour."
2. 6 eggs, lightly beaten. Popovers call for a lot of eggs: the liquid contributes to the steam released during baking, but mostly the eggs provide lots of protein for a firm structure to better trap that steam. The egg whites are the leavening agents: the proteins uncoil in response to heat and force out moisture. The yolks act as an emulsifier, providing a smoother overall texture to the baked popovers.
3. 2 cups whole milk. Milk provides the main liquid component and also contains a sugar called lactose, which bonds to those two proteins in flour to form gluten. As it evaporates during baking, it lets off steam, and this causes the batter to expand quickly in a hot oven. It also has protein and starch to contribute to that all-important weblike structure.
4. 1 teaspoon salt. According to MIT biochemistry Patti Christie -- another of Stewart's sources -- so many baking recipes call for a dash of salt to conceal any bitter taste, and many cooks might not realize that it also aids in the formation of gluten Per Christie: "Charged amino acids in the flour are going to interact with the ions in the salt, and that helps line up the gluten fibers." (I added a dash of thyme to my popover batter for flavor, but this is optional, and doesn't impact the chemistry.)
5. 4 tablespoons melted unsalted butter. There aren't a lot of fats in popovers -- which doesn't make them particularly healthy, mind you! -- because fats inhibit the formation of gluten, and with popovers you want that gluten (unlike for, say, scones, which contain tons of fats). But there is a little bit of melted butter involved. Most of this gets spooned into the individual cups prior to baking, to ensure a crisp browned crust, with a tablespoon or so going into the batter.
Here's a trick a friend of mine revealed: once you've greased the popover pan and put a bit of melted butter in each cup, it's a good idea to briefly pre-heat the pan until the butter sizzles a bit, then fill each cup halfway with batter before popping into the oven. Remember, that high heat is really important to a quick formation of the gluten web for trapping in the steam, and the pre-heating is an effective way to make sure this happens.
6. Finally, you need the right kind of pan. Sure, lots of folks use a standard cupcake tin, and they work just fine. But I'm a sucker for new culinary toys -- they're fun! -- and found a spiffy nonstick popover pan at Williams-Sonoma with six cups separated by rods, the better to let heat circulate around each cup. (I've used a scone pan from Williams-Sonoma for years, and it really does work well in terms of even heat distribution. And nobody paid me to say that.)
Put it all together, bake at 450 degrees for 15-20 minutes, then another 5-10 minutes at 325 degrees, and voila!
Important note: the baking times in recipes are often estimates, since temperatures vary from oven to oven. Also, we have a gas oven, which doesn't distribute heat as evenly (another reason specialty baking cans are helpful). The first batch, frankly, was a teensy bit over-crisp on the outside; I followed the recipe instructions to bake for 20 minutes at 450 degrees F before reducing the heat, and this proved a bit too long, especially since I preheated the pan. Our oven runs hot. You've got to be a bit flexible when cooking or baking: your kitchen is your laboratory, and sometimes you have to run the experiment more than once to figure out what it takes to get optimal results.
The Time Lord didn't have any complaints. We wolfed them down pretty quickly and spent the rest of the day in a popover-induced food coma.
Popovers are closely related to Yorkshire pudding, a traditional dish served in the United Kingdom and associated with the northern town of Yorkshire (although it didn't necessarily originate there; people seem to differ in their opinions on this score). Yorkshire pudding incorporates rich and fatty meat drippings from various roasts to produce what my fellow science blogger Joe Hanson called "a meat donut" on Twitter.
The dish was usually eaten before the main course, since it filled people up so the more expensive meats could feed more diners. Wikipedia directed me to this 1737 recipe for "dripping pudding" in a book called The Whole Duty of a Woman by "A Lady" and one William Kenrick:
Make a good batter for pancakes; put in a hot toss-pan over the fire with a bit of butter to fry the bottom a little then put the pan and butter under a shoulder of mutton, instead of a dripping pan, keeping frequently shaking it by the handle and it will light and savoury, and fit to take up when your mutton is enough; then turn it in a dish and serve it hot.
The British take their Yorkshire pudding seriously. Back in 2008, the Royal Society of Chemistry actually issued a ruling on the precise definition of a Yorkshire pudding, namely, that it can't be less than four inches tall. This came out of an appeal to its members on behalf of an ex-patriate Yorkshireman named Ian Lyness, who found he couldn't make a decent pudding in his new home in Boulder, Colorado -- probably because of the high altitude (baking, even boiling water, is tricky in such regions because of the change in air pressure). Mr. Lyness, you see, is a purist:
"I use batter mix that I pick up on my trips back to Blighty and my mum's old Pyrex dish... I do not go for the silly little ones on the plate with everything else, but a traditional, big long pudding.... Coleman's English mustard is also essential accompaniment, I find. But I have been struggling badly here. Sundays from my kitchen window here I can enjoy the sight of rearing snow-capped mountains but on my plate there are apologetic little hillocks."
The Royal Society couldn't just let him suffer, so they sent out a call for help to its members, thousands of whom work in the food and beverage industry, and a fellow Yorkshireman, chemist John Emsley answered that call. Emsley also has strong feelings about what makes for a proper Yorkshire pudding. "I have seen many grim results from people who have tried to get their Yorkshires to rise.... These are carbohydrates + H20 + protein + NaCl + lipids. Some amateurs even place the batter in the fridge first. What kind of foolish act is that?"
Emsley is right on the refrigeration score. The batter for Yorkshire pudding -- and popovers -- mustn't be chilled. Adding cold eggs to butter and sugar means they won't combine correctly. Chilling the batter actually causes the ingredients to separate again, until you warm it back up to room temperature and apply a brisk whisking to recombine them.
Next year, I might throw caution to the winds and make some nice mutton and Yorkshire pudding for the holidays. And I'll try to use the Royal Society of Chemistry's official recipe, although it's pretty vague on the details. (The formula for the basic proportions is 1/3 cup of flour and 1/3 cup of milk/water mixture per egg.)
The Royal Society of Chemistry Yorkshire Pudding
1-1/2 TBSP of plain flour
Half milk, half water to make a thin batter
1/2 tsp salt
Put flour in a bowl, make a well in the middle, add the egg, stir until the two are combined, then start gradually adding the milk and water, combining as you go. Add the liquid until the batter is a smooth and thin consistency. Stir in half teaspoon of salt and leave to stand for 10 minutes. Put beef dripping into Yorkshire pudding tins or into one large tin but don't use too much fat. Put into hot oven until well-risen -- should tale 10-15 minutes. Serve. Always serve as a separate course before the main meal and use the best gravy made from the juices of the roast joint.
Mmmm, roast joint and Yorkshire pudding. That should make for a Happy New Year!