Although I am but an amateur food scientist, popovers interest me greatly, so let me try to apply some of my research to the problems that P.J. mentions here. I have to eat a low-sodium diet, and since bread with salt isn’t worth eating (and since removing the salt makes the yeast run crazy), I’m intensely interested in popovers, since they require no yeast or other leavening—only steam. (I can use a salt substitute for flavor, but salt substitutes don’t control yeast like salt does in yeasted bread.) A guy named Alton Brown is more more into food science than I am. In June, he devoted an entire episode of his TV show, Good Eats, to popovers and their cousins (yorkshire pudding, dutch baby). You can see his popover recipe here, and read a transcript of the show from this page (which also has YouTube links to video from the show). A couple of passages in the show jump out at me, such as this one discussing selection of flour:

Wow, that is tough, and that would be great if we were dealing with a high-gluten dough, like you would put into a French baguette, you know? That’s not where our ingredient list is leading us today. No, today, we need the perfect mixture of both worlds. We need a flour that is strong but tender, like, say, all-purpose flour.

Later in the same show, AB argues with “recipe writers” (RW1 and RW2) who say to let popover batter rest, and here’s the key exchange:

RW1: You won’t get enough gluten. AB: I don’t want much gluten. RW2: Your flour won’t be hydrated. AB: Yes, I know that, but you know what? If I wait, I lose all of my bubbles.

AB makes his batter in a blender for 30 seconds, using room temperature ingredients, then pours them immediately into the lubricated popover pan and puts them in a 400°F oven. The key points here are bubbles and gluten. With no yeast or chemical leaveners, popovers rise purely by steam. It’s what makes them “pop.” If everything goes right, then as the bazillions of microscopic air bubbles in the batter expand in the over, they eventually join and form one giant bubble that makes the popover’s hollow interior. That’s why AB uses a blender to mix the batter (the force of the blades makes lots and lots of bubbles). It’s also why he doesn’t let the batter rest, and that’s especially important with King Arthur Unbleached All-Purpose Flour. Here’s a message from Joe Salkowitz on the Bread-Baker’s Digest mailing list in the past week (it will eventually be available here as part of digest v108n43):

I remember going to a King Arthur presentation where they showed their flour mixed with water and another brand of flour mixed in the same proportions. The other brand looked like soup while the KA was a dough; demonstrating differences in flours.

Most of us here know that King Arthur All-Purpose Flour (never bleached, never bromated) is 11.7% protein by weight. More protein means more gluten development. Gluten traps air bubbles as created by yeast to give bread its crumb. This is exactly the opposite of what you want in a popover. If you get too much gluten in a popover dough, you essentially have bread dough—sheets and sheets of gluten throughout the entire baked good with air bubbles trapped between them. In other words, dinner rolls. But without yeast in the mix feeding on the carbohydrates and blowing air bubbles, and doing so faster and faster in the oven until the temperature reaches 120°F, the bubbles just never get big enough to do anything. You get, basically, unrisen dinner rolls. This gets aggravated because, following popover lore, P.J. rests her batter for 15 minutes. That's actually an autolyse stage—just letting hydrated flour sit for 15-20 minutes helps develop gluten. With a higher-protein flour like King Arthur All-Purpose Flour, this is going to make so much gluten that the air bubbles really get trapped between sheets of gluten in the popover pan or muffin tin. They can't merge into one giant bubble in the oven, and the popovers don't "pop." One might expect that the same technique with a lower-protein flour, say about 10.5%, would probably work fine. (Good Eats is produced in Atlanta, and southern all-purpose flours are notorious for having low protein to make "softer" biscuits and baked goods. Identical-looking bags of Gold Medal All-Purpose Flour purchased in Massachusetts and Georgia will have significantly different amounts of protein. King Arthur's all-purpose flour is always 11.7% protein, no matter where you buy it.) With all this on the table, let's see if we can find the hidden trap in P.J.'s recipe. She wrote:

Next, I figured I’d go back to the good old days and beat the ingredients by hand, with a wire whisk. Wouldn’t you know, that method yielded gloriously tall popovers—so long as I whisked the batter to just the right consistency. Completely smooth? No. Big lumps? No. Small lumps? Popovers with POP.

Given all this information, this actually makes perfect sense. Those "small lumps" are clumps of flour that didn't get completely separated and hydrated during mixing. Therefore, the protein in those lumps isn't available for gluten formation, and you get less gluten. Less gluten means the bubbles in the batter have an easier time of combining. Why is it hit-or-miss? Because for this to work, you need to have the right distribution of the "small lumps" throughout all of the popover or muffin cups. If one or two of the cups doesn't have enough lumps, those cups have a lot more gluten, and the popovers can't "pop." Why don't you taste the lumps? The autolyse stage is enough to hydrate the flour, I think, but not enough to let it contribute much gluten. So, putting it all together: You want less gluten, so resting the batter is a bad idea. The only reason it works for P.J. is that she has small lumps that keep all the flour from contributing to gluten development. You can make a smooth batter, but you can't rest it, because autolyse makes too much gluten. Conversely, if you do rest the batter, you can't make it completely smooth (or you'd have to use a lower protein flour). Since distributing lumps in a batter among various cups is always going to be hit or miss, I don't think that technique can be foolproof. To be foolproof, you'll have to make a smooth batter without a lot of gluten development and bake it immediately. Susan's idea that gluten is getting torn up during mechanical mixing may be correct, but it's not relevant: you don't want too much gluten. Gluten keeps popovers from popping. You only want enough to let the heat of the oven set the outside of the popover and support the ever-increasing bubbles inside. So, to make this foolproof, you need a smooth batter with room temperature ingredients that you bake immediately in a pre-heated oven (letting the batter rest not only develops gluten, but also lets some of those bubbles dissolve, inhibiting "pop"). Once you get that done, I think the amount of eggs and butter to flour (that is, where the fats come from) is a matter of taste, not of success or failure. But make a smooth batter and don't let it rest and see how that turns out. (I wish I had the time to try all these variations, but I have to be satisfied with the chemistry part of it.) WHOA, thanks so much, Matt. I think I'll try our Mellow Pastry Blend (10.3% protein) with room temperature ingredients, in a mixer with wire whisk since I don't have a blender, and immediately into the oven. I'll let everyone know how it goes- PJH Matt, I tried with the Mellow Pastry Blend, room-temperature ingredients, whisking vigorously with electric mixer, baking immediately. Lovely - the popovers rose very evenly, no tilting. I like this! Thanks- PJH

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