Well-browned, crisp crust
Filling that's smooth and silky from edge to edge
Bright, complex, lingering lemon flavor
If tart, citrusy flavors are the rays of sunshine that brighten lemon bars, then thickeners are the storm clouds that cover them up.
And therein lies a culinary catch-22: For bars with lots of lemon zing, you need lots of lemon juice. But the more juice you use, the more flavor-dulling binders—such as eggs and starch—are required to keep the filling firm and sliceable. My task was to find a way around this problem.
Laying the Foundation
With lemon bars, it's easy to overlook the crust and focus on the wobbly, creamy, lemony layer. And that's exactly what most recipes do. But not mine. Instead of a nondescript platform for the filling, I wanted a crisp crumb with buttery sweetness.
The typical crust is modeled on a British shortbread cookie. I made a classic version, using the food processor to cut cold butter into a mixture of flour, confectioners' sugar, and salt. To ensure that every bite would have the same ratio of crust to filling, I did my best to evenly press the crumbly mixture into an aluminum foil–lined 8-inch square pan. (The foil would facilitate removing the baked bars from the pan.) I had to work carefully because, once compressed, the mixture stayed put, and it became difficult to fill in thinner areas or level out thicker spots. I popped the pan into a 350-degree oven and let the crust bake for 25 minutes. This is longer than most recipes specify, but I hoped that deeper browning would produce an especially crisp, full-flavored crust.
When a buttery scent filled the kitchen, the crust was dark brown, so I pulled the pan from the oven. I topped the baked crust with a placeholder filling made by whisking lemon juice and eggs together with sugar and salt before returning the pan to the oven for 30 more minutes.
The longer baking time had indeed helped develop a rich taste. Unfortunately, it didn't make the crust any crispier. After brainstorming with my colleagues, I realized why: The powdery sugar was producing a fine, delicate crumb that melted on my tongue. For a coarser, crunchier consistency, I needed coarser, crunchier granulated sugar. A side-by-side comparison of crusts made with both types of sugar confirmed it.
Finally, to make the dough easier to work with, I melted the butter in the microwave and stirred it into the flour. This created a pliable mass that was much easier to distribute evenly—with no adverse effect on the finished product. As a bonus, I no longer needed a food processor.
A Question of Proportions
After sampling unbalanced bars like the ones below, we experimented to find the ideal filling-to-crust ratio.
I now had a good base on which to showcase a bright, sweet-tart lemon filling. After my initial tests, I concluded that a filling that was twice as deep as the crust was most pleasant to eat (see “A Question of Proportions”); now I just needed to perfect the filling itself. I'd already fiddled with the simplest approach: whisking together lemon juice, sugar, salt, and a thickener—some combination of eggs, flour, and/or cornstarch—and baking until set. Unfortunately, by the time this filling was cooked at the center, its edges were curdled, as evidenced by pockmarks. A liberal dusting of confectioners' sugar, the baker's Band-Aid, disguised the unevenness, but nothing could camouflage the lumpy consistency. No matter how I tweaked the ingredients, oven temperature, and baking time, I couldn't fix this style of filling.
A more promising method required only marginally more work—the filling is precooked, poured over the crust, and baked until set. I gave it a try, cooking ⅔ cup of lemon juice, six eggs, 1 cup of sugar, and ¼ teaspoon of salt over medium heat. As soon as it reached a pudding-like consistency, I stirred in 4 tablespoons of butter for richness. I poured the filling over my baked crust and returned the pan to the oven. After 10 minutes, the curd barely jiggled when I shook the pan.
Less time in the oven had solved the textural issues since the edges and center of the filling now finished cooking at the same time: These bars boasted an incredibly smooth surface. However, their flavor was marred by egginess, and they lacked the requisite lemony punch. The former problem was relatively easy to solve. While developing Greek Chicken and Rice Soup with Egg and Lemon (March/April 2017), I learned that the sulfur compounds in egg whites are the source of eggy flavor. Reducing the number of whites by three took care of any egginess, but it also left the filling runny. That's because the proteins in the egg whites were providing structure when they set. After some experimentation, I found that I could replace that protein with the starch in 2 tablespoons of flour. This would dull the lemon flavor, but I'd get to that next.
Leaving a Sour Taste
As I'd known from the start, the flavor issue would be a challenge. Increasing the lemon juice was out of the question—I'd have to add even more flavor-dulling thickeners, which would defeat the purpose.
Science: Lemon Flavor That Lingers
To make our bars more lemony without adding more liquid, which would require a flavor-muting thickener, we turned to the science of flavor. When we chew, our brains register the five tastes (sweet, sour, salty, bitter, and umami) on the tongue and potentially a trillion aromas through a pathway in the back of the mouth that leads to the nasal passages. The dominant taste chemical in lemon juice, citric acid, is nonvolatile and registers only on the tongue. But lemon juice also contains a small amount of volatile compounds that give it subtle fruity-floral flavors as well.
With that in mind, we looked to lemon zest, which is full of the aromatic oils limonene, pinene, citral, neral, geranial, and linalool and would enhance the experience of lemon flavor in the nose. For a tangy boost from a nonliquid ingredient that would register on the tongue, we incorporated cream of tartar.
Together, these ingredients gave our lemon bars a complex lemony punch that was even better than if we’d added plain lemon juice.
I took a step back to consider how flavor in lemon juice (and all foods) works: When you take a bite, you encounter taste with your tongue and aroma through a channel in the back of your mouth that leads directly to your nose. With lemon juice specifically, you taste only the tartness of its citric and malic acids on the tongue, while all the fruity, lemony flavors come from volatile compounds that shoot into that back door to your nasal passage when you exhale. So to add more lemon flavor without more lemon juice, I'd have to consider both taste and aroma.
Incorporating lemony aroma would be easy: I could add lemon zest. Zest has even more volatile flavor chemicals than the juice, which is why it is so often added to foods to enhance lemony flavor. I found that 2 teaspoons of grated zest cooked into the filling (and later strained out) boosted its fruity flavor significantly. The trickier task was increasing that acidic punch in the filling without adding more liquid. What ingredient would help with that?
I flirted with the idea of purchasing powdered citric acid or grinding up vitamin C tablets (ascorbic acid). But then I realized I already had a truly sour-tasting powder in my pantry: cream of tartar. I whipped up two more batches of bars, one of which contained 2 teaspoons of cream of tartar. This was the magic ingredient: Tasters loved the bold sharpness of the bars containing cream of tartar, claiming they were unlike any others they'd tasted. And when they raved about the interplay of the tart, silky filling and the crisp, buttery crust, I knew I had a winner.