When you’re a test cook, being asked to develop a recipe for a classic item such as lemon tart can be daunting. After all, bakers have been producing handsome, crisp-crusted tarts filled with smooth, bracing, just-sliceable curd for centuries without my assistance. What could I possibly contribute? Well, actually, I had an idea: extra-virgin olive oil.
Admittedly, it’s counterintuitive: Butter is featured in both the crust and the filling of almost every lemon tart. But developing a recipe for an olive oil cake a couple of years ago reminded me that other fats offer advantages, too. Using a liquid fat in that cake made the mixing process easy and quick: There was no softening of the fat and no creaming until light and fluffy, yet the crumb was beautifully plush and fine and the cake had an intriguing hint of olive oil flavor.
In fact, the olive oil flavor in the cake was so subtle that I had to take care not to obscure it. I added a near-homeopathic dose of lemon zest—just ¼ teaspoon—to boost the oil’s fruity, floral notes and left it at that. And readers loved that olive oil cake, but they voiced one small objection: not enough lemon flavor.
They had a point. Olive oil and lemons have such an undeniable affinity that it seems a shame not to make the most of it. Maybe this lemon tart recipe was my chance to help the fruit side of the pairing shine. And maybe, as with the cake, olive oil would make the recipe even easier.
Years ago I developed a tart dough in which I defied convention by using melted rather than cold butter. I simply stirred 10 tablespoons of warm melted butter into the dry ingredients until the mixture resembled Play-Doh—no food processor necessary. Then, instead of rolling it out, I crumbled it straight into the tart pan and pressed it to an even thickness. There was no chilling or rolling required, and the crust baked up crunchy and rich.
Tweaking that recipe by substituting olive oil for the melted butter seemed like a no-brainer, but it didn’t work out quite the way I had envisioned it. Ten tablespoons of oil made the dough shiny with grease, and the baked crust was so tender and crumbly that I couldn’t cut it into wedges without it disintegrating.
I soon spotted my mistake. Unlike olive oil, butter isn’t a pure fat; it’s almost 20 percent water. So 10 tablespoons of butter is actually 8 tablespoons of fat and 2 tablespoons of water. It’s less fat overall, but that water is important because it interacts with the protein in the flour to form gluten, which provides the structure that holds the crust together. An absence of water produces a too-fragile crust.
When I substituted ½ cup of olive oil plus 2 tablespoons of water for the butter, the dough was appropriately moist but not slick, and I had no trouble cutting the baked crust into pristine wedges. There was a mere hint of olive oil flavor in the crust, which seemed perfect. But an empty tart shell, even one that’s so easy to make and so satisfyingly crisp, is a forlorn thing. I needed to move on to the filling.
I worried that the olive oil substitution would fail in this recipe because butter is a solid at room temperature and oil is clearly a liquid. How much of the sliceable nature of a lemon tart filling is owed to butter’s solidity?
To find out, I made two tart shells, and while they baked, I cooked two batches of filling. Each had 1 cup of sugar, 2/3 cup of lemon juice, 1 tablespoon of zest, three whole eggs, and three yolks. I whisked them over medium-low heat until they were thick, and then I took them off the heat and whisked ¼ cup of butter into one and ¼ cup of olive oil into the other. I strained each batch before transferring it to a shell and returning it to the oven. After 10 minutes, when both fillings had gelled, I set the tarts on the counter to cool. They looked remarkably similar, but I knew that any differences would show up when the butter had reverted to its solid state.
After 2 hours, I sliced. I was astonished to find that the olive oil version wasn’t softer than the butter version. Turns out, most of the firmness of lemon curd comes from the coagulation of the egg proteins (see “Why a Liquid Fat Doesn’t Make a Loose Curd”). But that’s not to say that the two fillings were exactly alike.
The olive oil filling was so tart that it made my forehead perspire, but the butter had softened the acidity in the other filling, leaving it milder but still lemony. Butter contains dairy proteins that bind to the lemon juice in two ways. First, the proteins bind to the acid, muting its sour taste. They also bind to other flavor compounds in lemon, such as limonene, reducing the lemon flavor even more. Since olive oil does not contain dairy proteins, it does not reduce lemon flavor. The universe seemed to be handing me an excuse to juice one less lemon, so I grabbed it and decreased the lemon juice in the olive oil version to ½ cup. The smaller amount of juice would make the filling just a bit firmer, too.
Finally, I considered adding some starch to the filling, figuring that it could help further temper the acidity and also slow down the bonding of the eggs’ proteins as they cooked, to prevent curdling. Cornstarch and flour each worked equally well, but because I already had flour in the crust, I went with that. The filling in the final tart was creamy and tender, but it sliced beautifully and each wedge had a perfectly clean point. The crust was crisp and remained so even after the tart was refrigerated for three days. Though there was a tiny bit of olive oil pepperiness on the finish, the tart’s flavor was mostly bright and lemony.
But if, by chance, this tart is too lemony for your taste, I have an olive oil cake recipe you might like.