Strawberry Rhubarb Pie

Sweet, tart, and jammy, strawberry rhubarb pie is the quintessential spring dessert. Come spring and early summer, I scour the farmers' market for those bright crimson rhubarb stalks, ready to load the trunk of my car with as much as I can fit. Rhubarb season is fleeting—and the only acceptable move is to take full advantage of the moment by buying pounds and pounds of it to turn it into crisps, pickles, compotes, and, of course, pie. 

In theory, making a strawberry rhubarb pie is easy. You chop up your fruit (though I should point out that rhubarb is technically a vegetable), toss it with sugar and your thickener of choice, encase it in pie dough, and bake until bubbling and golden brown.

The only problem? Strawberries and rhubarb both have low pectin levels and a lot of moisture—a recipe for a watery, mushy mess, especially if you don’t use the right kind and amount of thickener. There are also questions of how best to handle the fillings. What's the best size to cut the strawberry and rhubarb? Do they need to be macerated first to draw out moisture? What additional flavorings are good? And what's an ideal ratio of strawberries to rhubarb? 

In my attempt to nail down the perfect strawberry rhubarb pie, I baked a dozen, and did further experiments that zeroed in on some of the filling variables until I came up with my ideal version. I’m talking about a pie that balances the sweetness of strawberries with the tanginess of rhubarb, with a just-set, slightly saucy texture of softened fruit that still holds some of its shape.

The Right Ratio of Fruit and Dicing Decisions

Because I wanted a flavorful dessert that allowed both the strawberries and rhubarb to shine, I started with a 1:1 ratio by weight of strawberries to rhubarb, both finely diced. What I didn’t realize was just how much water the strawberries would contribute to the pie filling. Even with an adequate amount of thickener, the pies made with equal parts strawberries and rhubarb were invariably too loose. You might wonder, why not just use more thickener? We could, but simply increasing starch until the liquid is thick enough solves one problem while creating another. Stabilizing excess liquid with even more starch results in a stretchy, slimy filling that—while certainly thicker—is not appealing.

With this in mind, I adjusted the ratios to reduce water from the strawberries, opting for 2 parts rhubarb to 1 part strawberries. Instead of a fine dice—which I suspect exposed a little too much surface area that allowed more water to leach out and created a runny filling—I opted for 1/2-inch pieces. This was the ideal size of fruit: large enough for the pieces to remain distinct, but small enough to soften and cook down into a slightly jammy filling. 

While this fixed the texture problem, it threw off the flavor balance. I wanted the pie to taste distinctly like a combination of strawberries and rhubarb, but now it was too rhubarb-forward. To make up for the decrease in fresh strawberries, I folded in a few tablespoons of strawberry jam, which helped intensify the berry flavor without contributing any additional water.

Preventing a Runny Filling: What Kind and How Much Thickener Should You Use?

Take a look at most fruit pie recipes, and you’ll see that they typically call for all-purpose flour, cornstarch, potato starch, or tapioca starch as a thickener. This is because most fruit used in pie fillings (think berries, stone fruit, or apples) hold plenty of water that’s released during the cooking process. Without a thickener, you’d end up with a runny pie—but there’s only so much a thickening agent can do to help you achieve a just-set filling that’s neither soup nor sludge.

The best pies—ones with bright, fresh fruit flavor and a jammy texture—depend on proper moisture management, which can be achieved multiple ways. My testing already addressed the ratio of strawberries to rhubarb and how large to dice them, but I was simultaneously tinkering with other variables to zero in on my ideal flavor and texture. One of those variables: the type and quantity of thickener. As I wrote above, more thickener doesn’t solve all problems. In order to get my filling to work, I had to not only switch up the ratio of fruit but also experiment with a different kind of thickening agent. 

I had started off using tapioca starch, former Serious Eats editor Stella Parks’ preferred pie thickener, which sets fruit fillings without becoming overly gloppy. I used 5.5% of the total weight of fruit, Stella’s recommended amount of starch. But even after reducing the amount of fresh strawberries in my filling, it still wasn’t quite enough to absorb the amount of liquid unleashed by them. When I increased the amount of starch to 6%, the filling managed to somehow be both too loose and too gelatinous, underscoring the point that simply adding more thickener wasn't going to be the sole solution to the problem.

Curious to see if there were any other options for thickening pie filling, I noticed many recipes called for minute, or quick-cooking, tapioca pearl fragments. My first attempt with minute tapioca resulted in what one of my neighbors described as a “tapioca boba pie situation,” with little pearls nestled among the fruit. Cute? Yes. Delicious? Yes. But not exactly what I was going for.

Minute tapioca comes coarsely ground in the package, making it a little harder to evenly distribute among the fruit. I wondered: what would happen if I ground it up in a food processor with the granulated sugar and salt? A little fussy, perhaps, but the resulting filling was exactly the texture I was looking for: just set but jammy enough to slightly ooze out of the pie when cut into. Grinding the minute tapioca seemed to hit a sweet spot, texture-wise, that tapioca starch wasn't. Instead of the gluey gel created by tapioca starch, ground minute tapioca managed to set the filling's juices in a more pleasant, less stretchy way (this could have something to do with the relative fineness of the grind; tapioca starch is an even finer powder than minute tapioca ground in a food processor or spice grinder).

To Macerate or Not to Macerate?

In my ongoing question to manage moisture while preserving the fresh and fruity character of the filling, I also wanted to find out whether macerating the strawberries and rhubarb first might be a good idea. By tossing the fruit with sugar and/or a liquid like lemon juice or alcohol, water is drawn out via osmosis, which simultaneously removes excess water while softening the fruit and concentrating its flavor. 

Some recipes call for incorporating the macerating liquid back into the filling, but macerating 8 cups of fruit gave me about 1 1/2 cups of liquid—which is a lot to add back into a pie. Discarding the juices, though, would lead to a loss of valuable flavor and also a less consistent application of sugar in the filling, since some of it would go down the drain with the juices. I decided to reduce the liquid by half to form a syrup, and then fold it back into the filling, in the hopes it would intensify the fruitiness of the pie while removing unwanted moisture. The resulting pie was delicious, but macerating the strawberries and rhubarb with sugar broke down too much of its structure and led to a mushy filling once cooked. 

What worked best was allowing the fruit to sit with the sugar and the finely ground minute tapioca for 15 to 30 minutes before baking, which gives the starch time to hydrate and absorb any excess liquid before the heat of the oven draws even more out.

The Flavorings

I seasoned my first few pies with lemon juice and zest, but paired with the natural tang of rhubarb, the filling was a touch too sharp. In search of something more subtle, I tried a variety of flavorings, including lemon juice, orange juice, and rosewater.

I ultimately opted for vanilla paste, which lends a delicate, floral aroma that complements the sweetness of the strawberries and tartness of the rhubarb. If you don't have vanilla paste, you can substitute with equal parts vanilla extract. It won’t be as concentrated in flavor as vanilla paste, but will do the trick. If you have some handy, another great option is elderflower syrup, which has a bright, effervescent, and floral quality.

Baking and Cooling the Pie

With the filling figured out, all that’s left is baking. It's best to set the assembled pie in the freezer for 15 minutes before baking (not including the 15 to 30 minutes of macerating time), which not only allows the gluten in the dough to relax, but keeps the butter cold for the crispiest, flakiest layers. (You can read more about it in Stella’s pie crust recipe.)

Which brings us to what may be the most difficult part of this entire process: After you’ve baked the pie, you’ll have to allow it to fully cool, a process that can take up to four hours. Though you may be tempted to slice into the pie as soon as it comes out of the oven (or even within the first hour), it’s essential that the pie cools so the filling can set. After this much effort to manage moisture and perfect the filling, it's a shame to watch it flood the pie plate.