Cherry Blueberry Pie
(with a little side of math)
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The distinctive sweetness of blueberries partners wonderfully with the bold, tart flavour of sour/tart cherries in this delightful pie. The filling can be adapted to fresh, frozen, or canned cherries. My family-favourite shortcrust pastry recipe is also included, but you can easily substitute your own (or store-bought) if you prefer.
Also included are plenty of tips about finding and using sour cherries, working with tapioca flour to set the filling, and getting perfect, flaky pie crusts.
Today we're making pie, and talking about math, because they're both awesome. I mean, I'm going to talk about math... if you just want to make a fantastically delicious blueberry and sour cherry pie you can click that "skip to recipe" bit above. Just make sure to check out the Recipe Notes section. That's Diversivore Rule #1 - always check out the Recipe Notes.
You've probably seen people connecting pie to math before - notably to the mathematical constant pi (π). The words pie and pi have no linguistic connection; pi the number comes from Greek (as does the symbol, π), while pie the food seems to have made its way into English via the Latin pia (pastry), and, oddly, the word pica (magpie). The connection between magpies (which are black and white, long-tailed birds) and the pie we eat is a bit peculiar. It's thought to be related to the magpie's habit of collecting and stashing curious odds and ends, and the pie's original status as a pastry filled with miscellaneous items.1 The connection between the two pi(e)s may be nothing more than a linguistic coincidence, but it's been enough to bring the worlds of mathematicians and bakers together to celebrate a mutual love of... well, having an excuse to eat pie, I suppose. We've even got a 'pi(e) day' now (which I'm going to refer to as π-day from here on out), on which we celebrate the number by eating the dessert. It falls on March 14th, or 3/14, because pi is approximately 3.14. We get to eat some tasty treats and enjoy a bit of fascinating math. What's not to love?
Well, you might be surprised to learn that there's a bit of mathematical controversy lurking behind the scenes. As it turns out, there are those who think that π-day is all wrong. They argue that the day that deserves the glory should actually be June 28th - 2π-day.
1 Sweet fruit pies are much more modern than savoury pies made with mixed ingredients.
Yes, 2π-day is a thing, and it has been for quite a while. The basic premise is that June 28th is 6/28, and 2π = 6.28(...), so... well, you get it. But this isn't some arbitrary doubling for its own sake - the why of 2π day is much more interesting than the what. 2π is actually a very important number - so important, in fact, that some argue that it's superior to π for a number of reasons. We'll get into those reasons in a moment, but let's turn our attention to 2π day for a moment. From my own perspective, there are three excellent reasons that 2-pi day is actually better than pi day. Allow me to lay them out for you (with numbers... because... math!).
#1: Two pies are better than one. Obviously.
#2: There are far better fruit options for your pies if you're baking in June than in March. Cherries, strawberries, blueberries - all out of season in the oft-wintry month of March. It's less of an issue for meat pies obviously, but I like my fruit pies.
#3: Pi is a fine number. A great number even. Great enough that I memorized it to a truly useless 65 decimal places back in junior high (don't judge me - it was on a chart on the wall, and I was really, really bored in class a great deal of the time). But it's also something of a frustrating number, because in practice, it only gets you half of the way to where you want to be.
As you can probably see, point #3 is where the actual math is hiding - along with the actual controversy.
Let's back up a minute and talk about what pi is and where it comes from. Most of us are at least passingly familiar with pi, but I think many people never really learn exactly what pi is, or where it comes from. Even if your high school math days are (mercifully) far behind you, you probably know that it's 3-point-something, and that it has something to do with circles. You may also remember that the decimals in the number go on forever, which turns out to be a very important aspect. Pi goes on forever and ever and ever because it's what's called an irrational number. This means that you can't express pi as a ratio of two integers (i.e. as a fraction composed of 'normal' numbers like 1, 2, 553, or -76). 22/7 is a decent approximation of pi, and 355/113 is even better - but they're not exact. You can find irrational numbers by taking the square root of certain integers (e.g. √2 and √3), but you can't find pi this way (π2 is also irrational). So where exactly does the number come from? Well, pi is a ratio - but not a ratio between integers. Instead, pi is the ratio of a circle's diameter to its circumference. Imagine a straight line drawn right through the center of a circle. That's the diameter. Now imagine picking up that line and wrapping it around the outside (circumference) of the same circle. You'll need more than one to go all the way around - in fact, you'll need exactly π (3.14159...) of those lines. That's pi.
So why then did I say that pi only gets us half of the way to where we want to be? Well, the problem is that diameter thing. As it turns out, it's a whole lot easier to talk about the construction and mathematics of circles and angles when you use the radius (i.e. a line drawn from the center of a circle to any point on its edge). A circle is the only shape with a single, constant radius - but there are infinitely many shapes with a single constant diameter (like the fascinating Reuleux triangle). It's also much easier to draw a circle with the radius: take a compass, measure/define the radius by moving the two arms apart some distance, then spin that bad boy around the pointy end. Ta-da, you've got a circle. If you want to start out with a diameter you'll have to add extra steps in order to cut that diameter in half to get the radius. This might sound a bit nitpicky in the era of computers, but math is a heck of a lot older than computers. The ancient Greeks laid the foundations for an incredibly important body of mathematics simply by working with a compass and a straight edge. Given all of this, it makes sense that the radius of a circle is much more important than the diameter - but this has some ramifications for pi. Allow me to demonstrate with a little pi(e) illustration I've put together:
I adapted this animation from a version found here on Wikipedia.
The pie itself is actually a different recipe of mine... so I guess I did sneak two pies into this 2π-day article.
Because pi is the ratio of the diameter to the circumference, expressing it in terms of the radius (which is half the diameter) only gives you half of the circumference. This means that you only get half of the way around the circle. So π pieces of pie only gets you half a pie, while 2π pieces of pie gives you a whole pie. And that's just awkward.
2π-day seems to make a lot more sense now, right? If you bake a pie to celebrate π-day, you're actually baking 2π worth of pie (unless you can somehow bake half a pie, I guess). But let's pause for a second and think about this, because we still have a problem: do you make one pie, or two pies on 2π day? 2π is equal to one pie, not two pies (fun fact: if you read this out loud to someone they'll probably start looking very annoyed with you). Now, I personally think it makes sense to bake a single pie because... well, it's a lot of work to bake two pies, and I'm only putting up one recipe after all. But no matter what, we're stuck with the fact that one pie ≠ one π.
This is where tau comes in.
Tau is another Greek letter. It looks like this: τ (which, you'll note, is conveniently similar to π). Most importantly, when it comes to circles, tau is equal to 2π. This isn't a coincidence of course; tau was specifically chosen to represent 2π as a single term, because... well, because it makes math a little nicer. Back in 2010, physicist/programmer/educator Michael Hartl wrote a math essay called The Tau Manifesto. In it, Hartl makes a case for the inadequacy of pi. He argues that tau is superior in many respects, and that it's the more logical choice for a circle constant. I'm not going to get into the details here (you can check out the manifesto for that), but I will say that this has actually turned into something of a full-on debate in the math world. It's mostly in good fun, but things have been known to get a little bit heated when this whole pi vs. tau thing comes up. There's nothing wrong with pi, for the record; you don't get better or more correct math by using tau. You simply get math that's a bit more elegant.
I would argue that there are three basic camps on this whole debate: pi fans, tau fans, and people who think the whole thing is a bit crazy. Putting aside esoteric arguments, there is a legitimate point to be made about making math education easier by changing the circle constant to tau - but that's a tough sell. I mean, Esperanto isn't the universal common language, the Dvorak keyboard hasn't replaced Qwerty, and the metric system is still (arrrrgh) not used in the USA. What I'm saying is that humans aren't big on changing things for the sake of elegance, simplicity, or even utility.
At this point you might be wondering if I'm a tau proponent. The truth is, my interest in mathematics is much too recreational for me to really care all that much. But I do like the pie-making produce that's available in June a whole lot more than what I can get in March, so I guess you could call me something of a tau-utilitarian. A... u-tau-litarian, if you will (I'm not even sorry). Then again, you can't bake a tau, so you're still stuck talking about your pie in terms of pi, so maybe I'm still a pi-pusher. Perhaps I'm just one of those annoying people who likes to stir up a bit of playful controversy without actually taking a position on the subject. I'll let you decide.
Recipe Notes
Alright, enough pi. Time for pie.
If you're an experienced pie-maker, there's not a whole lot I'll need to tell you here. I would encourage you to read the blurb below about tapioca flour, as it's not all that well-known, and it's a total game-changer when it comes to fruit pies.
You can use storebought pastry to make this if you like, but it's not actually too tough to make the crust. There are a few tricks to it, so I'll dive into these in a bit more detail below.
Sour/Tart Cherries
Once upon a time, this section would probably have been fairly unnecessary, but sadly sour cherries (aka tart cherries) have fallen off of the radar for a lot of people. While they remain deeply popular in many recipes (e.g. cherry pie), and across many cultures, many people are unaware of the difference between sour cherries and their more common sweet cousins. The cherries we buy for eating are sweet cherries (Prunus avium). While sweet cherries are undeniably delicious, they're generally not great for baking (though there are occasional exceptions - clafoutis, for example, is made with fresh sweet cherries). That distinctive, tangy, and instantly recognizable cherry flavour that we associate with cherry pie and cherry candy isn't really present in sweet cherries. Instead, it's the sour cherry (Prunus cerasus) that delivers that flavour in a big way. As you'd probably guess, sour cherries are much tangier and less sugary than sweet cherries, though some newer varieties are sweet enough to eat out-of-hand. The two traditional varieties that are generally encountered are the deep red morello cherry (shown below) and the scarlet Montmorency cherry. The former has dark red flesh and juice, while the latter has bright yellow flesh.
Sweet cherries are fairly easy to find (in season of course), but fresh sour cherries can require a bit of hunting. They're somewhat regional and sporadic as a commercial crop, and often aren't carried in your average grocery store. I'd personally love to see this change, as sour/tart cherries are a wonderful crop that grows very well in many parts of North America, and the industry has been hurting in recent years. If more home cooks (re)discover the joy of pies, jams, and other recipes made with fresh sour cherries, it would go a long way toward raising the fruit's profile (and demand). If you want to find sour cherries, try searching the web for local growers and markets that sell them. Grocery stores that cater to Eastern European, Turkish, Iranian, or Russian communities are also good places to check, as sour cherries are particularly popular in those parts of the world. Canadian readers can also check out the Canadian Cherry Producers map showing commercial growers, while my American friends can check out the US Tart Cherries directory. If you're in Europe, you may have an easier time tracking down fresh sour/tart cherries, but you may still have to hunt around a little bit.
What if you can't get fresh? Frozen is the best bet alternative here. As with fresh sour cherries, you're best off looking for these at Eastern European/Central Asian grocery stores, or by searching for any producers that are relatively close to you. Canned cherries are also a good alternative, and may be the easiest to find. These are simply sour cherries that have been preserved in a light syrup (avoid any canned in a heavy syrup, as they'll be gooey and cloyingly sweet). They're a great ingredient to have around for baking projects, and the beautiful red syrup is great with ice cream. It's important to note that canned sour cherries are not the same as canned cherry pie filling; you cannot use cherry pie filling in this recipe without making substantial changes to the other ingredients.
Finally, it's worth mentioning that there are actually several other kinds of sour cherry out there in the world. Few of these are likely to be available on a commercial level, but many are grown in backyards and gardens. Nanking cherry (Prunus tomentosa) is a popular cold-hardy cherry tree with beautiful, tart, bright red fruit. Mongolian cherry (Prunus fruticosa) is a very hardy, dwarf cherry species that's sometimes found in hedgerows, etc. If you've got one of these cherry trees in your yard, use them! I'm particularly fond of the Nanking cherry myself, as it was a popular plant in Alberta where I grew up. Lastly, I want to mention the Cornelian cherry (Cornus mas). This lovely tree is not a true cherry (it's actually a dogwood), but the red, ripe, olive-shaped fruits taste like a cross between a sour cherry and a cranberry. They're a popular ornamental tree in a lot of urban areas, and they can produce huge quantities of fruit. While I've eaten Cornelian cherries, I've not yet tried baking a pie with them, so I can't give direct advice on that subject here. If you've ever done it, I'd love it if you left a comment below!
Tapioca Flour
Take a look at the slice of pie below. See how the fruit is nice and contained in the pie, rather than spilling out all over the plate? That's the magic of tapioca flour.
Tapioca flour is made from the cassava root (aka yuca or manioc), and it's something of an old-school pie ingredient. While it might not be as common in home kitchens as flour, cornstarch or gelatin, it is (in my opinion) the very best ingredient for thickening and setting pie filling. Unlike cornstarch or flour, tapioca flour cooks into a clear, soft, jelly-like substance. This means that it won't impart a granular, gritty texture to your pie filling. Unlike gelatin, it sets very nicely at room temperature. Tapioca also doesn't become cloudy after cooking, and it isn't affected by acidity. It's great for setting any fruit pies, but it's at its very best with pies made with berries that produce a lot of juice. These juices can cause pies made without thickeners absolute disintegrate when cut into, but with tapioca and sugar, they become a thick and deliciously gooey filling.
Tapioca can be found in a number of forms. I like to use tapioca flour, also called tapioca starch. Bob's Red Mill is popular and easy to find in North America. Instant/minute tapioca is also fairly easy to find, though you'll want to grind it finely with a spice grinder (or similar) to break it down into a fine grained powder. The larger pieces used to make instant tapioca will still work to thicken your pie, but you'll end up with little white chunks of tapioca in the mix too.
The instructions in the recipe below should be all you need to know even if you're new to using tapioca in your pies. That being said, it's worth noting that it's important to let your pie filling become hot enough to bubble throughout, as this will activate the tapioca and allow it to set. This shouldn't be an issue if you follow the baking instructions as given, but it helps to cut your pastry in such a way that you can see when the center of the pie is bubbling. You can also learn more about using tapioca flour in this Epicurious article if you're curious.
If you can't find tapioca flour, you can substitute the same volume of cornstarch. I recommend whisking the cornstarch with the added water used in the recipe before combining it with the fruit, as this helps prevent clumps from forming.
Pastry Making
This is based off of my mom's pie crust recipe, which means it might be the single most tested recipe on this entire site. My mom makes a mean pie, but she'd be the first person to tell you that crust can be a bit of a finicky thing to make. It's not that it's difficult mind you - it's just that there are three important factors to consider.
- Use cold ingredients. Your shortening can be at room temperature, but not hotter. Cold butter, and ice cold water are must-haves. Heck, if you've got the time to throw your flour in the fridge, it wouldn't hurt.
- Don't overwork the dough. The idea is to get it to a point where it's just barely combined so that little pockets of butter/shortening remain, making the pastry flaky and soft. Working the dough for a long time also develops more gluten, making for tougher crusts.
- Don't obsess over looks. Good advice in life, and good advice with pastry. The reason a pastry like this is called shortcrust is because it has a high ratio of fat to flour, which means that it's 'short' in baking parlance. This makes the dough tasty, but fragile. The delicate structure makes it easy to tear, especially when you're transferring it to the pie plate. If you tear your pastry, just grab a bit of scrap and patch it. It might not be gorgeous on your first attempt, but it'll still taste great. Patching can look a bit unattractive, so I know it can be tempting to ball the dough back up and roll it out again, but don't! This will overwork your dough, making it dense and tough.
I might sound a bit hypocritical given that I clearly went way above and beyond for the decorating on this pie with a pattern of cherries and blueberries , but that's because a) I've got a fair bit of experience making crusts, b) I wanted to make it stand out, and c) I'm a sucker for punishment. Seriously, I hand cut all of the leaves for my Swedish apple cake topping too. I don't know why I keep doing this to myself. Funny enough, patterned tops like the one in the photo are actually easier in some ways, as you don't have to undergo the nerve-wracking challenge of transferring a big, delicate slab of shortcrust to the top of the pie. Decorations are fun (to me, anyway), but simple vented crusts and lattices both look, and more importantly, taste fantastic too.
One quick tip that has saved me a lot of pie heartbreak: roll the dough out on a largely flexible surface like a baking sheet, or piece of wax paper. This allows you to get under it, move it, and carefully get it into your pie dish with the minimum of tears (I'll let you decide how to pronounce that last word).
Lastly, I've never been one to do egg-washes on my pies, but you can certainly do that if you're so inclined. Some people like to use an egg wash on the bottom crust in order to make it less prone to sogginess, but I don't feel that it's necessary thanks to the tapioca flour thickening up the filling.
Note: Nutritional Information is given for a single slice (1/10th portion of the total recipe).
Nutritional Summary
Ingredient Pages
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Cherry Blueberry Pie
Ingredients
Crust
- 2 cups all purpose flour plus more for dusting (see note)
- 3/4 tsp salt
- 1/2 cup vegetable shortening (very cold)
- 1/2 cup butter (very cold)
- 4 tbsp ice water
Filling
- 2 cups sour cherries pitted (~300 g) (see note)
- 3 cups blueberries (~450 g)
- water (see instructions for quantity)
- 1/2 cup sugar
- 1/4 cup tapioca flour (aka tapioca starch) (see note)
- 1/8 tsp salt
Instructions
Filling
- You can make this pie with fresh, frozen, or canned sour cherries. If using fresh, pit the cherries and try to collect as much of the juice as you can during the process. If using frozen, allow them to thaw, making sure to keep the juice that collects. If using canned, drain the cherries well but keep about 1/4 cup of the liquid.
- Combine the cherry juice with enough water to make 1/2 cup of liquid total.
- Combine the cherries, blueberries, juice/water, and sugar in a bowl. Stir to combine, then add the salt and tapioca flour, stirring once more. Let the mixture stand for 20 minutes or so while you make the crust.
Crust
- Combine the flour and salt in a large bowl, then use a pastry cutter (or butter knives) to blend in the cold butter and shortening. Stop once the mixture forms crumbs about the size of small peas.
- Add the ice water one tablespoon at a time, mixing a little as you do.
- Shape the dough into a loose ball, but try not to handle it too much. Divide the ball into two portions.
- Liberally flour your working area, then roll out the two dough portions. It helps to dust the dough with flour while you work too. Work the dough until it fits into 9.5 inch pie pan. Roll the top out until it's large enough to easily cover the pie, then set it aside.
- Preheat over to 400°F (200°C).
- Fill the pie and top with the remaining crust. Cut vents or designs into the dough and trim the edges. If you want to, you can use an egg wash or sprinkle the dough with sugar at this point.
- Bake for 35-40 minutes, until the crust looks flaky and lightly browned, and the filling is bubbling in the center. Remove from the oven and cool for an hour (or more, if you can manage) then serve.
Notes
Nutrition
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Comments
This made me lol! I never knew all that math controversy ! This is a solid pie recipe whichever day you choose to celebrate, I like the short ingredient list and the cherries add delicious tartness to the blueberries, thanks!
I love the math lesson and the pie (pi?) looks amazing! Thanks for sharing the tip about the tapioca starch and the berries…I’ve seen too many a berry pie where the filling just turned into an oozing puddle and the pie has basically collapsed. Your pie looks perfect!
I made this pie yesterday. It was a big hit. I have a question about the tapioca flour: the filling never set. I suspect it’s because I didn’t bake it long enough?
Hi Lisa! Glad you liked the pie, but I’m sorry it didn’t quite set for you. It can be a bit difficult to predict exactly why one batch of filling does or doesn’t set up, but we can trouble-shoot a bit. Was it very runny, or more just… goopy? Was it served hot? The hotter it is, the more movement there is. Did the pie sit very long before serving? A longer resting time (e.g. overnight) helps the starch really gel the liquid of the filling thoroughly. Beyond that, it could be an issue of excess water in the fruit – some of the really big blueberries we’re seeing in stores now are very juicy, and will shed excess water when cooked. Baking time could also be an issue, but that’s even trickier to try explore from my end. I will say that you want to ensure that the filling is clearly hot enough to be bubbling in the oven. You could certainly try a longer baking time to see if it works better for you! I hope that gives you something to work with. Good luck!
Math was never my strong asset so you might have lost me somewhere in the dissertation 🙂 , however, as always, I enjoy reading your posts very much! It might not be a surprise to you that I love making pies and also eating them 😉 . This is a beautiful pie, Sean, well done. And thanks for the story of the 2pi!
I only made my first pie a couple of months ago and this flavour combination also sounds super legit! Now where to find sour cherries…
Pi was always 22/7 because we weren’t allowed to use calculators in school in Brunei. Thank you for the tip on using tapioca starch! I find it also thickens pastry cream very well – much better than corn starch. You are so right when it comes to not overworking the pies.
So interesting, thank you for the math lesson. Who knew! I love the idea of adding cherries to the blueberry pie too, definitely on my list. Plus that crust looks so perfectly flaky.
Sean, that pie looks amazing!!! It’s only about 8:00 AM and I want it for breakfast! I agree that 2 pie day is much better than one haha!
LOL. Have you had a bit of extra thinking time lately, Sean? TBH you had me at pie but that was a fun math lesson. I’m down for 2 pi day!
Also, I’ve never tried tapioca starch in fruit pies before but I’m going to give it a try next time. I didn’t like how my last cornstarch pie turned out.
Absolutely delicious! The proportions are terrific! I used frozen cherries and following your directions, the mix was perfect! I especially love your math explanation! Very visual!
Thank you Donna! I really appreciate you taking time to comment. It makes my day. Cheers, and happy pie making!