It’s summer. Berries and stone fruits abound, and so the season of pies continues. And we continue to think deeply about the science of pie. There has been intense interest in pies these past few months: first at the Science of Pie event; next at the World Science Festival’s Scientific Kitchen workshop at Pie Corps in New York; and most recently the New York Times Pie Issue. But we believe you can never know too much about pie. Here are 10 more things we think you should know…
1. A bit of high school chemistry goes a long way when baking pies.
The ideal gas law (PV=nRT) tells us that the volume of an air pocket gets bigger with increasing temperature. In the oven, molecules get more energy and start moving faster and faster, causing air pockets to get bigger and bigger; this can result in an inflated pie that collapses once you cut into it. At the same time, apples lose water, most of which gets converted to steam. Consider that a water molecule takes up about 1700 times more volume in the gas phase than in the liquid phase: if your crust were completely impermeable to water and all the steam got trapped inside, your pie would become much larger than your oven! Luckily much of that steam can escape through the crust and through steam vents. (This is also a good reason to be sure to avoid air pockets when you lay your crust into your pie tin!)
2. There is an art to cutting your fruit for a pie filling.
The way you cut your fruit is important. Smaller pieces of fruit will cook more quickly, but they also tend to lose more liquid since they have a higher surface-area-to-volume ratio. The geometry of your fruit pieces is also important for packing the filling into your pie. After placing your fruit slices into the center of the pie, pat them down to make sure they all like flat. This will create a pie with a lovely cross-section of layered fruits and, more importantly, will help to avoid air pockets that can expand in the oven.
3. Sometimes the best pie is a day-old pie.
Temperature is important for pie texture. Eating your pie the day after you bake it allows plenty of time for the pie to cool down and the filling to “set”. Because molecules flow more quickly past each other at higher temperatures, hot pie filling straight from the oven will be more runny; as the pie filling cools, starchy molecules like cornstarch and flour spend more time interacting with each other. As the pie cools, the pectin molecules of your fruit also spend more time interacting with each other. This results in a more solid, gel-like filling that will take longer to seep out of the pie when it is cut and served on a plate.
4. Think of butter as a gas.
Butter is really just a bunch of teeny tiny water droplets dispersed in a matrix of fat. In the oven, these water droplets convert from liquid to gas. This means that the chunks of butter you can see in your dough are really just big pockets of air waiting to happen. More air = flakier crust! While butters with the highest butterfat content are generally synonymous with the highest quality butter, when it comes to baking pie a slightly lower fat content, and higher water content, may be a good thing.
5. Wash with egg for a darker, more delicious pie crust.
All those lovely color and flavor molecules in a nicely browned pie crust are the result of the Maillard reaction, a chemical reaction that occurs between amino acids, which comprise proteins, and sugar molecules like lactose or glucose. Brushing an egg (protein) on your pie crust before baking is a great way to add extra color and flavor. For extra browning, mix some heavy cream into your egg wash (more protein plus lots of lactose).
6. Turn up the heat!
Maillard reactions happen faster at higher temperatures. Keep your oven hot (375F or so) to brown your pie that extra bit more. Another strategy is to start off at 400F, then turn down the temperature to 350F.
7. Bake your pie in parts.
A major challenge in baking pie comes from its complexity: you’ve got a crust that should be brown and crisp together with a filling that largely contains water. When contending with fruit pie fillings, one strategy is to prebake the bottom crust to help prevent it from becoming soggy. In this process of “blind baking,” don’t forget to prick holes in the bottom of your crust so the water vapor can escape. Filling your pie crust with pie weights or dried beans during this process can also help prevent layers of your wanton bottom crust from puffing up. Pie master Bill Yosses suggests taking this sequential baking process an extra step further: after the bottom crust has baked, it can be stitched into the sides of a crust using extra dough to “glue” the bottom to the sides. In the spirit of experimentation, this could be an interesting new method to try.
8. Create a pie crust with your “perfect” texture.
Typical attributes of a “perfect” pie crust include: flaky, tender, brown, and a little crispy. While the optimal texture of a pie crust is a deeply subjective and personal matter, here is a rough guide to how you can tune your pie crust texture simply by considering how you work your fat into your flour. For taste, color, and texture, we prefer butter, but shortening or lard can also be used.
- You want your fat to be solid when working it into the flour. Remember those little chunks of fat will become pockets of air in your crust! In a liquid form, it would coat the flour too evenly, resulting in a less flaky crust.
- Because butter melts around 30–32 degrees Celsius (86–90F), it can be tricky to make sure it remains solid while you work it with your hands (about 35 degrees Celsius or 95F). Prior to making your dough, cut your butter into small 1 x 1 cm cubes and place in the freezer for about 10-15 minutes.
- For a crust that has more form and larger flaky holes, work your very cold butter into the flour until you have a distribution of butter pieces with various sizes: some should appear the size of peas, others the size of almonds. When you work your butter in to achieve these sizes of chunks, much of the butter will get worked in so the rest of the dough will appear as coarse wet sand.
- For a tender and flaky crust you need a decent coating of fat around your flour. To achieve this, try the two-step method: (i) Divide your butter in half: cut one half into small cubes, and keep the remaining half in stick form. Place both halves in the freezer to ensure they are very cold. (ii) Work the stick of very cold butter into your flour by grating it in with a coarse grater. Work in thoroughly with your hands until the mixture has the texture of a coarse sand. (iii) Add the remaining half of your butter in cubes and work in with your hands until the largest pieces are about the size of peas. The theory here is that completely coating the flour in oil helps create a more “tender” crust.
- If you want to avoid getting your hands messy, or want to minimize heating of your butter, use a pastry cutter, or two knives held side by side, to work the butter into your flour.
9. Different types of flour create different types of pie crust.
What flour is the best flour for pie crust? This is a contentious question that has a variety of answers depending on personal preference, but the type of flour you use can have a major effect on the final texture of your crust. The protein content of flour, based on the type of wheat the flour was made from, will affect the extent of gluten formation in your dough. While springy networks of gluten proteins are great for chewy breads (bread flour has particularly high protein content), they can make pie crust dense and tough. Flours with lower protein content, such as pastry flour or cake flour, will create less extensive gluten networks and can produce a more tender crust. However, the pie crust ultimately needs to be formed into a dough, which can make it a challenge to work with a fragile dough that can result when using a low-protein content flour.
10. Almond extract tastes great in a fruit pie.
What more can we say? Nuts and fruit taste great together! A bit of almond extract is a delicious complement to apples and apricots alike.