Apple Pie

In the last post, we discussed how sugar preserves the structure of cooked fruit. This comes in handy for fruit pie fillings, which often become a mushy and wet (but nevertheless delicious) mess. In this recipe, apples are tossed with sugar and drained. The drained liquid is cooked into a thick syrup that’s added back to the apples and baked. The apples maintain some crunch, not much water leaks into the pie, and the syrup adds an extra punch of flavor. Let’s take a look at the recipe and then discuss the science! You can also watch me make this pie and explain the science on YouTube.

Apple Pie

Makes 1 9-inch pie

This apple pie filling is thick, flavorful, and slightly crunchy. It’s great in pie, but I also like it mixed into plain yogurt or oatmeal for a quick snack.

Ingredients

7 apples (about 1 kg), peeled, cored, and sliced
2 Tbsp (30 mL) lemon juice or distilled white vinegar
1/2 c (100 g) granulated sugar
1/2 c (110 g) brown sugar
2 prepared pie crusts
2 Tbsp (16 g) cornstarch
1/4 c (60 mL) cold water
1/2 tsp table salt
1 Tbsp (14 g) butter
1 tsp apple pie spice
2 Tbsp (30 mL) milk

Method

Combine apple slices and lemon juice in a large bowl. Toss to coat.
Mix the granulated and brown sugars and add a heaping 3/4 c (168 g) to the apples. Reserve the remaining sugar. Toss the apples to coat. Transfer the apple mixture into a colander set on top of a bowl and drain for 3 hours.
Grease the pie pan, roll out both crusts, and line the pie pan with one crust. Refrigerate the crusts.
Chop up 3 drained apple slices, squash the chunks with the flat side of a knife, and add them to a saucepan. Measure the drained liquid and, if necessary, add water to make 1 c (237 mL). Add the liquid to the saucepan and bring to a boil. In the meantime, whisk the cornstarch into the water.
When the syrup boils, slowly add the cornstarch mixture while stirring constantly. Bring the syrup back to a boil and add the remaining 1/4 c (42 g) sugar and the salt. Boil for 2 minutes, stirring constantly.
Remove syrup from heat and stir in butter and apple pie spice. Toss apples in the syrup. Cool for 30 minutes at room temperature. Preheat oven to 375°F. (Note: if you would just like to prepare the filling, pour the hot apples back into the saucepan and simmer over low heat with the lid on until apples have softened, about 20 minutes.)
Pour apples into the prepared crust. Add the top crust, pierce to vent, brush with milk, and bake at 375°F for 50 minutes. If the crust starts to brown too much, cover it with strips of aluminum foil.

Sugar science

Apple pie was the first type of pie my mom ever made, from a recipe in the back of a picture book about apple trees. I don’t remember the book, but I know exactly what her photocopied recipe looks like: notes scribbled in the margins and crinkles from the many times she pulled it out of the recipe binder. The pie was a treat every time. My favorite part was the inside of the crust, which was soaked with all the juice the apples released as they baked. Unfortunately, that juice also ran out of the pie when it was sliced. Not that we especially cared.

This pie is a little different. The filling is thick, the apples are crunchy, and the pie is a burst of fall flavor. To better understand the following discussion, I suggest reviewing the recent posts about sugar, especially the section “Sugar preserves the structure of fruits” here.

Sugar drains the apples.

In the second step of the recipe, we coat the apples in sugar and drain them for three hours. When the sugar touches the apples, it dissolves in the water at the surface, and the concentration of water outside the apple suddenly becomes a lot lower than the concentration inside. As a result, water diffuses out of the apples and drains into the bowl. The effect of the sugar is immediate—when I pour the apples into the colander, they’re already coated with syrup, and I hear liquid dripping into the bowl before I even clean up the kitchen. I usually get one full cup of liquid by the end of the three hours.

In a concentrated sugar solution, water diffuses out of the fruit and sugar diffuses into the fruit.

The loss of water improves the pie in two ways. First, it prevents the fruit from releasing more water as the pie bakes. In recipes where the apples are added directly to the pie, their cells break and release water, which boils out of the pie. In this recipe, a lot of the water has already been drained from the apples, so there’s not much left to release. To make the most of the flavor in the drained liquid, we boil it down, thicken it with some cornstarch, and add it back into the pie. The syrup compounds the intense, concentrated flavor of the drained apples to ensure deliciousness in every bite. We’ll talk more about the chemistry of starches in the future, but it’s important to boil the syrup so that the cornstarch doesn’t continue to thicken in the oven.

The draining step also prevents a space from forming between the filling and the top crust. Typically, apple slices sag as they soften in the oven, leaving a gap under the crust. In this recipe, however, the apples are already limp when they’re drained. The stiffness of the apple is determined by the rigidity of the cell wall scaffolding that provides its structure. When the apple is full of water, the scaffolding is solid, like a full water balloon. When we drain the apples, however, the cells lose water, the scaffolding becomes rubbery and flexible the way a water balloon becomes squishy as it leaks, and the apple becomes limp. We see the same thing in thirsty plants: their water balloon cells don’t have enough water to keep the structure stiff, and the plant wilts.

Without the draining step (low sugar concentration), the cell wall structure breaks down, the fruit becomes mushy, and water is released. With sugar (high sugar concentration), the pectin glue holds the deflated cell walls together even as the fruit cooks.

Sugar preserves structure and texture.

Although the drained apples are bendy, they’re crunchy because pectin, the glue that holds the scaffolding together, continues to hold the structure intact. (This always catches me by surprise when I sneak a bite!) But usually, in the oven, the pectin disintegrates, the cells separate from each other, and the apple becomes mushy. In this recipe, however, the draining step reinforces the pectin and keeps the apples crunchy even as they bake. As the apples sit and water diffuses out, the sugar we added diffuses in. Inside the apple, the sugar acts like a waterproof finish for the cell wall scaffolding. It holds onto water molecules so they can’t dissolve the pectin that glues the cell walls together. Thus, even when the apples are heated, the scaffolding remains, and the apples stay crunchy.

The acidity of the lemon juice and brown sugar also keep the pectin intact. As we discussed in the context of proteins, acids mediate the electrical interactions between molecules. In this case, the acids neutralize some of the negative charges on the pectin, allowing it to be packed more tightly together (this is similar to the effect of salt on gluten). The more tightly the pectin is packed, the harder it is for water to squeeze in and dissolve it.

Conclusions

The draining step for this pie filling removes water and adds sugar, leading to improvements in texture, flavor, and structure. It requires some extra time and planning, but to me, the results are worth it. The aroma of the syrup as it cooks and the intense apple flavor that permeates every bite of the crunchy filling is the epitome of fall!

References

BeMiller, J. N. An Introduction to Pectins: Structures and Properties. American Chemical Society, 1986.

Corriher, S. O. Bakewise; Scribner: New York, 2008.

Corriher, S. Getting the Texture You Want When Cooking with Fresh Fruit. Fine Cooking, 2001, 46.

Figoni, P. How Baking Works, 3rd ed.; John Wiley & Sons, Inc.: Hoboken, 2011.

Freeman, S.; Quillin, K.; Allison, L. Biological Science, 5th ed.; Pearson: New York, 2014.

Apple Pie