Fruit pie fillings are made from cooked or uncooked fresh or dried fruit. The fruit is assembled in an unbaked bottom crust, with or without a top crust made from a pastry dough or crumb mixture. Crust(s) and fillings bake together. 

Thickeners are used in juicy fruit pies so the liquid that come from fruit when baked gels. Without them you will have baked fruit in a puddle of juice. Better to have the juices remain around the fruit to make use of all  the fruit's flavor.

Typical thickeners for fruit pies include the obvious thickeners (flour, cornstarch, tapioca, arrowroot) and the not-so-obvious (ClearJel and potato starch) and several combinations (flour-cornstarch, tapioca-cornstarch). All are available in the supermarket, except for the combination thickener which you mix yourself with equal parts. The amount needed varies with the kind of fruit and the quantity of sugar used. However, I must warn you that everyone has their favorite thickener and will swear by it as the very best! But, certain thickeners are best used with certain types of fruit.

In general, fruit fillings thickened with arrowroot and tapioca, were clear and bright in appearance, and the flavor of the fruit came through clearly. Of the two, tapioca showed a bit more thickening power and was therefore my favorite. When making a lattice-top pie, the tapioca on top of the fruit baked into hard bits, so first grind it into a fine powder and use ClearJel® or Potato Starch can be used as a good thickener.

Instant ClearJel Powder = ClearJel starch: This modified cornstarch is my favorite thickener for all fruit pies. (You can use it 1 for 1 with cornstarch). It is the secret ingredient that many commercial bakers use in their fruit pie fillings. ClearJel can be purchased from the supermarket or online.  

ClearJel makes a fruit filling that's clear, not cloudy; has absolutely no floury or starchy taste; and is freezer-stable; you can freeze an unbaked pie and then bake it later without a chance of the filling becoming runny. ClearJel easy to use: you don't have to first precook the fruit with it as you do with cornstarch, for example. All you have to do is whisk together the ClearJel and the sugar in a small bowl, and then sprinkle this mixture over the fruit. Stir or toss well and you're done. 

Unlike ordinary cornstarch, ClearJel works well with acidic ingredients, tolerates high temperatures, and doesn't cause pie fillings to "weep" during storage. It is an especially good choice if you're canning homemade pie fillings, since it doesn't begin thickening until the liquid begins to cool. This allows the heat the be more evenly distributed within the jar during processing. 

All-purpose Flour is a favorite when making apple pies. The rule of thumb is about 1/3 cup of flour to each quart of fruit, but each recipe will vary. Most recipes call for sprinkling it directly on the cut fruit before putting it in the crust. I also sprinkle an extra tablespoon or two on the bottom of the crust before pouring in the filling which helps jell the fruit juices that seep to the bottom.  

Cornstarch is also a good thickener to use with fruit to make a filling with because it does not impart it's own flavor and yields the smoothest texture. It also does not thin when reheating a slice of pie. 

Cornstarch has twice the thickening power of flour, but like flour, it imparts a slightly starchy taste, especially true in juicy summer fruit pies; that's why I recommend cooking part of the fruit and cornstarch before baking a pie. But cornstarch doesn’t work when your filling is high in acidity (cherries) or if you plan to freeze the filling at any time because it loses its thickening powers over time if stored. 

Quick-cooking (Pearl) Tapioca is often used as a pie filling thickener because it can hold more fruit juices than other thickeners without becoming rigid. But, it is not recommended for a lattice top pie or other open fruit pie, as it remains hard when exposed the hot air of the oven. However, it can be used with a two-crust pie. 

Tapioca has no flavor of its own and it cooks quickly. If you use tapioca, let the filling mixture stand for at least 15 minutes before you spoon it into the crust to bake. This will allow for more efficient thickening. When you prepare a pie filling, mix the thickener first with sugar and any spices to prevent lumps and uneven thickening, then add the fruit. Granules are gritty if under cooked but are clear and soft when fully cooked. 

Instant Tapioca / Tapioca Starch: Tapioca starch is a fine powder made from tapioca. It creates a perfectly smooth filling and imparts a high gloss for a tasty-looking result. It’s the perfect product to use with high-acid fruits or a lattice-type pie. If you find only pearl tapioca, just place it in a spice grinder, blender, or food processor and grind away. Now you have "instant" tapioca.

Arrowroot: Because it begins to thicken long before the boiling point of fruit fillings in pies, arrowroot is not a desirable choice. It imparts no flavor of its own but provides a high gloss, and just 1 tablespoon will firm up each cup of liquid. It’s fine for high- and low-acid fruit mixtures, but it creates a gloppy mess if mixed with any dairy product. However, it breaks down under high heat. 

Potato Starch: After making many pies, I found that potato starch works surprisingly well, for it was especially soft and clear, yet firm. This gluten-free starch is also used to thicken soups and gravies. Its main advantage over other starch thickeners is that it's a permitted ingredient for Passover, unlike cornstarch and other grain-based foods. Liquids thickened with potato starch should never be boiled. Supermarkets often stock it among the Kosher products. 

ALL ABOUT FRUIT FILLINGS: Some examples of fruit fillings and tips. 

Apples are popular to use in pies. I like to use 2 - 3 different apple varieties in pie baking. Pick a contrast in flavor (one sweet, one tart) and texture (one that bakes firmer and one that softens during baking) for really nice results. For instance, use 1/2 Granny Smith and 1/2 of a sweeter variety like Jonagold, Gala or even McIntosh (these break down more and make for a different type of pie). 

Try the The Ultimate Apple Pie Recipe.  It freezes really nicely. Slice apples thinly for apple pie. Thick slices promote air space and create a gap between the fruit and the crust.

Blackberries Unlike blueberries, whose acidic skin reeked havoc with the starch, blackberries can be added with the starch and sugar from the start. Try my delicious Fresh Blackberry Pie.

Blueberry skins contain oxalic acid, which attacks nearly all starch thickeners if it were UNCOOKED. Peeling blueberries is an unheard-of task and would wreck their taste and shape. 

Adding the berries to cooked starch seemed the only way to guarantee a consistently thickened pie. See the Blueberry Lattice Pie Recipe. Without the sugar, the starch paste was almost thick enough to bounce. Even with the sugar, the paste was too thick. But I realized that as soon as the starch thickened, the blueberry's acidity couldn't affect it. 

Cherries, used in the Cherry Pie Recipe, need to be pitted if using fresh. Canned cherry filling is good, too. Drain the cherries and save the juice for the filling recipe when it calls for water. 

Peaches, must be peeled first before using as the filling as in the Classic Peach Pie Recipe. This is best done by blanching them: slicing a cross in the bottom and then plunge them into boiling water, two at a time. You will see the cut begin to peel, immediately remove them and then plunge into ice water to chill quickly. Then after a few minutes you can pick them up and with a paring knife, peel them. They are blanched, so you don't need to worry about them browning at this point.

Freezing Fruit Pies (For more about storage options, see Baked Goods & Candy Storage).

Filling boil-out

During baking, fruit pie filling will boil too much before the rest of the pie is done, causing it to spill over the edges or make the crust soggy from the juices. Apples, because of the natural pectin content, will not run during baking. That is, the filling and the apples are bound or thickened by the natural pectin in the fruit as well as by the starch in the filling base. Cherries, especially the sour pitted variety used for pie filling, are more acid in content and require a larger amount of sugar to sweeten the filling. The acidity and the high sugar content will cause the fruit filling to expand during baking. This is true for other fruits that are of a higher acid content and have higher percentages of sugar in them.

Thus, while apple pies may be filled to the very top of the pie pan, and fresh apple or frozen apples are filled above the level of the pan, cherry pies, blueberry pies, and similar fruit pies are filled to less than the rim of the pie pan. The fruit will expand during baking and the filling will tend to run out of the edges of the pie or break through the top crust of the pie and run out of the cracks in the pie crust.

As indicated previously, pies should be baked in a hot oven. Baking in a cooler oven will allow the internal temperature of the filling to reach the boiling point before the pie crusts are fully baked and have reached a desired crust color. A hot oven allows for the bottom and top crusts to be baked before the filling reaches an internal boiling point. Note is made of the fact that fillings which have less than 50% fruit as compared with liquid and sugar base will tend to spread or run much more than the richer (more fruit) fruit fillings. Fruit fillings that have not been thickened sufficiently, either through a lack of thickeners used or improper cooking of the filling, will be thin and tend to run and expand during baking.