There are several alternatives, but the most common I know of are tapioca flour and arrowroot powder/flour.
That said, most cornstarch substitutes aren't appropriate for pies, because they break down under high heat.
The only substitutes for corn starch that I'd use in a pie are potato starch and tapioca.
Tapioca powder can be hard to find in some areas though; I usually get it at Asian grocery stores.
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ClearJel is a product you may want to look at.
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Did you whisk the cornstarch in water separately before combining it?
I think that reduces the starchy taste a little bit.
Otherwise, maybe try a roux?
I've never heard of using it in baking, but it will thicken things without adding a starchy taste.
See http://en.wikipedia.org/wiki/Roux
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We have a person with Celiac disease in our family, so generally we make the substitution the other way around, but generally you can substitute 2-3 parts flour for one part cornstarch to get the same thickening properties.
Unfortunately flour generally imparts more flavour than cornstarch.
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I like to use Xanthan gum.
It works really well and a little bit goes a long way.
It doesn't add any additional flavoring to the recipe either.
I highly recommend it.
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Any starch will work, so cornstarch, wheat flour, tapioca flour, potato starch, whatever.
The results will be slightly different, as different starches require different ratios with liquid.
My guess is that you can safely reduce the cornstarch in the pie, as the amount required to actually taste it (on top of your fruit, sugar, etc!) is probably quite a bit.
Most pie recipes I've seen have used wheat flour, though.
What kind of pie?
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Cornstarch will taste uncooked if it doesn't reach near boiling during cooking.
In the recipe maybe it would be better to cook the cornstarch first as if making custard, then stir it gently into the filling.
But if they are fruit pies, how about using more fruit and no thickener?
That's how I make apple pies, and they're fine.
Or arrowroot would thicken at a lower temperature than cornstarch so that might work.
It gives a clearer, more transparent glaze too, which could be prettier.
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Absorptive thickeners - nut meals, breadcrumbs - work well in some pie fillings, though the texture and color is different.
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One thing I do with apple pies (that I would suppose you could do with at least the pear half of this pie) is to mix some dried fruit (dried apples for an apple pie) in with the fresh fruit, to sponge up some of the juice - it also means that the filling collapses less than when it's all fresh.
I already upvoted @Aaronut's tapioca suggestion
- tapioca was the only thing we used for this purpose in my family.
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If you cook the cornstarch in the liquid when you add it that will improve things.
Another alternative is to use Agar Agar, which is a seaweed derived gel.
It's expensive, but clear and quite amazing.
Also , Heston Blumenthal uses it Happy Pie!
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I made a some pies a couple of days ago. The recipe called for cornstarch and it thicken the filling after it came out of the oven. The only thing is I could taste a bit of the cornstarch when I was eating the pies. I would just use less of it but I am afraid of making my pies to runny without the cornstarch.   Is there an alternative to cornstarch to thicken the pie filling or should I just use less and hope I still use enough?
Is there an alternative to cornstarch when used to thicken a pie filling
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<substitutions><baking><pie><cornstarch>
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Different starches like tapioca flour, arrowroot, wheat flour or potato starch can also be used as substitutes, although they may need different quantities and/or methods. Xanthan gum or agar agar can also thicken the mixture, or you could try absorbing some of the liquid with breadcrumbs, nut meals or dried fruit. To improve the taste of cornstarch, make sure it is mixed with water before adding and that the filling reaches boiling temperature. Make sure the cornflour is correctly dissolved and cooked. You can replace it with any starchy product.
Any starch will work as a replacement: flour, xanthan gum agar agar, tapioca, potato starch, arrowroot. You can also use something that will absorb the filling like breadcrumbs. Make sure to whisk the cornflour in water separately first. It also needs to boil during cooking. Try mixing dried fruit in with the fresh fruit.