I use tapioca for no other reason than it was the ingredient in my first recipe i made years ago. Very neutral in taste and glossy clear. Seems whenever this comes up the response is 50/50.

Some have a family recipe as a reason to use one or the other...

Rhubarb is also good in savory sauces like bbq, and salsa...and syrups for waffles and french toast...heated with honey and run smooth in a blender....

I like flour as a thickener in most pies, because I like the taste. I make a rhubarb pie with egg yolks and flour...sugar too of course, and use the whites to make a meringue topping.

I believe the first rule for thickeners is to know what they do and if that's what you want it to do. You can find lists of thickeners and the advantages and disadvantages of all of them, and that's the information you need to arm yourself with before making a choice. Things like:

Advantages of arrowroot -

1. Works well with cold acid fruits

2. Does not need to boil to thicken.

3. Does not need to be cooked to remove the "raw" taste.

Disadvantages of arrowroot -

1. When used in a sauce that is served hot, it does not keep long and does not reheat well.

Just because a recipes uses "X" doesn't mean it's the "best" choice. Much of cooking is based on old habits, available ingredients, not better science and information. There are a number that will work in this recipe, so it mostly comes down to personal preference. I'm a big fan of Ultra Gel and arrowroot, but here's some good "science" from King Arthur about thickening fruit pies.

http://www.kingarthurflour.com/blog/2013/06/21/fruit-pie-filling-no-runs-no-drips-no-errors/

-Grainlady

Interesting about King Arthur's PFE...I always use a generous pinch of salt and squeeze a wedge of lemon over an apple pie...unless the apples are really tart ( as in early windfalls from your own tree)...and for a blue berry pie, which I find needs a big boost unless made with wild berries, I grate some lemon rind too. A cherry pie always gets a little vanilla and a very little bit of almond, peach pies get a little cinnamon and lemon if they are very ripe.
I find that the amount of sugar and thickener is always a bit of a crap shoot with fresh fruit. I have gotten better at assessing the juiciness and sweetness of the fruit before I mix up the filling....but it's not always perfect. I would much rather have a pie too juicy that one that cuts like it has had jello added. And it's been a lot of years since I have made a "magic pie"....you know the kind...where you cut a piece and magically you still have a full pie left!

My WI aunt & cousins make theirs w/ an egg. So sorry, I do not have the recipe, but imagine it can't be hard to find....?

I've traditionally used flour for fruit pies, and still like it best, but in trying to go gluten free for Amanda, I've defaulted to either cornstarch or tapioca, whichever I have on hand. I can then keep a bit of the filling separate and just bake it without crust or make a crumb crust out of gluten free cookies or graham crackers for her dessert.

I also cannot add lemon because Mother is allergic to lemon, and so my blueberry pie usually has lime, apple pie has no acidic additive. Like LIndaC, a cherry pie usually gets a little almond extract and peach pies get cinnamon.

I'm still in the "experimental" stage where I'm figuring out which thickener works best in each instance, so I won't be much help, I'm afraid. I'll be following this, though, as I have about 20 pounds of rhubarb in the freezer from last year and my plants are getting bigger each year...

Annie

Hmmm, I think I am going to try cornstarch. I did a little more digging and a few sources mentioned cooking a cup of the fruit with 3/4 cup sugar and cornstarch on the stovetop first till thickened, then add that mixture to the rest of the fruit before filling the pie shell.

If all fails, then there is Strawberry Rhubarb dump cake, coffee cake, or some sort of torte.

Here's the recipe I use...Annie I'll bet you could sub 1 T cornstarch for the flour in this recipe.

Rhubarb Cream Pie

1 unbaked 9 inch pie crust...in a 9 inch pie pan

3 cups of rhubarb cut in 1/2 inch pieces

1 1/2 c. sugar

4 T. flour

3 eggs

Add'l 6 T. sugar

1/2 tsp vanilla, pinch of salt, 1/4 tsp. cream of tartar

Mix flour and sugar and pour over rhubarb in a large bowl,
mix well , separate the eggs and add the yolks to the rhubarb and mix well.
Place in pie shell and bake at 375 for 1 hour until center is bubbly and crust
is brown. Cool slightly.

Beat whites, salt, and cream of tartar until soft peaks
form, add the 6 T. of sugar in batches and continue beating until stiff peaks
form when you raise the beater. place merangue on pie and bake 5 or 6 minutes
at 425 until lightly browned.
This is Grandma Ann Campbell’s recipe….Mel Campbell’s mother.

I don't see a fail at all. Just heat the rhubarb alone, chopped, low and slow for 20 min or so before adding the starch. You could even take a 1/2 cup warm rhubarb out of the pot and stir in your cornstarch slurry to that first, then stir into the pot batch. Other fruits i just add off the heat depending what, ...i just make little flat rustic tarts with sometimes mixed fruits....a handful of frozen blueberries on the way into the oven...

Thanks, LindaC. My Aunt Ronni used to make a rhubarb custard, but mine is generally straight rhubarb. Now I'm thinking I need to make the rhubarb custard, Mom would probably love that!

Annie

The egg seems to take the "bite" out of the rhubarb....mellows it a bit. I find people who don't like rhubarb pie like it with the egg. me...I love rhubarb....sourer the better! I grew up with a bowl of stewed rhubarb on the table every evening during the season.
I have lots of other rhubarb recipe, most came from this forum when it was a GW forum...a couple of cakes, a pudding cake a chutney....can't remember what else...but I'll post if anyone is short of rhubarb recipes.

SO's favorite pie is strawberry rhubarb, and I've had my best luck with tapioca in that. Otherwise, it's always way too runny. I need to try and remember to order some of that King Arthur thickener, but in the meantime, I use tapioca. I'll have to try the egg version next time.

Pinkmountain, do you use quick-cooking, minute tapioca? Do you stir it in and let it absorb into the the fruit filling before filling the pie? I've read that when some people use tapioca, it doesn't break down and they end up with little tapioca clumps in the filling. Some people said that mixing the tapioca with the filling and letting it sit and absorb for about 10-15 minutes prevents that.

I use the quick cooking which I run through the food processor to try and get it ground up a bit. It's hard to grind to a powder completely. And yes, there is some tiny amount of beading in the pie, but I haven't noticed it "clumping." It seems minor to me, but I guess you can get tapioca powder but I haven't seen any where I shop. I'm kind of lazy, so when I make pies, I am more of the "throw it together" school so don't finesse it much. That's why I'm trying to figure out how to make Linda's pie without the meringue topping.

Cornstarch is not a good thickener for rhubarb...rhubarb is too acedic for cornstarch to work well.

FWIW, I use tapioca in juicy pies too - I crush tapioca pearls (that's what's on hand) to make 'minute' size & never notice any lumps in the results. I sprinkle 2 Tbs.over the bottom of the crust before adding filling

Yum deluxe!