Quick Answer
Both cornstarch and flour can be used to thicken rhubarb pie fillings. Cornstarch creates a glossy, translucent filling while flour makes a more opaque, matte filling. Cornstarch thickens at lower temperatures than flour, so it’s a better choice for no-bake and frozen pies. Flour lends a richer flavor. Ultimately, it comes down to personal preference on the texture and appearance you want for the filling.
What is Rhubarb Pie?
Rhubarb pie is a classic springtime dessert made from rhubarb stalks, sugar, and a thickener baked in a pie crust. Rhubarb has a tart, sour flavor that when combined with sugar makes a sweet-tart filling. The stalks have high water content, so a thickener is needed to create a thicker consistency for the filling rather than a runny sauce. Popular thickeners used are flour, cornstarch, tapioca starch, or arrowroot powder. These thickeners gelatinize when heated to create a smooth, thick filling.
Purpose of Thickener
Thickeners serve several important purposes in rhubarb pie filling:
- Absorb water released from rhubarb during cooking
- Bind ingredients together into a viscous, spreadable consistency
- Allow the filling to hold its shape when sliced
- Prevent water separation (syneresis) as the pie cools
- Give the filling a glossy or matte appearance
- Contribute starch flavors and texture
Both flour and cornstarch can perform these functions. The differences lie in how they interact with the rhubarb, their flavor, and the final texture they produce.
Cornstarch vs. Flour Differences
Thickening Power
Cornstarch is a pure starch, containing only the thickening agent amylopectin. It has twice the thickening power by weight compared to flour. Only 1-2 Tbsp cornstarch is needed per pie compared to 1/4 cup or more of flour.
Gelatinization Temperature
Cornstarch thickens at lower temperatures, around 160-180°F, whereas flour needs to be cooked to at least 190°F to fully thicken. This makes cornstarch better for no-bake and frozen pies that won’t reach as high of temperatures.
Transparency
Cornstarch makes a glossy, translucent filling while flour gives a more opaque, cloudy appearance. This is because cornstarch gel is clear and allows light to pass through.
Texture
Cornstarch gives a smooth, velvety texture. Flour has more graininess and a less slippery mouthfeel. The protein in flour gives it more structure.
Flavor
Cornstarch has a neutral taste and won’t influence the flavor of the filling. Flour has a distinctive wheaty, raw dough flavor. Some people enjoy this flavor in rhubarb pie while others consider it distracting.
| Properties | Cornstarch | Flour |
|---|---|---|
| Thickening power | High | Moderate |
| Gelatinization temp | 160-180°F | At least 190°F |
| Transparency | Translucent gloss | Opaque, cloudy |
| Texture | Smooth, velvety | Grainy |
| Flavor | Neutral | Adds wheaty flavor |
Best Uses
Cornstarch
Cornstarch is the better choice when:
– You want a crystal clear filling
– Making a no-bake or frozen pie
– Desire a smooth, slippery mouthfeel
– Want the purest rhubarb flavor to shine through
Flour
Flour works best when:
– A heartier, less delicate texture is desired
– The wheaty, doughy flavor complements the pie
– You don’t mind a matte, opaque appearance
– Making a baked pie that will reach over 190°F
Substitution Ratios
If substituting cornstarch for flour, use:
– 1 Tbsp cornstarch per 1/4 cup flour
When substituting flour for cornstarch, use:
– 3-4 Tbsp flour per 1-2 Tbsp cornstarch
Since cornstarch is more powerful, less is needed when swapping it for flour. It’s difficult to achieve the same thickening otherwise. When using flour instead of cornstarch, more is needed to compensate for the lower thickening capability.
Tips for Usage
– When using cornstarch, it’s important to fully dissolve it in cold water first to prevent clumping before adding to the hot fruit mixture.
– With flour, whisk it thoroughly into the rhubarb to prevent lumps from forming when cooking.
– For the best results, pair the thickener with an acidic ingredient like lemon juice, which helps create the proper gel structure.
– Overcooking starch-thickened fillings can make them thin out. Remove from heat as soon as the desired thickness is reached.
– Letting the pie filling cool completely before adding to the pie shell will thicken it further as it sets.
Conclusion
While both cornstarch and flour can thicken rhubarb pie filling, cornstarch creates a glossy gel with a smooth texture and neutral taste. Flour lends a matte finish and wheaty flavor with a heartier mouthfeel. The best choice comes down to the specific qualities you want in your pie filling and whether you prefer a delicate or robust texture. Taste testing different ratios is the best way to find your ideal rhubarb pie consistency. Follow proper substitution ratios to achieve the right results with whichever thickener you choose.