This article was in the Dallas Morning News today. I would post a link to it but they don't keep the stories online for very long. I had no idea of some of these differences. On South Beach, I try to keep the amount of cornstarch small but I do still use it. Whole Wheat flour also works as a thickener.
Both arrowroot powder and cornstarch are starch thickeners, powders used to give body to pie fillings, puddings, gravies and sauces without adding their own flavor. But they each have their pros and cons.
Arrowroot is a good thickener for an acidic liquid. (Cornstarch loses potency in acids.) Arrowroot is also a good choice if your sauce is mildly flavored. It holds up well to freezing, which cornstarch does not. When you're looking for a high-gloss finish, use arrowroot.
On the downside, it's more expensive than cornstarch, and it's not a good choice for sauces or gravies with dairy products: They turn slimy.
To thicken with arrowroot powder: Mix it with an equal amount of cold water and then whisk it into your hot liquid for about 30 seconds.
To thicken with cornstarch: Mix it with an equal amount of cold water before whisking the slurry into hot liquid. Simmer the liquid, whisking constantly, for about 1 minute before it thickens.
Laura H. Ehret