Not all starches are created equally.
Some sources, including the Argo cornstarch website, indicate that cornstarch and potato starch may be substituted for each other on a 1:1 ratio, while others say that potato starch has more thickening power (meaning it makes a thicker sauce than an equal quantity of cornstarch would). To see, we made a simple pan sauce using a recipe that called for thickening with cornstarch. We made the sauce a second time, substituting potato starch for the cornstarch, and tasted the two side by side. The potato starch made a thicker sauce. The reason, our science editor explained, is that potato starch granules are four to five times larger than cornstarch granules. When potato starch granules swell in hot liquids, they take up more space, restricting the liquid’s flow more than smaller cornstarch granules, which results in a more viscous sauce. A few tasters found the potato starch sauce to be slightly “gooey-slimy” or “stringy,” while others perceived it as more “velvety” than the cornstarch sauce. In the course of making the sauce, we also noticed that while cornstarch needed to be simmered in the sauce for a couple of minutes to achieve maximum thickness, the potato starch thickened more quickly, even before the sauce came to a full simmer. We repeated this test with a blueberry Danish filling and got similar results.
**THE BOTTOM LINE: ** Potato starch thickens at a lower temperature and to a greater extent than cornstarch. For every tablespoon of cornstarch, use 2 teaspoons of potato starch.