Let's get saucy.
All of your favorite saucy dishes—gravies and stews you want to sop up with crusty bread, stir-fries that envelop the rice or noodles on your plate—probably rely on one of two common cooking techniques: Whisking a scoop of cornstarch into a cooking liquid to make a slurry, or cooking flour in some kind of fat to make a roux. Both of these thickening techniques are used give certain dishes that gorgeous, glossy finish that makes them so irresistible—but they’re not quite as interchangeable as you might think. While similar in purpose, the techniques work best within different recipe contexts and are most commonly used in different cuisines. Want the secret to perfecting a stellar sauce? Here’s all you need to know about when to use a flour roux vs. a cornstarch slurry.
Let’s start with the basics: A roux is equal parts flour and fat, and is usually the first step of preparing a dish. In classic French cooking, there are three types of roux: white, blond, and brown. According to The New Food Lover’s Companion, white and blond roux are traditionally made with only butter and flour. A white roux is cooked just long enough for the mixture to lose its “raw” flour taste—the blond roux is cooked a little longer, until it turns golden.
A brown roux is (you guessed it) dark brown—almost mahogany in color—and can be made with pretty much any fat you have on hand: Think oil, beef fat, drippings, and lard. It’s cooked for much longer and works better with hearty, meatier flavors; some brown roux take close to an hour to slowly deepen in color, and they have a more pronounced, pleasantly nutty flavor. If you’re a fan of flavor-packed Creole and Cajun dishes, such as gumbo, a well-prepared roux is the foundation.
Foods thickened by the addition of a roux look cloudier—almost opaque—like a traditional Thanksgiving gravy. Deploying this thickening method works best in slow-simmered foods with more fat, so that you avoid that starchy, unpleasant taste of uncooked flour. If you have leftovers, roux-thickened sauces can stand up to the refrigerator and freezer better than those thickened with cornstarch (which can turn gummy). Make a roux when you’re going for rich, creamy, luscious, sauces.
While a roux is all about patient simmering, cornstarch thickens sauces in a snap. It’s added at the end of the cooking process, and unlike the slightly nutty, luscious flavor and texture of a roux, a cornstarch slurry is neutral flavored, making it perfect for vibrant Asian-inspired dishes where bright spices and seasonings are the focus. It also works better for more acidic sauces, and unlike the opaque creaminess of a roux, cornstarch slurries stay translucent, giving that chicken and veggie dish a gorgeous sheen.
To make a slurry, simply combine cornstarch and cold water in a separate bowl, whisking to get rid of any lumps, and slowly pour the paste into your dish as it cooks. Don’t skip this step—adding straight cornstarch directly into a hot pan of food is a recipe for a lumpy mess. It only needs a few minutes to simmer to start thickening and voila, you’ve got that crave-worthy sauce ready to go. Make a cornstarch slurry when you’re making a bright sauce or soup containing more acid, where the flavors of spice, fresh herbs, and other flavoring agents are intended to be the true stars—or, if you’re in a hurry and don’t have the time to slow-simmer a roux.