What to look out for when making the classic sauce thickener.
A roux only has two ingredients—fat and flour—but can go awry in many ways. Making a good roux is one of the backbones of cajun and creole cooking, but it's also useful for all kinds of sauces, from gravy to bechamel. A roux basically takes the consistency of a liquid from thin and drippy to a classic sauce consistency that coats the back of a spoon. It's not the only way you can achieve that thicker saucey consistency. A cornstarch slurry, simple roasted flour, or an uncooked combination of flour and butter known in French cuisine as a beurre manie can be used the same way in certain cases. But for a gumbo, for example, you're going to want to make a roux. Once you've made it a couple times, you'll know what to look out for, but here are some things that can go wrong that you'll want to avoid.
You Didn't Measure the Flour and Fat
Yes, a roux is just flour and fat. Butter, oil, and drippings from meat all work as the fat. It depends on what flavor you want. But the ratio between those two things really matters for a roux, because it determines the thickening power of your end result. Too much flour and your sauce will be too thick. Too much fat and it won't be thick enough. The ratio will depend on what you want to use your roux for, but the classic roux for thickening sauces is a one-to-one ratio of flour and butter.
You're Turning Up the Heat Too Much
A roux is one of those stovetop dishes that benefits from patience. It's tempting to turn up the heat to try to nudge it along, but more often than not, that will just burn your flour and you'll have to start over again. There's no coming back from a burned roux—it'll add an acrid, unpleasant note to the dish that you just worked so hard to make.
You're Cooking It Too Much or Too Little
One of the tricky parts about roux is that it's better for different things at different stages,. A blond roux is one where the roux is just barely browned. It'll smell a little nutty, and have the consistency of wet sand. This roux is useful for bechamel or cheese sauces because it also thickens the most of any type—the more you cook a roux and the darker it gets, the less thickening power it gets. If you keep cooking the roux, it'll turn into a brown, peanut butter color, which is great for lighter gumbos and many sauces and stews. Keep pushing until the roux is a very dark brown, and that's the color you want for gumbo. That roux doesn't thicken as well as the other kinds, and it is also made best with oil or drippings, since butter can burn at high temperatures. It's a process that takes patience to get to the right stage without turning up the heat and scorching the whole thing. If you need a visual guide for what the roux will look like, try this video of Emeril Lagasse making roux.
You're Adding Hot Liquid to a Hot Roux
If you add a cold roux to a cold liquid, it won't dissolve or thicken. Likewise, adding a hot roux to a hot liquid will result in a lumpy sauce. You want to either cool the roux down and then add it to simmering liquid, or add cold liquid to the hot roux you just made. And yes—if you're worried about time and the roux process intimidates you and you want to get it over with, you can make roux ahead of time and keep it in the refrigerator or freeze.
You're Adding Your Liquid All at Once
Once you have your roux where you want it and you're adding your stock or milk or whatever else, it's important to make sure you combine them gradually. If you're thickening a sauce with rroux, add a little roux at a time and whisk until you get the consistency you're looking for. If you add it all at once, you can quickly get a sauce that's way too thick. If things get way too thick you can always add more liquid, but it'll also lessen the flavor in the dish that you're working toward.