Throughout the year, we have established many fundamentals of cooking. We learned what mirepoix is and used it to flavor stocks and slow roasts. Now, we will take this one step further. How do we turn our stock or pan drippings into a sauce or gravy? How to we prevent our casserole or pot pie from being to runny? How can we recreate the thick stickiness of many Asian dishes at home? The answer is starch, mainly from flour and cornstarch.
Before we look at these two ingredients, both of which contain starch, or long chains of glucose, we have to understand how they thicken things. Starch is very good at absorbing water. As this happens, they release some of their starch molecules, creating a thick liquid. For this to work effectively, the starch molecules, which are clustered, must be separated using fat or water. This creates what we call a roux or slurry. We then add this to a liquid and bring it to a near boil, the temperature required for the starch to effectively release.
I mentioned a roux and a slurry. These are two of the most basic thickeners but they serve different purposes and required different amounts of time to do their magic. Thus, they also have different applications.
A roux is made of flour and fat, traditionally butter, but oil, pan drippings, or even rendering from cooked sausage or bacon work great. To make a roux, we whisk flour into the hot fat to create a paste. Traditionally, this is done at a 1:1 ratio (by weight or volume), however, I prefer a 3:2 flour to fat ratio as insurance. Because raw flour does not taste very good, we need to cook our roux, stirring constantly, for a minute or two until it smells toasty and starts to leave streaks. At this stage, we call it a pale or blond roux and it has its maximum thickening potential. You can cook roux even darker (slowly or else it will turn bitter). A brown roux will have a nice nutty flavor but will only have about half the thickening power of a blond one. Traditionally, gumbo requires a brick red roux which can take an hour or more to achieve. While tasty, it will have a fraction of the thickening power of a blond roux so adjust accordingly.
Now it is time to thicken our liquid, whether it be stock for brown sauces or milk for bechamel. Add the roux to the liquid or the liquid to the roux. The key here is to add it slowly while stirring to prevent clumping. We also, ideally, want one of the two elements to be cold while the other is hot. This will also prevent lumps. To properly thicken, we need to add one ounce (or two tablespoons) or roux for every cup to cup and a quarter (8-10 oz) of liquid. Once combine, we slowly bring the mixture up to a boil, stirring here and there. Once boiled, we can kill the heat if we are happy with the texture or boil for a minute or two until thicker. We need to be careful as overcooking can lead to the starches breaking down. You will notice that as the mixture cools, it will go from a thick liquid to an almost solid jell. This is ok as once reheated slowly, it will return back to its normal state, perhaps aided by a little more liquid. Rouxs are perfect for thickening cream soups, making gravy, creating pie fillings, or building a lovely cheese sauce.
There is a variant of a roux called beurre manie, which is flour kneaded into softened butter at a 1:1 ratio. This can be made in advance and kept in the refrigerator. When the time comes to add it to say, a pan sauce, whisk in one once (two tablespoons) per cup of liquid and simmer until thick. The advantage is that it takes a lot less time (and equipment) to thicken. The downside is that it lacks as much thickening power as roux and also has less flavor. It will also not withstand reheating the way a roux thickened mixture will.
Finally, we can always use a cornstarch slurry, or equal parts cold water and cornstarch (or any starch) mixed together. This can be added directly to the pan or mixed into a sauce before it is added to the food (again, 2 tablespoons of slurry per one cup of liquid). When brough to a simmer, it will thicken. Again, this is great because it requires no advanced cooking and can be added right at the end of a dish. The disadvantages are that it is a weaker bond so will not withstand repeated reheated. This makes them ideal for last minute pan sauces and many Asian dishes that rely on a light, sticky, gravy.