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The sauce thickens
(article, Hank Sawtelle)
What's the best way to thicken a sauce without making it lumpy? Cornstarch or flour? And as a paste, or just dump it in?
— Melissa T., Denver, Colorado
Dump it in? Really now, Melissa. Who cleans up after you?
A sauce that's too thin or too thick is a huge bummer, and the fact that you care about this is good news. Many people are content to follow a recipe and then dutifully spoon the output onto a plate, without taking a moment to consider the quality of the results. Recipes are bad at nailing thickness, and every cook's idea of the right texture is different. Luckily, it's not difficult to adjust a sauce to suit your taste.
These dishes rely on flour or cornstarch as thickeners.
Most sauces are mainly water, and their thickness depends on how much other stuff is in the sauce to interfere with the motion of the water molecules. Starches — the long chains of sugars that plants make to store energy — are terrific thickeners when used correctly.
Flour and cornstarch contain dried starch granules, which are densely packed little balls of starch molecules.
Tucked away in the granules, the starch molecules can't do any thickening. But get the granules wet, and they start to absorb water. Add heat, and they swell dramatically with water, loosening up and forming a gel within each swollen granule.
Near the simmering point, the granules spill starch molecules into the sauce, thickening it suddenly. The increased thickness is caused by the long starch molecules interfering with each other and the swollen granules.
[%image reference-image float=right width=400 caption="A slurry of cornstarch and cool water makes an excellent thickening agent for sauces."] As anyone who's ever tried it can tell you, you shouldn't add dry starch directly to a hot pan of sauce — the granules will angrily huddle together in gel-covered lumps that can't be dissolved. (The exception to this rule is specially manufactured “instantized” flour, like Wondra, which is designed to dissolve directly in hot liquids. I don't know exactly how it works, but it's expensive and unnecessary.) The only remedy to the dumping situation is to pass the sauce through a fine-mesh strainer, pour another glass of wine, and try again.
So now that we've established the wrong way to thicken a sauce with starch, what's the right way? As you suggested, making a very thin paste (or slurry, if you want to sound fancy) of cornstarch and tepid water is a good way to separate the starch granules so they won't form lumps. Add water until you've got a cream-like consistency. The starch won't dissolve in the cool water, so the granules will settle to the bottom in time.
Give it another quick stir before adding it to your sauce in a stream, whisking as you go. Then bring the sauce to a simmer (this step is important) to see what the thickening results are before adding any more starch.
If you get impatient and add more starch before the sauce simmers, you may overshoot your target and end up with a gloppy mess. If this happens, you'll need to whisk in more liquid (stock, wine, etc.) to thin the sauce back out.
In his book On Food and Cooking, food-science writer [/author/HaroldMcGee "Harold McGee"] makes the related point that sauces thicken further as they cool. And since we don't eat boiling-hot sauces, he notes, “sauces should be thinner at the stove than they're meant to be at the table.” (Pre-warming your dinner plates in a very low oven is a restaurant trick that also helps keep food warm and sauces loose.)
Slurries (I love to sound fancy) work better with cornstarch than with flour, because flour has lumping tendencies even in cool water. So cornstarch is a better choice in a last-minute sauce emergency, which is good news because it has more thickening power, less graininess, and none of the raw flavor of flour.
h1. Other references
I've merely scratched the surface on the behavior and types of culinary starches. For further reading, I recommend any or all of the following:
Harold McGee, On Food and Cooking, pages 610-625.
Shirley Corriher, CookWise, pages 273-284.
James Peterson, Sauces, pages 113-116 (and read the rest of the book if you truly seek to become a Sauce Ninja).
Flour performs best when it's mixed with fat, most commonly in a roux (butter or fat cooked with flour), which requires some planning ahead. Besides being an essential component in many classical French sauces, roux is also a common thickener in good old American gravies. For gravy, flour is cooked with roasting-pan drippings and then degreased juices (or stock, broth, milk, etc.) are whisked in.
A good measuring rule of thumb is about 1 tablespoon of cornstarch to thicken a cup of sauce, versus 2 tablespoons of flour per cup of sauce.
Whichever starch you use, be sure to taste the sauce after thickening and adjust the seasoning if necessary, as starches can mute tastes, particularly salt.
Finally, starches aren't your only thickening option, especially if you have a few extra minutes at the stove. If there are vegetables in your sauce, puréeing them with a blender (standard or immersion) will thicken the sauce by suspending tiny veggie bits in the water. Similarly, mixing tomato paste into a tomato-based sauce will thicken it by suspending more tomato particles in the sauce.
Reducing — simply simmering the sauce until there is less of it — helps thicken most sauces, and can dramatically affect those with sweet ingredients (for example, port wine or balsamic vinegar). Keep in mind, however, that reducing will concentrate the flavors (sweet, sour, salty) of the sauce, and may drive off some of the aromatic components (especially delicate herbs). So taste and adjust as necessary.
p(bio). Based in Portland, Oregon, Hank Sawtelle has engineering, legal, and culinary degrees. Email questions for the Ask Hank column to AskHank@culinate.com.