The sauce thickens

How to avoid lumps

March 11, 2009

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.

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.

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.

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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 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.

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.

Based in Portland, Oregon, Hank Sawtelle has engineering, legal, and culinary degrees. Email questions for the Ask Hank column to

There are 8 comments on this item
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1. by Laura Parisi on Mar 11, 2009 at 1:01 PM PDT

also, if you’re in a jam, and you don’t have cornstarch, it is possible to thicken with flour without first creating a roux (the better way to do it): slowly sift it into your sauce and whisk like hell. Eventually the bumps go away. It’s not ideal but it works if you’re in a bind.

2. by KAB on Mar 11, 2009 at 3:07 PM PDT

I just took an Indian cooking class from a “native cook” (like a native speaker?) and she said that in parts of India they use onions to thicken many of their dishes. The onions are sauteed for up to half and hour over medium heat (similar to caramelizing them) until they’re virtually falling apart, then simmered with the dish. Brilliant!

3. by Matthew Amster-Burton on Mar 12, 2009 at 5:19 PM PDT

Laura, another thing to do in that case is use beurre manié, a paste of butter and flour.

4. by Bavaria on Mar 13, 2009 at 5:49 PM PDT

I’ve had some sauces made with potato starch that were very good--kind of a middle ground between the robust flour sauce and the clear cornstarch sauce. The trouble is, I don’t know where to get potato starch. Any ideas?

5. by Liz Crain on Mar 16, 2009 at 2:30 PM PDT

Great story! I need to think of some good questions now...

6. by mad hatter on Mar 18, 2009 at 3:13 PM PDT

Good point about buerre manie, a paste of soft butter and flour but remember flour should be added uncooked like this just til the sauce thickens (reaches the boil) and then taken off the heat. Otherwise it tastes “floury” (if that’s a word)! Jane Johnson

7. by Caroline Cummins on Mar 20, 2009 at 7:08 PM PDT

And what about arrowroot? Any good in sauces?

8. by Hank Sawtelle on Mar 24, 2009 at 10:15 AM PDT

Great root-starch questions

@Bavaria: I have used potato starch also in the past. It is a very powerful thickener. They sell it at my regional grocery chain (QFC) in the same section as the cornstarch. It’s also available on-line. I have also used “potato flakes” - instant mashed potato type things - as a thickener for soups and sauces. Just whisk the heck out of it and they should break up. They can also be used as a crust, e.g. on seared fish.

@Caroline- arrowroot is terrific. I thought about discussing it in the article. I like it because it dissolves quickly and it’s clear when it dissolves (that’s why it’s used in cherry pies, etc.). One thing to keep in mind is that it can be overcooked and over-whisked, in which case it will lose its thickening power. So definitely add it at the end and don’t simmer it for long times.

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Ask Hank

Hank Sawtelle has engineering, legal, and culinary degrees, and an intense curiosity about food and cooking. Follow Hank’s blog, Sous Vide Jones.

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