Sometimes you need to make a sauce, soup or stew thicker.  This is where thickening agents come in. 

Cornstarch, flour and arrowroot powder are commonly used as thickening agents, but what is the difference and does it matter which one you use?

Cornstarch is prepared from corn. To use this as a sauce thickener, you have to make a slurry with cold liquid.  That is, adding equal parts of a cold liquid (ex: water, broth) to cornstarch.  Whisk the mixture together and add it to the hot, simmering sauce.

If you add cornstarch directly to a sauce, you'll get a big clumpy mess.

The cornstarch particles behave like little sponges as they soak up the liquid and expand.

A few key characteristics of cornstarch:

  • It gives a glossy appearance to the liquids it thickens, so it is usually used with sweet sauces and pie fillings rather than savory gravies and sauces.
  • Cooking too long will cause cornstarch to break down, affecting its thickening ability.  Sauces cooked too long with it will eventually thin out.
  • Acidic ingredients, like tomato sauce, will affect cornstarch's thickening ability.
  • Cornstarch doesn't freeze well. It will take on a spongy texture when frozen.

Flour is a common thickener for gravies and savory sauces. 

To ensure a smooth sauce, use white, all-purpose flour, rather than whole wheat or bread flour.

There are many ways to use flour as a thickener:
  • Dredge stew meats in it prior to adding to liquid before cooking. This will thicken the liquid as it cooks.
  • Whisk some flour with cool water (the slurry) and stir it into sauces. Don't just throw flour directly into the sauce. You'll end up with lumps.
  • Make a roux by adding flour to some kind of fat, like melted butter, lard or oil, and cooking to the desired level. Lightly cooked roux is great for thickening light gravies or corn chowder. Cookingn longer with make it darker, which is great for gumbo and many French dishes calling for a roux. Whisk in the roux while cooking.
A few key characteristics of flour as a thickener:
  • Unlike cornstarch, flour must be cooked (i.e. bring to a boil and then cooked for 3 minutes) to lose its "raw flour" taste and release its thickening properties.
  • Flour will turn thickened liquids cloudy, so flour is best used when opacity is OK. This is usually the cause for gravies and creamy sauces.
  • Flour will continue to thicken after cooking, so turn off the heat right before the sauce is the desired thickness.
Arrowroot powder, or arrowroot starch, is another thickener, but it has some different properties than cornstarch and flour. Arrowroot (or tapioca) is often used as a substitute for flour when doing gluten-free cooking. Arrowroot can thicken sauces, gravies, soups, jams, and helps create flaky, moist, baked goods.

To use arrowroot powder as a thickener, make a slurry by mixing the powder into cold water and whisk until smooth. Stir into the hot, simmering sauce to be thickened towards the end of cooking.

A few key characteristics of arrowroot powder as a thickener:
  • Do not use with dairy or cream sauces. It will come out slimy.
  • Arrowroot powder will not turn liquids cloudy when thickened, so when transparency is the goal, arrowroot will be a better choice than cornstarch or flour.
  • Arrowroot thickens at a lower temperature than flour or cornstarch, so it can be used towards the end of cooking.
  • Arrowroot's thickening properties are not affected by freezing or acidic ingredients.