~ How to: Make a Roux & Slurry (to Thicken Foods) ~
At some point in time, cooks of all levels of expertise encounter a hot food, usually a liquid, that needs to be thickened. Whether it's a soup, a stew, a sauce or a gravy (sometimes even a pudding or pie filling), knowing or not knowing how to do this can and will make or break an otherwise great recipe. This fundamental (and easy) task is one I always try to teach in my cooking classes. Why? Because I've come to learn it confounds a lot of home cooks.
I grew up with a grandmother and mother who each kept two small canisters in the cabinet next to their stoves: one for flour and one for cornstarch. A week rarely went by that I didn't watch one of them drop some flour or cornstarch into a measuring cup, whisk in some cold water and use it to almost instantly thicken whatever happened to be simmering on the stovetop!
On Friday, I was preparing ~ A Summer Comfort Food: German Potato Salad ~ for my next Kitchen Encounters segment on WHVL-TV. In this recipe, I whisk up a slurry and add it to the hot bacon, onion and potato mixture. Within seconds, like magic, the slurry thickens and enrobes the potato salad in a thick, creamy sauce!
When the cameras and lights were turned off, one of the crew members asked a "slurry" question, which resulted in a short but informative discussion about thickening hot liquids and food in general, and, the differences between a roux and a slurry. After posting the potato salad recipe, which can be found in Categories 2 or 13, I decided that perhaps this topic deserved a blog post all of its own.
What is the difference between a roux and a slurry?
A roux is simply equal parts of flour and fat (clarified butter or butter, oil or animal fat) that are cooked in a pan until they are completely combined, thick and smooth. The mixture is then cooked briefly, just long enough to remove any raw flour taste. More complex rouxs require a chef's full attention as they are cooked, whisking constantly, for a lengthier period of time, until the roux turns the desired shade of light tan to reddish brown. "Roux", in French, is the word for "reddish-bown", and, the darker the color, the nuttier and more complex the flavor. Once the roux has been prepared, hot liquid is whisked into it, in increments, returning it to a simmer prior to each addition, until the desired consistency is reached. Why return to a simmer each time? Because a roux's full thickening power is not realized until the hot liquid is brought to a simmer.
A bit about slurry (sometimes referred to as "whitewash"): In the home kitchen, when home cooks are looking to thicken a soup or a sauce at the last minute, they make a slurry. A slurry is simply a 1:4 ratio of flour, cornstarch or arrowroot to COLD water, broth or juice that is whisked together in a small bowl until they are completely combined, slightly thickened and smooth. This ratio is not "etched in stone", and, if a recipe gives you a specific ratio, use it. Scientifically, here's what happens in a slurry: the liquid hydrates the starch in the thickening agent, which prevents it from clumping when added to the hot liquid. The slurry is then added directly to the simmering dish being prepared and cooked briefly, just long enough to remove any raw flour taste. Occasionally, seasoning may be added to the slurry.
Flour, cornstarch or arrowroot? Which one works best?
Flour is hands down the most common starch used in American slurries, and, cornstarch is hands down the most common starch used in Asian slurries, and, I use them almost exclusively in this manner. The biggest difference between the two is the "look" of the dish being prepared at the end: flour produces a mat/flat sort of cloudy finish, while cornstarch produces a glossy/shiny sort of glassy finish. It has also been my experience that cornstarch does not do well when used in dishes that contain a lot of acid, and/or are going to be frozen, which makes arrowroot the substition for cornstarch in those culinary applications. Arrowroot is completely tasteless and becomes completely clear when cooked, but, it does not work well used in dishes that contain cream, because it tends to get greasy/slimy, particularly if the dish being served requires reheating.
Tip: If you've prepared a dish that started out with a roux and you feel it needs a slurry at the end for additional thickening: use cornstarch or arrowroot to prepare the slurry. Why? Because a chemical reaction occurs that will prevent the same starch used twice to thicken it any further.
Special Equipment List: small pot or skillet; measuring container or bowl; whisk
Cook's Note: Besides roux and slurry, there are other methods of thickening too. They require thoughtful use, but: Try adding some cooked and pureed starchy vegetables or smashed beans to the dish. Ground up cereals, grains, nuts and seeds can work too!
"We are all in this food world together." ~ Melanie Preschutti
(Recipe, Commentary and Photos courtesy of Melanie Preschutti/Copyright 2012)