Cooking Class: The thick of things

This winter, wow your friends and family by fortifying your hearty soups with a 'slurry.'

soup 390 (photo credit: Thinkstock/Imagebank)
soup 390
(photo credit: Thinkstock/Imagebank)
When you’ve made a soup but you find it’s too thin, there’s a quick and easy solution – stir in a slurry. A “slurry” is simply a fancy word used by cooking professionals for starch dissolved in liquid.
If you try to thicken a hot soup or sauce by stirring in flour, you get instant lumps. “With a slurry,” writes Shirley O. Corriher, author of Cookwise, “you stir cornstarch into cold water or stock to separate the starch grain by grain. When you are ready to thicken the sauce, stir the slurry again and then stir it into the pan.”
What kind of starch to use is up to you. Some homey, old-fashioned recipes call for making a slurry from all-purpose flour, but most chefs find that flour is a better thickener when blended with fat.
For a fat-free, quick and easy way to thicken soups and sauces, cornstarch or potato starch are the most common choices. They provide sauces with a translucent, glossy finish.
If you’ve done any Chinese cooking, you’re probably familiar with this technique.
Many, if not most, Chinese sauces are thickened with a cornstarch slurry. French chefs use this method too; for example, to turn meat stock into a quick brown sauce.
Corriher explains the science behind the slurry: “Starches normally do not dissolve in cold water. As the water is heated... the granules expand or puff. The hotter the water gets, the more the granules absorb and the more they puff... Finally the granules pop, starch rushes out into the sauce, and suddenly it thickens.
“The way this looks when you are cooking is that you are heating and stirring... and nothing happens... You may mix up a little more cornstarch into cold water and add it. Then, all at once, when you get to the temperature when the starch granules pop, you have... glue because you have added too much starch.
“The secret of working with starch-thickened sauces is to bring them to a gentle boil first. Then decide whether they are thick enough and add more starch in cold water if necessary.”
Instead of limiting yourself to flour and cornstarch, Corriher recommends trying other starches, such as powdered tapioca, pure wheat starch, arrowroot and potato starch.
Which one to choose depends on what you are making. Grain starches such as wheat and corn set up into an opaque gel that is firm enough to cut with a knife and are useful, for example, if you want a thick cream pie filling.
Sauces made with grain starches can be reheated without becoming thinner.
Root starches such as arrowroot and tapioca form a clear, thick glossy coating.
They may become thin when reheated and thus are best added to sauces at the last minute. Potato starch falls somewhere in between and makes a firm, clear gel.
Starches differ in their thickening power. You need less cornstarch than flour to achieve the same thickness. To make 1 cup sauce of medium thickness, you would use 2 Tbsp. flour, 1 Tbsp. plus 1 tsp. cornstarch or arrowroot, or 21⁄4 tsp. potato starch.
Starches are not the only way to thicken sauces and soups. You can also use bean flour. Not long ago, I decided to add chickpea (garbanzo bean) flour to my vegetable soups to boost their protein content. The chickpea flour thickened the soups lightly and added a subtle bean flavor.
Thickening soups with chickpea and other bean flours is, in fact, an age-old technique. The bean flours not only enhance the nutrition but also enable the cook to make satisfying soups in minutes.
Maguelonne Toussaint-Samat, author of Ethnocuisine de Provence, writes about old- fashioned soups called les soupes au baton, or “twig soups,” made with chickpea, fava bean or dried pea flour. As the soup thickens, it is stirred with a twig from a bay leaf tree, which adds flavor.
Sometimes the soup is enriched with meat or sausage and might be flavored with herbs and onions. When the soup is thick enough so the twig can stand upright, the soup is drizzled with olive oil and served very hot.
Rozanne Gold, author of Radically Simple, makes a similar soup in her chapter on five-minute soups. She boils pressed garlic in water with olive oil, cumin and salt, thickens it with chickpea flour and serves the soup with sliced green onion, a drizzle of olive oil and lemon wedges.
Given the popularity of chickpea flour in the cuisine of India, it’s not surprising that it’s used to thicken sauces. A chickpea- flour-thickened curry sauce for vegetable dumplings, for example, might be flavored with ginger, garlic, cumin seeds, turmeric powder, mustard seeds and chilies and enriched with yogurt.
When thickening with bean flour, you need a larger amount than with cornstarch or wheat flour. The chefs at Bob’s Red Mill in Milwaukee, Oregon, recommend using 1⁄3 cup white bean flour for 2 cups chicken broth to make a quick, creamy-textured soup. Other bean flours are used in different proportions.
The beauty of thickening with a slurry, whether of starch or bean flour, is that if your sauce or soup still isn’t thick enough, you can simply add more.
Starches should be kept very dry in an air-tight container. When starches are exposed to air over long periods of time, writes Corriher, they can lose some of their thickening ability. “If you are using cornstarch that has been on the shelf for a couple of years, you may need to use more of it to thicken than the recipe indicates.”
Makes about 2 cups This sauce is a popular alternative to traditional brown sauce, which requires hours of simmering. It is delicious when made with homemade stock, but it can be made with packaged broth as well. If you like, prepare a double or triple quantity and keep it on hand in the freezer. To turn it into a tasty sauce for meat or chicken, add a little fortified wine such as Madeira and season it to taste with salt and pepper.
You can keep the sauce up to 2 days in refrigerator or it can be frozen for several months.
✔ 1 to 11⁄2 Tbsp. vegetable oil or olive oil
✔ 1 onion, diced
✔ 1 carrot, diced
✔ 3 cups meat or chicken stock
✔ 2 garlic cloves, crushed (optional)
✔ 1 fresh thyme sprig or 1⁄4 tsp. dried leaf thyme, crumbled
✔ 1 bay leaf
✔ 2 tsp. potato starch, arrowroot or cornstarch
✔ 4 Tbsp. cold water In a heavy saucepan, heat oil over medium- high heat. Add onion and carrot and saute, stirring often, until well browned.
Do not let mixture burn. Add stock, garlic, thyme and bay leaf. Bring to a boil, stirring, and simmer uncovered over low heat for 20 to 25 minutes.
Strain into another saucepan, pressing on vegetables. Skim as much fat as possible from surface. Simmer over medium heat until reduced to 2 cups.
Spoon potato starch into a small bowl.
Add cold water and whisk to form a smooth paste. Gradually pour into simmering sauce, whisking constantly. Bring back to a boil, whisking constantly. Simmer for 1 or 2 minutes, until thickened.
Makes 4 servings This soup is lightly thickened with chickpea flour. If you prefer a thicker soup, mix another 1 or 2 Tbsp. chickpea flour with 3 to 6 Tbsp. water, stir into the simmering soup and cook another minute or two. For a heartier soup, stir in 1 or 2 cups shredded cooked chicken or diced tofu. For especially good flavor, add herb-flavored sea salt.
✔ 62⁄3 cups vegetable or chicken broth or part broth and part water
✔ A 110-gr. (4-ounce) piece pumpkin or butternut squash (optional)
✔ 1 cup diced mild radish or kohlrabi (optional)
✔ 1⁄2 cauliflower, divided into medium florets
✔ 2 carrots, sliced
✔ 110 gr. (4 ounces) green beans, ends removed, halved
✔ 1 large onion, chopped
✔ 2 medium potatoes, diced
✔ 1/4 tsp. hot red pepper flakes, or to taste
✔ 3 or 4 Tbsp. chickpea flour
✔ Salt and freshly ground pepper to taste
✔ 1 Tbsp. chopped parsley
✔ 1 to 2 tsp. chopped green onion (optional)
Bring 6 cups broth to a boil in a saucepan. Add pumpkin and radish, return to a simmer and cook for 4 minutes. Add cauliflower, carrots and green beans, return to a simmer and cook for 5 minutes.
Remove cooked vegetables to a bowl with a slotted spoon.
Dice pumpkin. Add onion, potatoes and pepper flakes to broth in saucepan, bring to a boil and cook for 25 minutes or until tender.
Spoon chickpea flour into a small bowl and gradually stir in remaining 2⁄3 cup broth. Stir this slurry into simmering soup.
Return to a simmer, stirring, and cook for 2 or 3 minutes or until lightly thickened.
Return cooked vegetables to soup and heat through. Season to taste with salt and freshly ground pepper. Serve sprinkled with parsley and green onion. 
Faye Levy is the author of the Fresh from France cookbook series.