Cornstarch (Recipe: hot and sour soup)
When I was in high school, I did my homework and made good grades in English, history, geography, Latin and math.
I did pretty well in science, too. Right up until junior year chemistry.
As a food writer, I'm sorry now that I didn't "get" chemistry. If I did, I'd understand some of the science of baking powder/baking soda, cornstarch/arrowroot, and why cream of tartar helps beaten egg whites mount.
I'll bet they did well in high school chemistry.
Cornstarch (also called cornflour) is a white powder ground from the endosperm of the yellow corn kernel.
A gluten-free alternative to flour, it's used primarily as a thickener. Gluten reduces the thickening power of flour, so you need only half as much cornstarch as flour to thicken the same amount of liquid.
Most often, for thickening stews and stir-fries, I reach instead for the arrowroot on my spice rack (I substitute equal amounts of arrowroot for cornstarch), but more recipes for baked goods call for cornstarch.
Why? According to Harold McGee, "Arrowroot and potato starches come from below-ground storage organs, cornstarch and flour from seeds, and the two different kinds of sources produce starches with different qualities. Briefly, the root starches have larger granules and longer starch molecules that gelate and thicken at lower temperatures, and are more efficient at thickening, but that break down on prolonged heating or freezing: so you need less root starch to thicken, but the consistency isn’t as stable. Root starches also have a more neutral flavor than seed starches."
There are a couple of tricks to working with cornstarch in cooking. Generally cornstarch is added to a sauce in a slurry -- mixed with a liquid (water, stock, etc.) -- to prevent lumps from forming. I use one tablespoon of cornstarch to three tablespoons of liquid.
Cornstarch begins to thicken when heated to 203°F, and it thickens quickly, turning from milky white to transparent. When the slurry turns clear, stop stirring, as any agitation will interfere with the setting (thickening) process. If the starch network that sets and traps the liquid doesn't have time to congeal, liquid is released and thins the sauce.
If you're adding cornstarch to a sauce (or pie filling) and it does not thicken, try adding more liquid, not more cornstarch. Counterintuitive, I know, but the problem could be that there wasn't enough liquid in your slurry to allow the starch granules to enlarge to their full capacity.
You can add cornstarch to flour to make cake flour, and cornstarch is just what you need for dulce de leche cornstarch shortbread cookies, but I don't bake often, so most of the time I use cornstarch to thicken (sauces, soups, etc.).
While you can store cornstarch almost indefinitely in an airtight container, the quality will degrade over time. If your cornstarch is old, you might need more of it to yield the thickening power your recipe requires.
How much more, exactly? How old? I don't know.
Ask someone who did better than I did in high school chemistry.
Hot and sour soup
Adapted from Great Food Fast, this is the first soup I've made where the flavor deepens when the soup rests for a couple of hours before serving. Serves 8 as an appetizer, 6 as a main.
32 oz chicken broth, homemade or low-sodium store bought (I use Swanson 99%)
2 Tbsp reduced sodium soy sauce
1 Tbsp dark soy sauce
1/2 tsp red pepper flakes
8 oz large button mushrooms or fresh shiitakes, stems removed, caps thinly sliced
4 Tbsp rice vinegar
2 Tbsp cornstarch
1 large egg, lightly beaten
7 oz firm tofu, drained*, cut into 1/4-inch cubes
2 Tbsp finely grated peeled fresh ginger
Sriracha, to taste
3 scallions, thinly sliced
In a large stockpot, combine the broth, soy sauces, red pepper flakes, and 2 cups of water. Bring to a boil over medium heat. Add the mushrooms, reduce heat to low, and simmer, until the mushrooms are tender, 10 minutes.
In a small glass measuring cup, whisk together 3 Tbsp rice vinegar and the cornstarch. Add to the pot, and simmer, stirring, until the soup is slightly thickened, 1 minute or so.
Drizzle the egg slowly into the pot, stirring with a fork to break up the egg into shreds as it hits the hot liquid. Add the tofu, and stir. Remove the pot from heat and let it sit for a minute.
Place the ginger in a small sieve over the pot, and press down on the solids to allow the ginger juice to fall into the soup. Add the remaining tablespoon of rice vinegar, and season the soup with sriracha, to taste.
Set the soup aside, covered, for an hour or two (or more) until ready to serve. Bring the soup back up to heat. Stir in the scallions, and serve hot.