Which pantry item starts out yellow, but ends up white and green?
Cornstarch -- a white powder ground from the endosperm of the yellow corn kernel. When cooked in a sauce or stir-fry, or baked in a pie, cornstarch turns clear. When manufactured into biodegradeable trash bags, it turns "green."
Fortunately, cornstarch doesn't turn food green!
Called corn flour in other parts of the world, cornstarch, a gluten-free thickening agent, was patented in 1841; though first used as a laundry starch, it's as popular today in the culinary arena.
I've been learning more all the time as I research my way through my pantry. Most often, for thickening stews and stir-fries, I'll reach for the arrowroot on my spice rack, but more recipes for baked goods call for cornstarch. I wondered why.
According to food science guru 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."
And from The Accidental Scientist I learned that too much of a good thing -- stirring -- can negate the thickening process. Generally cornstarch is added to a sauce in a slurry: i.e., mixed with a liquid (water, stock, etc.), to prevent lumps from forming. (Generally I use one tablespoon of cornstarch to three tablespoons of water.) Cornstarch begins to thicken when heated to 203°F, and it thickens quickly, turning from milky white opaque to transparent. After that point, stop stirring, as any agitation will interfere with the setting process. When the starch network that sets and traps the liquid is broken, 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.
Gluten also reduces the thickening power of flour. You need twice as much flour as cornstarch to thicken the same amount of liquid, but an equal amount of arrowroot or cornstarch will do the trick.
While you can store cornstarch almost indefinitely on the pantry shelf, 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.
If it's really old, relegate your cornstarch to the laundry room, where it will work wonders on your wrinkled shirts.
Beef stir-fry with bitter melon
Nancy Soohoo was my neighbor in Boston for nearly 30 years, until she passed away a few years ago, but language kept us from getting to know each other for half of that time. It was her small plot in the community garden down the block that brought us together. She’d taken over her godmother’s garden plot, where she grew several varieties of squash, peppers, basil and bitter melon. The melons hung down inside a chicken-wire frame, and when she invited me to view her garden, I felt like I'd entered a cave dripping with stalactites. All the proportions in this recipe, which Nancy gave me with the help of a translator, depend on the size of your bitter melon (they can be as small as a cucumber, or as large as a giant zucchini), so think of it as a method more than a recipe. Salting bitter melon helps to reduce the bitterness, but it’s still an acquired taste. If you haven't acquired it, use a couple of zucchini or seedless English cucumbers instead, and stir-fry them with the meat. Serves 4, as part of a Chinese dinner.
1 small bitter melon
3 Tbsp peanut oil
1 clove garlic, sliced
1/2 lb flank steak, sliced thin
2 Tbsp black bean sauce (use the Asian sauce made from fermented black beans, not the Mexican variety)
1/2 tsp sugar
2 tsp cornstarch dissolved in 1/3 cup water
Cut the bitter melon in half lengthwise, and scrape out the pits. Mix 1 Tbsp salt with 4 cups of water in a large bowl, and soak the melon (or cucumber) for 5 minutes. Drain, rinse, then sprinkle 1/4 tsp salt directly on the cut side. Place in a pot of boiling water and boil for 5 minutes. Drain, then cut into julienne strips. Heat a wok, and add 1-1/2 Tbsp peanut oil. Add the melon, toss, and add 1/2 cup water. Cook 8-10 minutes, until the melon is tender. Drain and place on a serving plate. Add 1-1/2 Tbsp oil to the wok, and stir fry the garlic for 15 seconds. Add the beef, and continue to stir. Add black bean sauce and sugar, and stir 1-2 minutes more. Pour in the cornstarch solution, and stir until thickened slightly. Place the meat on top of the bitter melon, and serve hot with steamed rice.
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