A Guide to Cornstarch
Cornstarch is a staple of any well-stocked pantry. If you cook even somewhat frequently, then chances are good you’ve encountered cornstarch at some point in your culinary journey. And I’d even bet that you have a box of the stuff sitting in your cupboard right now.
You probably already know a thing or two about the properties of cornstarch. You likely use it as a thickener for sauces, or in batters and dredges for fried foods. But let’s take a moment to appreciate cornstarch holistically–what it is, what it isn’t, where it comes from, and, most importantly, the many things you can do with this powerful ingredient.
What Is Cornstarch?
Cornstarch is a fine, odorless, flavorless white powder derived from the endosperm of the corn kernel. Like rice grains, corn kernels consist of multiple layers: the outer protective pericarp (the “hull”), the germ, the endosperm, and the exposed tip cap, which is the point at which the kernel attaches to the cob. The starchy flesh of the endosperm comprises around 82 percent of the kernel’s weight, and contains all of the native starch in the plant—in other words, all of the magic. In a popped corn kernel, the endosperm is the fluffy white part that tastes so good when doused with butter and salt.
These days, cornstarch is made by a process called wet milling. Shelled corn is cleaned and steeped in large tanks in a warm, acidic solution of water and sulfur dioxide. This solution softens the kernel, which makes it easier to mill. The water is boiled off, and the milling process loosens the hull (pericarp) and endosperm from the germ. After passing through a series of grinders and screens, the endosperm is isolated and processed into a slurry, which contains mostly pure corn starch. When dried, this starch is unmodified; it can be refined even more to make modified starches intended for specific cooking applications.
A Relatively Recent Discovery
Humans have been using starch in both cooking and non-cooking applications for thousands of years, dating as far back as predynastic Egypt. But cornstarch didn’t come to prominence until the 1840s, when Thomas Kingsford isolated cornstarch from corn kernels by using an existing process for extracting wheat starch. Kingsford’s method involved soaking the kernels in an alkaline solution, then grinding them—the beginnings of what would become modern wet milling.
At the time, Kingsford intended the product for use as laundry starch: The practice of starching clothes added a soft but crisp texture to fabrics and improved their resistance to being soiled. It wasn’t until the 1850s that cornstarch became the useful food additive it is today.
The Difference Between Cornstarch, Corn Meal, and Corn Flour
While we touched on this earlier, it’s important to distinguish the various granulated corn products you can find at the store. Here’s a broad breakdown:
Cornstarch is flavorless and odorless, and is used mostly to alter the texture of foods. It’s almost pure starch.
Corn flour is a fine, ground powder made from dried whole corn kernels. It has an earthy, sweet taste. It contains parts of the outer hull as well as the germ and endosperm. It’s commonly used in baking and fried applications.
Corn meal is essentially the same as corn flour, but ground more coarsely. It imparts a more gritty texture to baked and fried goods. It’s often a key ingredient in cornbread and Johnnycakes.
How Does Cornstarch Work Its Magic?
Cornstarch is a starch, which means it’s is a collection of semi-crystalline granules of starch molecules called polysaccharides. Those molecules are amylose and amylopectin, and they exist in different proportions depending on the starch source. The key to nearly all of cornstarch’s functional properties is gelatinization—the breaking down of starch molecules with heat and water, allowing those molecules to bond with more water. As you hydrate and heat cornstarch, the starch granules swell and soften, and they lose their hard, crystalline structure. Eventually, those granules burst; amylopectin leaches out into the surrounding water, and the mixture thickens. If that same mixture is cooled, the mixture generally becomes thicker.
In general, the relative proportion of amylose dictates how strong the gel will be; the higher the proportion of amylose, the higher the gel strength. Cornstarch has a relatively higher proportion of amylose (around 25%) compared to other starches like tapioca flour (18%), and that increased gel strength is one the reasons it is so useful in cooking.
What Can You Do With Cornstarch?
Concentrated starch additives like cornstarch are important primarily for improving the texture of foods. Here are some ways we can use cornstarch; some may be obvious, some may be new to you.
At room temperature, if you mix cornstarch with water, you’ll create a milky liquid that, over time, will gradually separate into two distinct phases, with most of the cornstarch settling to the bottom and a slightly translucent liquid settling on top. This is because cornstarch at that temperature isn’t all that soluble in water; mixing it into liquid will create a suspension, where the particles of starch are dispersed in the liquid but not dissolved. This is why when recipes call for using a mixture of water and cornstarch to thicken a sauce, they will tell you to mix the combination thoroughly just before adding it to what you’re cooking, as mixing produces a uniform suspension of the starch in the liquid.
But if you apply heat to the mixture, the cornstarch dissolves and forms a semi-clear gel. Compared to flour, cornstarch thickens a mixture faster and has a cleaner flavor; it also produces a clearer gel. That mild flavor and clear appearance makes cornstarch a great thickener for dessert glazes, fruit sauces, puddings, soups, stews, and any instance in which you might want a slightly glossier appearance and cleaner flavor than flour alone could provide.
However, cornstarch’s gelling properties aren’t infallible. Under certain conditions, a cornstarch gel is suboptimal, and sometimes undesirable.
Heat Stability: Cornstarch begins to gelatinize in water around 144–162°F (62–72°C), and fully gelatinizes around 203°F (95°C). But you also may have heard that you shouldn’t boil cornstarch for too long, since the sauce will begin to thin. Why? Prolonged, excessive heat can degrade those swollen starch molecules, and eventually decrease the potential thickening power, which leads to a thinner sauce than you’d expect. For this reason, it’s wise to add cornstarch toward the end of cooking—as is customary in many Chinese stir-fry dishes.
pH Tolerance: Acids tend to inhibit the thickening power of cornstarch. Much like heat, certain acids like acetic acid (vinegar) or citric acid (lemon juice) can severely hamstring the efficacy of cornstarch: in combination with heat, at sufficient concentrations these acids can break starch molecules down into component sugars. Interestingly, according to this study, adding a small amount of acid (keeping the pH between 3.6 and 5.5) increases the viscosity of the mixture. Finally, if you’re keen on adding acidity to a cornstarch-thickened sauce, consider adding the acid after the mixture has cooled. Researchers found no decrease in viscosity if acid was added to a gelatinized mixture after it had cooled to room temperature.
Retrogradation and Syneresis: Cooling gels made with cornstarch can present a few problems. When gel made from cornstarch cools, its properties change: it gets thicker and turns very slightly opaque as the starch granules reassociate in a process known as retrogradation. In some extreme cases, and if left in the fridge (or freezer) for long enough, thickened mixtures of cornstarch may break or split and form a watery layer. This ‘weeping’ is known as syneresis, and occurs frequently with cornstarch as the starch and protein molecules contract, forcing water out of the swollen granules. The main takeaway for most home cooks is that cornstarch isn’t the best option for foods that will require long-term storage in cold temperatures. You’re better off thickening that huge batch of beef stew with a standard flour roux or some alternative starch like potato starch or arrowroot starch.
Improving Stability of Fat-Water Emulsions
Here’s another useful trick: Starches can help to stabilize fat-in-water emulsions. We see this process occur in pasta dishes that utilize starchy pasta water: the excess starch in the water acts as a thickener and improves the emulsion, producing a smooth, glossy sauce. Béchamel sauce is another prime example; the flour disperses the fat and facilitates a smooth emulsion of butter in milk. The same principle applies to cornstarch, but the effect can be even more dramatic. Remember that compared to wheat flour, cornstarch is almost pure starch, so its capacity to thicken, as well as its capacity to disperse fat droplets and keep them from coalescing, is greater than that of flour. This study also suggests that the smaller the granule size, the better the stabilizing potential of a starch in an emulsion. Cornstarch has a smaller granule size (20 microns) than wheat starch (25 microns), which suggests that it is a slightly better choice.
J. Kenji López-Alt
Kenji illustrates this effect beautifully in his 15-Minute Ultra-Gooey Stovetop Mac and Cheese and Nacho Cheese Sauce. The cornstarch yields a smoother, creamier, more velvety texture than flour or cheese alone.
Velveting is a Chinese stir-fry technique that involves coating small pieces of meat in cornstarch and quickly cooking them in a hot wok. This technique has two advantages: It produces a thin layer of hydrated starch on the exterior of the small pieces of meat, which, when eaten, we perceive as a kind of tenderness and juiciness; and it serves to thicken any sauce that the meat may be cooked in, as the surface cornstarch leaches into the sauce, binds excess water, and makes the sauce more viscous.
Crispness and Tenderness in Baked Goods
Cornstarch is a frequent support player in baked goods. And unlike flour, cornstarch is nearly pure starch—there’s no gluten, no fiber. All that added starch content means that cornstarch typically improves the water binding capacity of a dough. Because it has no gluten, cornstarch tends to soften the texture of a baked cookie, for instance, producing what we call a ‘short’ texture. For crisp baked goods, cornstarch is equally helpful because it inhibits gluten development (which sometimes leads to a tough texture), resulting in crispier crackers.
Of course, there are drawbacks to using cornstarch in your baking. First, cornstarch doesn’t have any gluten potential. So if you’re looking for cookies with more structure and less of a crumbly texture, then cornstarch isn’t necessarily the best ingredient to incorporate into the dough. Second, cornstarch has none of the proteins that wheat flour has, so it offers little to no browning. Finally, if you’re relying on cornstarch to thicken a baked pie filling, then you may want to look to another ingredient, as its thickening power deteriorates when subjected to prolonged heating. As Stella suggests in many of her pie recipes, like this cherry pie, tapioca starch is a better choice.
Crispier Fried Foods
One of the greatest advantages of cornstarch is its potential to produce crispy coatings on fried foods. In dishes like Korean fried chicken, Southern fried chicken, or General Tso’s chicken, adding cornstarch to the batter or dredge raises the proportion of starch relative to other components in the coating (tough gluten proteins, for example). Some recipes—like Kenji’s stir-fried crispy tofu—use cornstarch exclusively to achieve ultra-crispy textures. More starch means greater water absorption and enhanced gelatinization of starches; frying drives off water, leaving a porous, brittle network that we perceive as crunchy or crispy when we bite into it. So the more of a starch network we build, the crispier the end product.
Like in baking, frying with cornstarch has its drawbacks. Again, it’s tough to produce any significant browning using cornstarch alone, which is why using a blend of cornstarch and wheat flour is common practice. Plus, using too much cornstarch can result in an excessively brittle coating, which might be undesirable for a particular dish.
Like any pantry ingredient, cornstarch is just another tool in the toolbox we call our kitchen. Need to thicken a sauce on the fly, with minimal effort, without muddying the color? Use cornstarch. Want a crispy coating that stays crispy in a sauce? Cornstarch is a solid choice. It all comes down to problem solving: How do you get the texture you’re looking for, in the least amount of steps, with the least amount of effort? In many cases, just a spoonful of cornstarch will do the trick.