There is hot chocolateand then there is hot chocolate. These terms are used interchangeably, but there is undoubtedly a difference. Hot cocoa is more like hot, steaming chocolate milk, made from your favorite instant pack or a mixture of cocoa powder and sugar with milk or water.
Hot chocolate is a much richer concoction, similar to hot brownie batter, and so thick it coats the back of a spoon.
Both hot drinks are divine, but there is at least one less intuitive ingredient that contributes to the decadence of hot chocolate. If you’re looking for European-style or Italian-style hot chocolate recipes, the secret ingredient that almost always comes up is… cornstarch.
What is cornstarch?
This white powder is cornstarch (duh) during the refining process. Cornstarch comes from the endosperm of the grain, which is the main energy reserve of the young plant, according to Norman Wagner, a chemical engineer at the University of Delaware. Its chemical formula is C6HtenO5. A molecule composed of carbon with a certain ratio of H2O is a carbohydrate, so the endosperm is full of carbohydrates and energy for the growth of the nucleus. Wagner likens starches in plants to fat in humans: both are stored energy.
Amylose and amylopectin are the two main chemical components of starch. Both are polysaccharides, meaning they are complex sugars made up of multiple sugar molecules rather than a chain of single sugars (monosaccharides). These complex sugars are long chains of molecules, says Wagner Reversebut are rolled up tightly like balls of yarn.
What does hot chocolate cornstarch do?
It’s easiest to see cornstarch work its magic when you make what some call oobleck, a simple slurry of room temperature cornstarch and water. This combo makes a suspension, meaning the cornstarch molecules don’t dissolve and instead float in the liquid. So it’s part solid and part liquid. When you knead this mixture vigorously, the water molecules are trapped in the polysaccharide chains, creating a slimy but solid substance as the molecules seize and cling together. Kneading it slowly keeps it in this strange fluid state.
In fact, the oobleck is a type of non-Newtonian fluid. While water is a classic Newtonian fluid that maintains a constant viscosity no matter how hard it is agitated, the properties of non-Newtonian fluids change depending on the force applied. Ketchup is a non-Newtonian everyday liquid that can come out of a bottle when shaken or squeezed, but doesn’t completely soak our fries.
There are several types of non-Newtonian fluids:
- Thixotropic: force applied over time decreases stickiness (e.g. honey)
- Rheopectic: force applied over time increased viscosity (e.g. heavy cream)
- Shear thinning: The greater the force applied, the greater the less viscous the substance (for example, tomato sauce)
- Thickening by expansion or shear: the greater the force applied, the greater the more viscous the substance (e.g. oobleck)
Although cornstarch in hot chocolate does not create chocolatey oobleck, it does create another type of non-Newtonian fluid.
It all comes down to temperature, says Wagner. Cornstarch, which inside the corn endosperm is a tight tangle of molecular chains, does not unfold in cold water. “If you put the starch in cold water, the water can’t get inside, it stays like a granule, and then you get the oobleck,” says Wagner. Reverse.
Hot water, on the other hand, loosens the carbohydrate chains. “If you put the starch in hot water, the water soaks in and the starch swells,” says Wagner. The cornstarch partially dissolves and the swelling carbs form a “sweet, gooey mess.” The result is a non-Newtonian thixotropic fluid that becomes less viscous the more it is stirred, but retains a pleasantly thick mouthfeel when drunk. The same concept applies if you add cornstarch to hot milk or similar liquids like almond or soy milk.
What’s the right way to use it?
It is possible to create your own instant mix to keep at home by mixing sugar (refined or confectionery), cornstarch and cocoa into a shelf-stable combo. Salt and vanilla are also good additions to homemade powder. Too much cornstarch can turn the drink into a sticky paste. Some recipes recommend about a teaspoon of starch to a cup of milk for optimal thickness, but we haven’t tested them.
A sufficient amount of cornstarch also keeps the mixture together. At the bottom of a cup there is usually a layer of undissolved sugar and cocoa powder that has settled. “That starch can provide thickening of the liquid to make the viscosity really high, so those little particles don’t settle to the bottom so quickly,” says Wagner.
Melting some of your favorite chocolate into this drink makes for an even more sumptuous treat.
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