For a slick, supple mouthfeel, there’s nothing like a suspension of fine droplets of oil in water (or vice versa)—what scientists call an emulsion. Cream, butter and chocolate are emulsions, as are gravy, vinaigrette and cheese. But when an emulsion breaks, the results can get ugly: a layer of clear fat floating on top of the gravy boat, a salad dressing that comes out of the bottle all oil and no vinegar, a plate of nachos covered in greasy goo.
Making one means overcoming some powerful forces of nature. The repulsion between water and oil is electric. A water molecule is unbalanced, electrically speaking, in such a way that a polar charge develops among its atoms. As a result, groups of water molecules form exclusive cliques, aka droplets. Oil molecules, in contrast, are nonpolar and hydrophobic. It takes a surprising amount of force to persuade a polar liquid to mingle with a nonpolar one at an intimate level.
A blender is not always up to the job. The human tongue can detect particles (including liquid droplets) that are just seven to 10 microns across, but blenders generally cannot do better than 10 to 12 microns. When the cooks in our research kitchen were working out a recipe for eggless mayonnaise, they relied on a rotor-stator homogenizer instead. This countertop machine spins a small blade (the rotor) at up to 20,000 rpm within a slotted metal sheath (the stator). Tremendous shear forces rip the droplets down to just a few microns.
For another challenging recipe—a kosher, dairy-free veal “cream”—we tried even bigger iron: an ultrahigh-pressure homogenizer. Our model, which is about the size of a large sink, pressurizes the mixture to as much as 25,000 psi, then slams it into a metal wall to smash it to submicron bits. The result is delicious.
In the finest emulsions, the particles are just a few nanometers in diameter—so tiny the emulsion turns clear. Mountain Dew is a nanoemulsion, for example. To make a transparent nanoemulsion of essential oils from thyme and bay leaf for a chilled chicken soup, our cooks needed a handheld tool because the quantity of liquid was so small.
The solution was an ultrasonic homogenizer, which transforms several hundred watts of power into high-frequency sound waves that induce minuscule bubbles to form in the liquid. These cavitation bubbles then implode, tearing droplets apart as they do. The high-pitched tool gives new meaning to whine and dine.
This article was originally published with the title Making Liquids Go Bipolar.