At home you harden the ice cream by sticking it in your freezer. At a factory the soft goo is squirted into cartons and run through a hardening tunnel at –40 degrees F for a couple of hours. Besides being extremely expensive, says Windhab, hardening causes microstructural mischief. He remembers the day a manufacturer first described the process to him. “I said, ‘That’s nonsense what you’re doing!’”
Courtesy of Erich Windhab/ETH Zürich
When ice cream comes out of the high-tech freezers at the Federal Institute of Technology in Zürich, it is much firmer and colder than normal—around 5 degrees Fahrenheit—with finer ice crystals and smaller bubbles. That means it’s very creamy and will stay creamy, without being too fatty.
For smooth, creamy ice cream, he explains, you want the ice crystals and air bubbles to be as small as possible. Big ice crystals make the ice cream feel coarse, while big bubbles make it collapse in your mouth—a cold and watery mess. The ice crystals and air bubbles, however, tend to grow as large as possible. It’s the path of least energy: Big crystals have a lower surface energy, and big bubbles have a lower internal pressure than small ones. When ice cream finishes freezing in a hardening tunnel, far from the leveling agitation of the dasher, this insidious process gets a head start. The product that emerges may taste delicious, but it is often perilously close to having a major texture defect.
The peril increases sharply as the ice cream travels in and out of delivery trucks, in and out of supermarket freezers, which are often overfilled (“Always take from the bottom of the freezer,” says Windhab’s student Matthias Eisner), and finally—unless you are one of those finicky microstructuralists who always eat the whole pint at one sitting—in and out of your freezer at home. With each heat shock, small ice crystals melt, small air bubbles consolidate, and water and air molecules migrate through the serum. Back in the freezer, they attach themselves to the bigger crystals and bubbles, which thus grow bigger still. Pretty soon you’re feeling the grainy ice crystals or even looking at a layer of frost.
Manufacturers have been battling heat shock for decades. Many add a stabilizing agent, such as locust bean or xanthan gum, which thickens the serum and slows the diffusion of water and air. But Windhab’s invention, which you can now test by picking up a carton of Dreyer’s or Edy’s Grand Light, doesn’t rely on additives. In his ice-cream freezer, the mix is pumped into a space less than an inch thick along the wall of the freezer barrel. There, instead of being scraped and whipped by the blades of a furious dasher, the serum is methodically kneaded and churned between the threads of two large parallel screws, rotating at no more than 15 rpm. Because the screws add so little heat, the ice cream is extruded at the other end at a temperature of around 5 degrees F—fully frozen, with no need for hardening, and yet still plastic. “You can make pretzels out of it,” Wildmoser says.
The extruded ice cream starts out with a microstructure so fine, says Windhab, that it can resist heat shock for several months longer than its conventional competitors. But Dreyer’s is selling Grand Light on a different basis—as a low-fat ice cream that tastes as creamy as the real thing. Creaminess, as a mouthfeel, is not the same as fattiness, Windhab says; it’s viscosity without stickiness. And it’s the air bubbles in ice cream that, by interrupting contact with the tongue and palate, prevent stickiness. The function of the tiny fat globules, which sit on the surface of the bubbles and link them, is just to stabilize the foam. “If you manage to create the air cells very small, so that the foam is stable longer in your mouth, you don’t need the fat globules,” Windhab says. “The air itself is giving it creaminess.”
Sounds like magic—but it is very much science, which was why it took Windhab a long time to persuade the food industry, with its deep roots in culinary empiricism, to take him seriously. “People say, ‘These guys at the university, let them play,’ ’’ he says. But ice-cream science isn’t all fun and games. In Windhab’s lab there is a large trunk freezer with a stern sign: “Don’t Eat!” “That’s a problem we have,” says Eisner. “People are always eating our samples.”