The Physics of...Ice Cream

Scientists revolutionize one of the world's most complex foods.

By Robert Kunzig|Sunday, June 27, 2004

Many people are pretty happy with their scraped-surface heat exchangers, but Erich Windhab is not one of them. A tall, cheerful German with a beard, longish hair, and a shiny suit, he does not believe in complacent adherence to tradition. There is no denying the lingering appeal of scraped-surface heat exchangers, particularly at large family gatherings on hot summer days, but Windhab is not sentimental about their output. He likes the stuff his graduate students make better. “When Hans and Matthias are producing, everybody at the Technopark lines up with bowls,” he says.

A scraped-surface heat exchanger is what someone with a Ph.D. in chemical engineering calls an ice-cream freezer. At giant food companies like Nestlé or Unilever but also at university labs like Windhab’s—he’s a professor at the Federal Institute of Technology in Zürich, Einstein’s old shop—there are many Ph.D.’s consumed by the science of ice cream. “People laugh when I tell them,” says Hans Wildmoser, who just completed his degree under Windhab. And yet ice cream, of course, is serious business: Sales of frozen desserts, most of which are ice cream, total about $20 billion and 1.5 billion gallons a year in the United States alone. Americans consume more than 20 quarts per capita every year, second only to New Zealanders.

Sadly for manufacturers, we seem to be saturated: Consumption has actually been dropping lately in both the United States and Europe. The ice-cream frontier lies in places like Asia and especially China, with its billion-plus souls struggling along on about two quarts a year each. Yet as the industry lifts its eyes toward these promised and mostly tropical lands, the view is marred by an old nemesis: heat, or more specifically, heat shock.

Heat shock is the subtle damage that comes long before complete meltdown. To understand the Zürich team’s new scheme for countering this phenomenon, you have to understand what an amazingly complex foodstuff ice cream is—so complex that scientists can’t decide what to call it. An emulsion? A foam? A colloid? Ice cream is all those things, says Douglas Goff, a physical chemist at the University of Guelph in Ontario: It’s a composite structure of water-ice crystals, air bubbles, and milk-fat globules suspended in an unfrozen serum, which contains sugar, flavoring, and milk proteins, and sometimes less appetizing additives.

The industrial freezers that produce this miracle are just high-powered stainless-steel versions of the old hand-cranked device. In the home freezer, you pour the liquid mix of ice-cream ingredients into a cylindrical container that sits in a barrel full of ice and rock salt; the salt makes the ice melt at a temperature low enough to freeze the ice cream. As ice crystals form on the inside wall of the container, you scrape them off and into the liquid with a hand-cranked metal dasher—hence the name scraped-surface heat exchanger.

The goo thickens as water freezes out of it and gets foamier as the dasher beats air into it—commercial ice creams are anywhere from 20 percent air (“superpremium”) to 50 percent air (not so premium). The home process takes about half an hour, and the result is slurpy. An industrial freezer does the job in 30 seconds, using liquid ammonia or Freon at –27 degrees Fahrenheit—and the result is still soft. All scraped-surface heat exchangers bump into a physical limit: As the goo gets more viscous, and you continue to dash it at 200 rotations per minute (don’t try that speed at home), you reach a point at which friction is adding as much heat as the wall of the freezer is removing. Meanwhile, as water freezes out of the mix, the remaining serum becomes a more concentrated solution of sugar and protein, with globules of fat—so its freezing point keeps dropping. The goal of reasonably hard ice cream keeps receding because you can only freeze about 50 percent of the water.

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!’”

NOT-SO-SOFT SERVE

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.”

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