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
 | To keep ice crystals out of ice cream, some researchers have tried adding antifreeze proteins from polar fish. Chemist Douglas Goff has had promising results with winter-wheat proteins. “In terms of labeling, it’s easier to suggest you’ve used wheat seedlings than fish blood,” he says. |
