“Did you know Penn State has professors of ice cream?” I wrote to Jane Alexander, my editor at Science 80. It was the only one-sentence query letter I ever wrote. She responded to the idea the same as I did. “You gotta do the story!” she said on the phone. And so I did.

The Cold Facts About Ice Cream

By Terry Dunkle

My father always bought the cheapest ice cream. Plopped out of a trigger scoop on Saturday nights, it refused to melt, and was so puffed with air that it made all seven of us burp. There was a clown on the carton. Every bite produced a flash of cheap vanilla that lingered on the tongue until bedtime.

My enlightenment came on a fresh summer morning when I was 12. Sitting on the front stoop of a corner grocery in Jersey Shore, Pennsylvania, I opened the flaps of a pint of snow-white vanilla and plunged a wooden spoon into it. The ice cream felt incredibly cold on the teeth—there was an element of terror in the beginning. Then a pleasant collapse and the vanilla came up, slowly at first, rising exponentially, until the whole mouth belonged to it, was filled as a cave with warm vapors. Rapidly it faded, preparing me for a fresh surprise.

The ice cream, Hershey's, was by no means the best I have eaten. Every city in America has two or three brands at least as good, just as every city has clown ice creams. The difference is not always price. A survey in Chicago, for example, rated one of the cheapest ice creams best, the costliest “awful.” It is a question, says Philip G. Keeney, of how well the manufacturer understands the science.

Keeney, a professor at Pennsylvania State University, has published dozens of papers on ice cream flavor and structure. During the past 27 years he has influenced a great many of the largest ice cream companies in the world, for he has taught more than 1,000 of their technicians and executives in Penn State's annual course in ice cream technology. He does consulting work on three continents and has testified before Congressional hearings. “If you haven't heard of Phil Keeney,” one executive told me, “you really don't know your ice cream.”

At our first meeting, Keeney, sitting behind a Formica-topped table under portraits of cows, was wearing a red, blue, green, orange, yellow, pink, and purple checkered shirt, brown corduroy pants, and Earth shoes. A haystack of pewter hair covered his ears and the stems of his big plastic-rimmed bifocals, which magnified his eyes. He is 56. We were eating ice cream cones bought at the campus creamery, on the other side of the building. Keeney held the cone with his fingertips and licked it rapaciously, with thoughtful pauses between licks. It was vanilla.

“You'll notice I lick,” he said. “Most people do. It's fortuitous. The tip of the tongue is where we detect sweetness. That's the first flavor note I perceive when I eat an ice-cream cone. Then the texture. If the ice crystals are bigger than 20 microns—a thousandth of an inch—touch receptors in your gums and the roof of your mouth will detect them. Sometimes, if you do the bite test, you can actually hear them. Just bite down with your teeth. Go ahead.” He waved the cone at me.

The ice cream yielded without a whisper. It tasted smooth and fresh. There were notes in it of fresh milk and cream.

“Four days ago that ice cream was grass,” Keeney explained. “We have certain advantages here. Our longest milk route is seven miles; the other one's six. We measure the distance from factory to retail in feet. Your cone's dripping.”

We both took a lick.

“Now, you say fresh,” Keeney said. “Was it in the aftertaste? That's where the true glory of an ice cream shows. See, as the ice cream begins melting in the mouth, some of the volatile compounds begin to boil. Vanillin, for instance, will emit vapors almost as soon as it hits the tongue. The vapors waft up behind the soft palate and get into your nose by the back door, so to speak. The sensation shifts from taste to a combination of taste and smell.” He clipped the cone with his teeth. “That's what flavor is, really. Without smell, an apple and an onion taste alike. Try it next time you have a cold.”

Vanillin (accent on the van), the most common form of artificial vanilla, is a simple compound often made by treating wood pulp with sulfuric acid. It strikes a single flavor note. Real vanilla, more complex, plays a delicate arpeggio as it melts in the mouth. Extracted from the bean of a tropical orchid, it contains not only natural vanillin but also dozens of other volatiles, each with a different boiling point.

“Real vanilla only costs about a nickel extra per half-gallon,” Keeney said, “but your average manufacturer uses a lot of artificial. The customers won't pay the difference. They think, 'Ice ceam's ice cream.' They just don't listen to their taste buds.”

He popped the last bit of cone into his mouth and, chewing noisily, picked up his attaché case and walked out of the room. “Got a class,” he said from the hall.

I am standing in the compressor room of an average-sized ice-cream factory, surrounded by engines squeezing hot ammonia gas into liquid. Their sound, like the rumbling of locomotives, shakes the concrete floor and vibrates the cords in my neck.

I follow the ammonia pipes along a catwalk and down a flight of stairs to a blue metal door labeled FREEZER ROOM. Inside, it is quiet. The room shines with stainless steel and pink ceramic tile. Giant storage tanks feed into a tangle of pipes and machines receding toward the other end. No workers are in sight. A Quantobatch 4600/1 computer, red numerals glowing on its panel, instructs the ingredients to mingle and flow to the pasteurizer, which cooks harmful bacteria, then on the homogenizer, which pumps the mix to 100 times the pressure of an automobile tire and sprays it through a tiny valve to shatter the fat globules so the cream won't float to the top. The mix is ready.

The freezer, a block of stainless steel as big as an upright piano, has three shining cylinders jutting from its face. It works on the same principle as an old-fashioned hand-cranked freezer, but using liquid ammonia instead of rock salt and ice. As the mix travels down each cylinder, the ammonia squirts into a chamber surrounding it and suddenly boils because of the pressure drop. The ammonia cools the mix by drawing heat from it through the cylinder walls. As frost forms, spinning dasher blades keep shaving it off and stirring it back into the mix. Within 30 seconds the mix becomes stiff and smooth as toothpaste. As the ammonia recycles, the ice cream shoots continuously into a two-inch pipe feeding a filling machine.

The filler spits exactly enough ice cream—Chug!—into a half-gallon carton that only a moment ago was a flat sheet of cardboard. The carton advances. A lever jumps up—Clack!—closes THIS TAB FIRST, and—Clack!—retreats. Metal guides worry the ears together, and a box of ice cream marches into the hardening room.

This has been happening—Chug! Clack! Clack...Chug! Clack! Clack!—for several minutes now, a half-gallon springing into existence with every heartbeat. No one else is here. The operator, standing behind a picture window overlooking the floor, is talking on the telephone.

The first factory-made ice cream in the world, marketed by Jacob Fussell of Baltimore in 1851, probably contained only fresh milk and cream, sugar, eggs, and natural flavoring. These are the classical ingredients. Today, a typical supermarket brand contains “milk fat and nonfat milk, sugar, corn sweetener, whey, mono- and diglycerides, guar gum, Polysorbate 80, carrageenan, and natural and artificial flavor,” to quote one label. Many lists are longer. Because recipes fluctuate with the ingredient market, the same brand may have a different composition from week to week. The trick is to make them all simulate the same ice cream.

Essentially, ice cream is air, oil, and ice crystals suspended in water. The water never completely freezes, because an antifreeze—sugar—is dissolved in it, making a stronger, more cold-proof syrup as more water freezes out. At its optimum dipping temperature, about eight degrees Fahrenheit, a half-gallon of ice cream contains nine tablespoons of liquid.

Hanging in the liquid are billions of tiny air bubbles and quadrillions of even tinier oil droplets. Because oil and water don't mix, these fat droplets would rather clump together and float to the top, but they can't. They are jacketed with negatively charged proteins, which make them repel each other. Similar forces keep the bubbles in place. Ultimately, ice cream is held together electrically.

The simplest way to give customers more ice cream for their rmoney, says Keeney, is to pump in more air. It's easy. Just turn a silver knob on the freezer and the ice cream swells. The manufacturer must be careful, however, not to go over the legal limit of 50-percent air. By federal law, a half-gallon of ice cream must weigh at least two pounds, four ounces. Most brands weigh close to the limit, but expensive ones like Häagen-Dazs may be nearly twice as heavy.

Originally, air got into ice cream as an accident of stirring, but today it is ushered in by pumps and metering valves. “Ice cream is a frozen foam,” says John Speer, president of the ice-cream manufacturers' lobby, on the phone. “My God, you need that air in there! Try squeezing the air out of a loaf of bread sometime, see what you get.”

It's true, some air is needed in ice cream or it won't collapse readily in the mouth. The bubbles should burst like tiny balloons when the heat comes up. But too much air can insulate the interior, slow the meltdown, and make the ice cream feel strangely warm—more like Dream Whip than ice cream. Expensive, quicker-melting ice creams feel colder.

Occasionally, overinflation backfires. The bubbles overstretch their skins and burst while still in the carton. The ice cream shrinks from the walls of the containers, prompting phone calls from angry consumers.

Recently, Keeney conferred with a manufacturer who had been trying to ship cylindrical cartons of ice cream into Colorado. Whenever the trucks climbed into the Rocky Mountains, the air bubbles expanded, puffing up the ice cream and blowing off the lids.

“Try underfilling,” Keeney told him. “But don't sell that ice cream at sea level.”

Milk fat, the prime ingredient of ice cream, adds smoothness and richness by lubricating the palate and suffusing it with dozens of volatiles. Found in cream, it is one of the most complex of all natural oils, comprising more than 150 different fatty acids. To make a quart of cream, a half-ton cow must eat 30 pounds of feed and filter 2,500 gallons of blood through her udder. Because of this and because cream keeps only a week or two under refrigeration, it is by far the costliest ingredient.

“You don't have to get your milk fat from cream,” Keeney told a class of 85 ice-cream technicians. “In fact, you could make a pretty decent ice cream without any milk fat at all. Any kind of oil will do—vegetable oil, mineral oil, even motor oil, if that's what customers want.”

Pencils were scratching on notebooks.

“But it's illegal,” Keeney continued. “If you want to label it ice cream, you've got to use milk fat. You can use butter, which is just milk-fat globules pressed into a cake. Butter will keep for months without much deterioration, and sometimes it'll give you a little more milk fat for your money. Go ahead and use it, but be careful: The longer any milk product is stored, the greater the chance it'll lose flavor notes or develop foreign ones. Some of these stinky compounds in stale milk can be tasted in parts per trillion. That's one drop in all the ice cream produced in the United States in a week. You're gonna be a lot more confident with fresh cream, believe me.”

Last summer, Keeney went into a local supermarket and stuck a thermistor probe into the heart of a half-gallon of ice cream. The result was a long chart recording with a squiggle of green ink down the middle. “You can see how the temperature fluctuates,” he said, running a finger along the line. “This causes a serious problem: heat shock."

Heat shock, he explained, is the culprit in crunchy ice cream. "Every time the temperature in the display case goes up, even a tenth of a degree, some of the ice crystals melt. When it drops, the melt water refreezes—but not at its original site. It migrates to larger crystals and freezes there. So, as the ice cream sits in the cabinet, the little crystals fade away and the big ones get bigger and bigger. The same thing happens in your refrigerator—only worse, because you're always opening the door and taking the carton out."

To fight heat shock, manufacturers add stabilizers—carrageenan, guar gum, sodium carboxymethylcellulose, and other gel-forming compounds—to trap the melt water like a sponge, preventing it from migrating. Unfortunately, even the weakest gels, by slowing the release of volatiles, can muffle important flavor notes or alter their sequence. “The less stabilizer, the better,” Keeney said.

Until recently, warehousing at 10 or 20 below zero has kept the need for stabilizers at a minimum. “You're going to see this change,” Keeney said. “As energy costs go up, so will storage temperatures. I can make you an ice cream right now that'll keep for six months at 20 above. The trouble is, you won't like it. It has a funny mouth feel. Gummy. I suppose the public could get used to it gradually, though.”

Could they?

“Well, maybe,” he said. “Look at what's happened to fat content in the past 20 years. It used to average 12 percent; now it's close to 10, and hardly anybody complains. Well, hell, look at whey. We've been putting whey in ice cream for two decades now, and nobody even asks what it's for.”

Whey is the cloudy, bluish fluid left over when milk is made into cheese. For centuries it has been dumped down drains. Emptying into lakes and streams, it causes algae and bacteria to flourish and use up oxygen, smothering the fish. Since the 1960s, strict water-pollution laws have made throwing it away expensive. Rather than paying to have it treated, the cheese companies dry it and sell it to the ice-cream makers, who use it to raise their milk-solids content to the legal minimum.

Unforunately, whey also adds salty, metallic flavor notes. “Nobody uses whey for positive reasons,” Keeney said. “They use it because it's cheap and it's allowed. A lot of guys in the industry will get mad at me for saying that, but they all say the same thing when nobody's around. In a way, ice cream has become the sewage-treatment plant of the cheese industry.”

A dozen of Keeney's students gather in the freezer room of the campus creamery to try out the ingredients we have been hearing about all week. We have whey, two kinds of vanillin, carrageenan, mono- and diglycerides, guar gum, corn sweeteners, sodium carboxymethylcellulose, and butter. We have Crisco oil, to make the non-dairy ice cream that has become standard in Great Britain. We have plenty of air. Our freezer, almost identical to the one in the factory, feeds a drainpipe opened over a milk can sitting on the floor.

“Now, I want you all to taste these,” says Keeney, standing at the freezer barrel. White plastic spoons peek out of the breast pocket of his sport coat, and a little paper hat sits on top of his hair. Our own hats nod in agreement.

Keeney signals the creamery superintendent, who throws a switch and begins dumping mixes into the hopper. Hissing and humming, the machine extrudes a long, wet column of ice cream that keeps breaking off and splatting in the can. We press forward with our spoons.