The Scoop on Ice Cream

You think ice cream is simple? It's really a complex chemical cocktail, each lick of which sets off a physical and sensual explosion.

By Lawrence E. Joseph|Saturday, August 01, 1992
Take a lick of vanilla ice cream: sweet and cool, as expected, but with little fragrance. Surprising, since vanilla normally has such a distinctive scent. But wait a beat; then suddenly there’s the familiar aroma, the one that, if you go back long enough, schoolgirls once used as perfume.

That wait was just long enough to allow all hell to break loose inside your mouth. The ice cream is now literally exploding. Air bubbles are bursting from the heat of your tongue. A volatile compound, in this case vanillin, is boiling. Sweet scents are wafting their way up your nasal cavities, where olfactory cells send your brain the belated good news.

Take another lick, and touch off another explosion.

Ice cream is virtually the only food we eat frozen, which means that its flavor, which we define as a composite of taste and smell, is only fully released upon melting, explains Arun Kilara, a 43-year-old professor of food science at Penn State and one of the world’s acknowledged authorities on ice cream. Kilara is the leader and principal lecturer of an annual two-week industry seminar on ice cream run by Penn State’s Department of Food Science. The seminar was started a century ago as a winter course on milk processing for local dairy farmers. Today it is widely regarded as the foremost forum on the science of ice cream, and it has been presented to thousands of dairy professionals on four continents and to tens of thousands more in the form of a correspondence course. Perhaps its proudest graduates are a pair of ice cream devotees named Ben and Jerry, who founded their eponymous business after taking the correspondence course. Years later they shut down their Springfield, Vermont, plant for two days, so that all 100 employees, from floor cleaners on up, could attend a special version of the course, taught right on the premises.

To mark the one-hundredth anniversary of both the course and the Penn State creamery--the department’s laboratory--ice cream makers from the likes of Kraft General Foods, Nestlé, Carvel, Borden, and Häagen-Dazs have come for a packed schedule of lectures, lab sessions, and nightly homework assignments. Their mutually incompatible goal: to scoop the competition in the yearly $9.3 billion U.S. market for frozen dairy products, which include ice cream, ice milk, frozen yogurt, sherbet, and other cold delights. In terms of gallons produced, ice cream makes up about 56 percent of this total; it is so popular a dessert that the average American family eats it once a day. It is also eaten in nearly every other country in the world, though Kilara adds that in regions where, as he puts it, the milk culture is lacking, particularly in China, ice cream is considered rather more of a delicacy than it is here.

Though it’s a bitterly cold winter day, Kilara is handing out samples of his trade: creamery brand Peachy Paterno, named for the legendary football coach who stops by for a cone before each home game, and Teaberry, a western Pennsylvania delicacy faintly reminiscent of Tetley. Four days ago this ice cream was grass, Kilara says, intoning the standard freshness slogan of the creamery’s 150,000-gallon-per-year production facilities.

The tasting today is preliminary to a tour of the plant. After finishing their ice cream and donning white gauze hairnets, the members of the group are led by Kilara into the scrubbed-down, hosed-off, stainless- steel-fixtured room where milk is piped in fresh from the farm. In one vat cottage cheese is curding, in another cream cheese is being squeezed. Kilara takes the opportunity to expound the basic dairy lesson: it all begins with milk.

Cow’s milk averages about 87 percent water. Another 4 percent consists of milk fat, with the balance composed of protein, lactose (milk sugar), and ash. Fresh from the cow, each drop of milk contains about 10 million fat globules, each with an average size of .1 micron, or .000004 inch, and each wrapped neatly in its own phospholipid membrane.

Milk comes from the udder superbly emulsified, says Kilara in homage to the cow. He explains that an emulsion is a stable mixture of two liquids that usually don’t mix--like the fat and water in milk--and are held in suspension by substances that prevent them from clumping together. Emulsification is so elegantly achieved inside the cow because the membranes surrounding the fat globules are negatively charged and so repel one another, thus naturally keeping themselves nicely dispersed.

Milk fat is the essential substance of ice cream, responsible for its consistency, richness, and taste. (It’s also, of course, the chief culprit responsible for adding inches to our waistlines.) One of the most intricate and exquisite oils found in nature, milk fat mixes more than 150 fatty acids into a smooth blend. Though other fats--vegetable oil, mineral oil, even tallow--could theoretically be used in ice cream, in order for a product to be called ice cream its fat must come only from milk. Federal law requires that ice cream include at least 10 percent milk fat, two and a half times the fat content of the milk from which it’s made.

Not surprisingly, few true ice cream connoisseurs are fond of the industry’s use of fat substitutes, such as the complex protein found in NutraSweet’s Simplesse. The search for the perfect fat substitute, Kilara says, is like a contemporary version of alchemy--lots of useful discoveries, but they’ll never turn lead into gold notes. While some protein-based fat substitutes approximate fat’s texture, or mouth feel, he explains, they cannot dissolve flavor compounds in the same way. When fat melts, flavors are released in a characteristic sequence, giving ice cream its distinctive bouquet. In low-fat and fat-free formulations, the flavor release is faster, shorter, and simply less pleasurable.

Along with milk, cream, flavorings, and sugar, the typical ice cream mix also includes stabilizers--compounds that bind water molecules and keep them from freezing--and emulsifiers, which facilitate the dispersion of the milk fat. Curiously, the same industry purists who demand strict federal standards of identity for frozen dairy desserts also insist on the right to include all sorts of exotic chemicals in the products they deem pure. Not so much in the case of sweeteners, which, except in specific dietetic formulations, are usually simple mixtures of cane or beet sucrose and dextrose corn sweetener. But what about all that other gunk? Are such stabilizing gels and gums as those made from locust beans, guar gum, carrageenan (Irish moss extract), crystallized methylcellulose, gelatin, and alginates (obtained from ocean kelp) truly needed in such a basic food as ice cream? Why make chemical cocktails out of what should really just be snowy white vanilla?

The answer is to prevent heat shock, the trauma that ice cream suffers when it is allowed to melt and refreeze. There’s nothing to beat fresh ice cream, Kilara says with a smile. We used to make it at home in New Delhi. But few people make homemade ice cream these days, and not all that many buy it fresh-dipped. With the rise of the suburbs the market has shifted to packaged products, which are shipped, stored, and subjected to all kinds of temperature stress. That’s where food science comes in, to make a product that can survive distribution and still be as good, or almost, as if it were made in the kitchen.

When ice cream suffers heat shock during transport or storage, its ice crystals grow larger, making the refrozen product grainier. The threshold of perception is 20 microns, or about one-thousandth of an inch, at which point the offending particles can be sensed in the mouth. Stabilizing gels and gums minimize the traumatic effects. Although rarely accounting for more than half a percent of the mix by weight, these hydrocolloids--a group of molecules that can bind many times their weight in water--employ complex branch structures that work like gummy tentacles to trap water molecules and prevent them from clumping together. The more stabilizer added, the smaller the ice crystals and the smoother the ice cream.

If stabilizers are so necessary, why don’t Breyers and Häagen- Dazs, to take two top-notch examples, need them? Breyers is absolutely first-rate fresh from the package, says Kilara. They superheat condensed skim milk, which breaks down the milk protein so that it acts as a stabilizer, then add it to their mix; that process also gives Breyers its distinctive cooked flavor. But though the ice cream keeps well initially, after several days in and out of the freezer its ice crystals expand and puncture the air cells like needles popping balloons, he says.

Häagen-Dazs is different. The high fat content, about eighteen percent, has a stabilizing influence. And it has much less air than other ice creams. Kilara explains that federal standards permit ice cream to be up to 50 percent air, a doubling in volume known as 100 percent overrun. But superpremium ice creams such as Häagen-Dazs are much denser, as low as 20 percent overrun, and therefore suffer less from air-cell collapse. The additional fat in superpremium ice creams also acts as an insulator, which is why expensive, more fatty ice creams are warmer to the bite. Finally, since superpremiums are usually sold in pint containers, rather than typical half-gallons, the contents may be gone long before heat shock takes over.

Heat shock poses the greatest threat to high-water, low-fat frozen desserts, many of which would not be possible without the new- generation stabilizer microcrystalline cellulose. MCC is a highly purified cellulose gel that fights heat shock by seeding the dessert mixture with millions of tiny particles; these particles serve as nucleation points, solid centers around which minute ice crystals tend to grow. Food scientists cannot yet explain why MCC, unlike other stabilizers, amplifies flavor, a mixed blessing since stale ingredients and impurities may show up. Certain milk off-tastes can be sensed in concentrations as low as a few parts per trillion--equal to one drop per all the ice cream produced in the United States in a week.

After stabilizers are added, the mix is pasteurized--heated at 175 degrees for 25 seconds to kill bacteria. But since heating melts the milk fat, the mix must then be mechanically homogenized, or broken down into tiny particles. Homogenizers are high-pressure pumps that force the mix through a small valve in which the fat globules are broken up and dispersed. Pressures reach about 2,500 pounds per square inch--70 times the pressure of an automobile tire. Then the mix is cooled to just above freezing and stored in refrigerated tanks for at least three to four hours, or preferably overnight. This gives the milk fat a chance to recrystallize into a uniform texture. (Crystallization gives off heat, which must be removed before the next stage of the freezing process.)

Near the storage tanks is a stainless-steel tub filled with yogurt, a product some ice cream traditionalists like Kilara view with a touch of frustration. The type of yogurt packaged in a cup and culturing in their vat has strict federal standards of identity. The first is acidity, which gives yogurt its tart taste and comes from bacteria that are converting milk sugar into lactic acid. Federal law says at least .9 percent acidity must be present in cup yogurt. The second standard is the presence of bacteria, microflora that are believed to aid in digestion and perhaps also to produce antibiotics that inhibit some disease-carrying organisms. But for frozen yogurt, alas, there are no such standards.

There is a transference of the perceived health properties of cup yogurt that just isn’t warranted with most frozen yogurt, says Kilara, weary of battling the yogurt mystique. He explains that marketing studies have often shown that a majority of American consumers do not find yogurt’s acidic taste appealing, but like the proverbial dose of cod liver oil, that very tartness has taken on a desirable medical connotation. What irks Kilara is not the healthfulness associated with cup yogurt, but consumers’ belief that these benefits exist in frozen yogurt as well. Apparently, consumers want to eat yogurt without really having to taste it. Frozen- yogurt-industry honchos have suggested a minimum acidity level of only .3 percent for their product, which Kilara curtly dismisses as below the threshold of perception.

The frozen-yogurt chain TCBY, which now officially stands for The Country’s Best Yogurt, used to stand for This Can’t Be Yogurt. That sums up the situation nicely, concludes Kilara.

At this point in the creamery tour the group stops to inspect a new dispensing machine for soft ice cream. This beloved treat of many a childhood outing is made with less fat and sugar than hard ice cream, is served at 18 degrees instead of 0 degrees, and accounts for almost 25 percent of the annual gallons sold. But health regulations require that dispensing machines be cleaned daily, a cumbersome process that takes a minimum of two hours. The new machine, called the Taylor Labor Saver, circumvents that requirement by automatically sterilizing its mechanism and contents. It’s expensive but well worth the labor savings, Kilara says.

From the storage tanks the ice cream mix is usually fed through the flavor tanks, although some flavorings, like nuts and fruit, are added after the ice cream is frozen. This maintains the identity of the chunks and protects the machinery. Federal standards are again explicit regarding flavoring: to call a product, say, chocolate ice cream, its flavor must be derived wholly from a common, natural source, such as the cacao bean. Chocolate flavored indicates a mixture with more natural than artificial flavoring, while artificially flavored chocolate means the reverse. It is an irony of the business that while flavor is the most important factor when consumers choose ice cream, to ice cream makers, who do little more than select among suppliers’ offerings, it seems something of an afterthought.

At the center of the room sits the freezer, the icon, the challenge that defies rational expectations: ice cream mix must be whipped and frozen simultaneously. Ice cream freezers are essentially ammonia- cooled churns, stainless-steel barrels with whirring blades inside called dashers and a compressor that forces air into the ice cream. At this stage two types of emulsifier--which enable the milk fat to disperse evenly through the water--may be added to facilitate whipping: monoglycerides, the natural products of fat hydrolysis, and polysorbates, which are not found in nature but are chemically similar to biological membranes in that their molecules are hydrophilic (soluble in water) at one end and lipophilic (soluble in fat) at the other. The longer the whipping, the better the added flavorings will dissolve. But add too much emulsifier or churn too long, and the ice cream mix will begin to collapse into buttery chunks-- just like what happens to whipping cream that is beaten past the point where it forms stiff peaks.

Each time a freezer barrel is injected with ice cream mix, which enters at about 35 degrees, the surrounding chamber is filled with liquid ammonia, which boils from the sudden pressure drop, thus drawing heat out through the barrel walls. During the mix’s minute or so in the freezer, its temperature will plunge past the ice cream freezing point of 27 degrees, to as low as 21 degrees.

The smaller the ice crystals, the smoother the ice cream, says Kilara. You get the smallest crystals when the drop in temperature is the most rapid and when agitation is most vigorous. He says that by the time the ice cream exits, 50 percent of its water should be frozen. But what goes on inside the freezer barrel can never be directly observed. Perhaps, for all its knobs and dials, the freezer should not be operated as a machine but instead played as an instrument.

You begin to manipulate the parameters of temperature, pressure, and air to achieve optimal stiffness and consistency, and you think all the adjustments add up, but when you check the product, it’s just not right, says Kilara. Indeed, he adds, the problem of inconsistency plagues manufacturers throughout the industry, yielding a product far less uniform than consumers might assume. Kilara has been thinking of turning to chaos theory for answers: There are repetitive patterns in seemingly random events . . . he mutters, idly twiddling a pressure valve.

The creamery’s freezer, with a capacity of 300 gallons per hour, feeds a filling machine that spits a precise half-gallon onto flat cardboard boxes, which are then mechanically bent, tabbed, and sealed. Each half-gallon, in accordance with federal law, will weigh no less than 2.25 pounds. The cartons are then sent to the hardening room, where the temperature drops to about -30 degrees, freezing another 40 to 45 percent of the water. (The water never freezes entirely because the remaining solution, mostly sugar, becomes a concentrated antifreeze.)

In the supermarket, freezers store ice cream at about -10 degrees, ten degrees colder than other frozen foods. That is why ice cream is not included in the frozen-food category, Kilara says. The frozen-food industry has tried many times to take us over, but that will never be.

At home, of course, freezers and habits vary enormously, and they all affect how your ice cream is going to taste. There’s one basic truth about ice cream--its quality begins deteriorating from the moment it is made, Kilara concludes. Over the product’s lifetime, ice cream’s air escapes, its fat clumps, its ice melts, and its water freezes.

Kilara’s best advice: Eat your ice cream quickly--and often.
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