A scientific search for fried potato perfection.


Written by Emily Sohn • Illustrated by Claire Emery


The perfect french fry means different things to different people. Circle the globe, and you’ll find as many opinions as there are countries. In France, chefs fry them twice. In Belgium, street vendors serve them with mayonnaise. French fry cravings fuel more than 25,000 McDonald’s restaurants in 118 countries. And the food’s popularity continues to soar.

Behind the scenes of this global fry affair, a group of scientists is hard at work, prodding and probing french fries, potato chips, and other fried foods to understand what makes some irresistible and others inedible. They already know that no fry is worse than a soggy fry, except maybe one that has been burnt to a crisp. The question they are trying to answer is: What creates the perfect balance?

Until the early 1990s, most frying research was observational. People would fry foods for different lengths of time and then record how their flavors and textures changed, according to Paul Singh, a food engineer at the University of California, Davis. He was one of the first to scientifically pick apart the process behind those various shades of golden brown.

Now, in labs around the world, more than 200 engineers and chemists dig into the structure of the french fry like geologists studying the layers of the earth. They are turning an art into a science, using mathematical formulas and predictive computer models to describe the development of a deep-fried potato. Their work may eventually propel the french fry’s already lasting and celebrated career into a golden age of perfection.

Strong economic and social pressures lurk behind the quest for the scientific secrets of the french fry. Fried food production is an international, multibillion dollar industry. Fast food chains and industries race to make food "better, faster, and cheaper," according to Michael Blumenthal, once an oil chemist at Rutgers University in New Jersey. Now he runs a New Jersey-based consulting company that feeds its fried food research results directly into the industry’s hungry hands.

Health is one major concern. Oil gives fried foods their smooth flavors and pleasing textures. But as much as 20 percent of a fry’s calories come in the form of fat, and a potato chip is more than 30 percent fat. Still, it’s the fat that makes them taste good, Blumenthal says, and that means they sell. Research into fried foods could inspire engineers to create new fryer designs or techniques that maintain flavor but reduce fat.

For a population plagued by diet-related health problems, such as heart disease and diabetes, this may be just the kind of science our lives depend on. And industries haven’t overlooked the profits they could stand to gain by developing a lower-fat fry.



Improving a process begins with understanding it, says Brian Farkas, a food engineer at North Carolina State University. And understanding the french fry is a challenging feat.

"It’s so complex, it’s beyond imagination almost," Farkas says. "That’s what makes it so cool." Farkas began his work on fried foods in 1990 as a graduate student in Singh’s lab, unwillingly at first. "Fried foods? I don’t even eat fried foods! Give me something else!" he pleaded with Singh, during the formative stages of his Ph.D. topic. But when he saw how few people were working in the field and how many interesting questions remained unanswered, the allure of french-fry science won him over.

"You start with a white stick of potato, and out comes this enormously desirable product," Farkas says. It’s golden brown. It’s crispy on the outside, soft and steaming on the inside. "It’s one thing to say, ‘Wow, that thing really heats up fast,’ or ‘Boy, it really gets hot,’" Farkas says. "But it’s another to actually tack a number on it and say, ‘Well, it heats up fast. Well, how fast?’"

So what exactly does it take to create a perfect french fry?

To begin answering that question, scientists are discovering that they must look at the potato before frying even commences.



In the elusive art of french fry preparation, every step is important.

"You cannot just take any type of potato and make a french fry out of that," Singh says. Each variety’s unique ratio of water to sugar affects the final outcome. Eat at a McDonald’s in India, and your french fry will taste just like one in Sacramento off U.S. Interstate 5. The company works hard to satisfy its customers’ number one demand: consistency. In farms around the world, it grows only one kind of potato under very specific conditions.

Most restaurants, including McDonald’s, use Russet Burbank potatoes, which tend to be large and elongated with few eyes and skin that peels off easily. Fast food potatoes require quite a bit of manual preparation, from peeling the skin to removing the eyes to dipping evenly chopped potato strips briefly in hot oil so they don’t stick together when frozen and shipped. Using pre-fried food also saves a restaurant money, reducing the amount of energy needed to finish the job.

When they finally make it to your rest-stop restaurant of choice, and you say, "Yes, I’d like fries with that," the processed potato strips are plunged into hot oil, and that’s when transformation begins.



The most important factor that distinguishes a potato from a french fry is its layers. A raw potato is 100 percent "core," with no crunchy crust to please the palate. A potato chip, on the other hand, is 100 percent crust, crisp all the way through. Somewhere in between lies the fresh french fry.

Once a potato strip is immersed in hot frying oil, a crusty front immediately forms on the raw fry’s surface and begins moving inward, like the hardening crystal edges of a lake in early winter. From that point on, drawing the line between the crust and the core of a frying potato is like shooting at a rapidly moving target. It’s nearly impossible.

On a typical day at McDonald’s, oil temperature in the fryer averages about 340°F. So when a cook grabs strips of icy potatoes out of the freezer and tosses them into hot oil, water in the potatoes immediately begins to evaporate. Bubbles and steam emerge, creating an enormous cycle of "heat transfer" between the potato and the oil. The process, Farkas says, may be the most important factor in producing the texture of the final fried product.

Heat transfer between potato and oil happens rapidly at first. As water evaporates from the surface of the potato, pores form on the potato that act as windows for the penetration of oil into the food. The more water the food contains, the more oil it will absorb, because more water evaporation opens more windows for oil to penetrate.

Temperatures at the crust quickly rise and approach the temperature of the surrounding oil. Protected by the crust, however, the core’s temperature hovers at about 212°F, the boiling point of water, even in the midst of much hotter deep-fat frying oil.

After exactly three minutes in the fryer, the crust of a McDonald’s fry measures between one and two millimeters thick, about the same thickness as a paper clip viewed sideways. But if a distracted worker or daydreaming cook leaves his french fries in the hot oil too long, his customers will send back piles of burnt crisps. They ordered french fries, not potato chips, and any real fry has a soft core.



Baking, steaming, or boiling will never create that essential contrast between the crust and core of a deep-fried food morsel. Crunchy cheese sticks, melt-in-your-mouth beer-battered onion rings, and golden crispy french fries share one main feature in common: They have been fried in tubs of hot oil.

One of the oldest cooking methods in existence, deep frying has universal appeal because it produces foods with a unique set of textures and flavors. Oil can get much hotter than water without boiling, so it cooks food faster and kills dangerous bacteria in the process.

But in the field of oil research, science still borders on art, say most oil researchers.

Still, their dedication has produced a wealth of useful information about how oils change during frying, and how different kinds of oil affect a food’s outcome.

Today’s fryers use liquid oils, like soybean and canola. But not too long ago, food was fried in solid fats like beef tallow, according to Blumenthal. The switch helped satisfy demands for more healthful food. At the same time, however, it added some messy problems to the process.

Oils break down during frying, and liquid fats break down faster than do solids. As it fries food, more oil makes its way into the food, creating a greasier, fattier product. Flavors suffer as well. Food cooked in old oil tastes soapy, for many of the same reasons a bar of soap cleans you: Water and oil combine to make soap.

Any fast-food restaurant or snack-food producer committed to consistency in flavor and color can benefit from oil that lasts a long time. The less often they have to change the oil, the less money they have to spend. So industry and fast-food chains process their oils with a technique called hydrogenization. By inserting hydrogen into a liquid oil’s chemical structure, they make the grease partially solid and more stable.

Some restaurant chains use semi-solid oils. Others, like Popeye’s and Church’s, use solid shortenings. McDonald’s has worked hard to find an oil that maximizes frying efficiency. Its oil is substantially different from the oil used by other chain restaurants, according to Blumenthal, and french fry connoisseurs consistently rate McDonald’s fries as the gold standard. Though the chain guards its recipe closely, McDonald’s admits to changing the oil in its fryers every 10 days.

But as processed oil gets old, not only does food start tasting bad and containing more fat, it may harbor more frightening problems. Partially hydrogenated oils break down into "trans fats," nuggets of nutritional nightmares that may raise cholesterol levels and increase risk for heart disease. Even more worrisome, Blumenthal says, are the fumes that emerge from the fryer as oil breaks down.

The noxious steam contains traces of a highly toxic substance called acrolein, the same substance formed by the breakdown of certain kinds of pollutants in the air. Acrolein causes watery eyes, skin irritation, coughing, and runny noses. With extended exposure, it causes cancer in lab animals, and may do the same in people, according to studies conducted by the Environmental Protection Agency. Fryer ventilation hoods do not compensate for acrolein levels that emerge from overused oil, according to Blumenthal, and measuring those levels in frying oil is difficult. For these reasons, his company has developed a series of "quick tests" so that restaurants can evaluate the quality of their oil and change it before it’s too late.

Small-restaurant owners may not be aware of how crucial oil quality is in inspiring droves of loyal fans. But the way McDonald’s makes french fries and the way your local Mom and Pop restaurant makes them are as different as "night and day," says Blumenthal. "Mom and Pop may do it by hand, and they won’t go through a number of the intermediate (processing) steps."

Because so many factors affect how a french fry turns out, seeking a formula for perfection is a daunting task. From picking the potato, to cutting it into strips, to frying it for certain lengths of time at specific temperatures in oil of various qualities, understanding the science of a french fry may be more difficult than making fries the old-fashioned way: by intuition.

And if producing a perfect french fry requires an elusive marriage between science and art, keeping a fry in the realm of perfection is even more confounding.



Once a large batch of golden fries has been doused with salt and placed under the heat lamp to await the next customer to zoom through the drive-through window, the food’s texture begins to change immediately. An abandoned basket of fries or bag of chips inevitably turns soft and soggy. The culprit is water that infiltrates the crust both outward from the center of the potato and in from the air.

"Customers don’t want soggy fries," says Clara Rovedo, a postdoctoral food engineer in Singh’s lab at UCD. She has studied how moisture moves through a fried food after it emerges from the fryer.

Though her friends find her work amusing, Rovedo sees moisture migration as an essential area of research. "It’s funny for them. They think it’s no big deal," she says. "But it’s so important. It’s the difference between (a food) being accepted and being rejected by the consumer."

To measure the textural changes of foods after they have been fried, Rovedo and her team use a 2-mm-wide metal probe to poke repeatedly at fried potato patties. With the help of a multifaceted device called a Texture Analyzer, which stands on a desk like a 2-foot-tall, three-dimensional Hangman board, they then calculate how much force it takes to poke through the potato patty, and they gauge the crispiness of the crust.

Instead if using irregular french fries, scientists use uniform potato patties to standardize their texture measurements.

The experiments require three people to make sure every patty gets measured the same way. Because texture changes with temperature and time after frying, the scientists run from fryer to Texture Analyzer, with stopwatches, thermometers, and cutting knives in hand. They wear special gloves and safety glasses to avoid the splatter of hot frying oil. They measure crust resilience, and they conduct "stress relaxation tests," which rate potato patties for their ability to bounce back after compression. They test each crust 15 times.

At the end of the day, Rovedo goes home smelling like french fries and frying oil. Satisfied by doing work she finds both interesting and beneficial, she can tolerate the residual odors. "It’s better than working with fish," she says.



Most fried-food scientists find their work just as interesting for its own sake as for the applications it has on our dietary options. "We don’t look at it as a food or a food system or oil and potato," Farkas says. "What we do here is fundamental thermal science that happens to be applied to frying."

These scientists can only speculate on what the future might hold for the french fry. Perhaps it will be lower in fat, cheaper, or less dangerous to produce. Farkas hints secretively at a completely oil-less french fry. Singh predicts more healthful versions of fried foods and better strategies for handling and transporting them.They leave it to industries to apply their ideas and experiments to product developments and technical improvements.

But for industry-consultants like Blumenthal, who forge connections between scientific research and industrial developments, the future of the fry is much more tangible. He mentions more uniform fries and fries with better textures and flavors. He is currently working to make better microwave french fries.

Blumenthal also predicts nutritionally fortified fries, based on a recent survey that revealed that one-third of the vegetables eaten by pre-teens and one quarter of the vegetables eaten by teenagers in the United States are potatoes, most of which are fried. "That’s unbelievable," he says. "There are not many nutrients in french fries. So if that’s the way society’s going, then just like bread, french fries should be fortified."

But secrecy agreements with industries and competition for patents lends a mysterious air to the future of fried-food research. "There are a lot of clever and patented tricks of the trade," but those who know them will not let their secrets slip, Blumenthal says. "I don’t think anyone would dare."

While scientists continue to learn about what makes a french fry good, perfection in the french fry world remains elusive. Still, most scientists and restaurant patrons agree that the french fry is already pretty good. And the best kind probably depends more on people’s individual taste buds than on any scientific rule.

Blumenthal’s favorite fries, for example, come out of his lab at Libra Technologies in Metuchen, New Jersey. There, fried potatoes take on more meaning than just an accompaniment to lunch. "The magic here is that right next door is a major analytical lab where we actually find out what is happening."

But the expert analyses of restaurant patrons worldwide have the most powerful impacts on economics and technology research. "McDonald’s are the best because they’re salty and good," concluded a hungry, blonde 10-year-old girl in a fast food joint recently. And as scientists continue to understand what makes some fries so much better than others, a crisper view of the french fry’s future will surely emerge.




WRITER Emily Sohn
B.A., Environmental and Evolutionary Biology, Dartmouth College, 1997; Graduate Certificate, Science Communication (Writing), University of California, Santa Cruz, 2000
Internships: Discovery Channel Online (www.discovery.com); Santa Cruz Sentinel, Santa Cruz, California; Dallas Morning News, Dallas, Texas
B.A., Environmental Studies, University of Montana, 1996; Graduate Certificate, Natural Science Illustration, University of California, Santa Cruz, 2000
Internships: Friends of Santa Cruz State Parks, Santa Cruz, California; W.W. Norton Publishers, New York, NY; KLB Exhibits, Missoula, Montana

Text © 2000 Emily Sohn
Illustrations © 2000 Claire Emery