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Chris Young, chef and scientist, co-author of Modernist Cuisine: The Art and Science of Cooking.
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The Frying Game

Chris Young - June 2011

From clam bellies to French fries, from chicken to shrimp and even to vegetables, Americans wave the deep-fried food flag no matter what the chorus of Cassandras says. Chris Young lays out what happens when food hits hot oil and explains the virtues of a bewildering array of deep fryers.

Plunged into hot oil, countless steam-filled bubbles erupt and envelop the food in a cloud of swirling steam and churning oil. Seen in microscopic detail, the surface of the deep-frying food is continually rocked by violent explosions that release plumes of steam—a telltale sign that water just beneath the surface is boiling. There is nothing gentle about deep-frying. Such cataclysmic upheaval is, however, essential for both the speediness of deep-frying and for creating the inimitable crispy, crunchy, or blistered texture of superb deep-fried foods.

Deep-frying works a lot like baking, only faster. In both techniques, a convecting fluid transfers heat to food: the oil in a deep fryer churns in response to differences in buoyancy between hot and cold layers, just as the air in an oven does. And as heat is transferred to food, moisture in the food evaporates. This effect is invisible in baking, but in deep-frying it is made visible by steam bubbles rising through the oil.

Oil is much denser than air, which makes heat transfer more efficient, as well as more even. Oil is also more viscous than air. The Jacuzzi-like jets of steam bubbles can stir the cooler oil near the food into the hotter surrounding oil. The turbulence conducts heat to the surface of the food two to three times faster than stagnant hot oil. If that seems counterintuitive, think about what you do when you get into a really hot bath. You try to stay as still as possible because stirring the water makes it unbearably hot. The temperature of the water is the same in either case, but flowing water feels much hotter because it hasn’t had time to cool off against your skin.

Whether a food is surrounded by hot air or oil, moisture is always evaporating from the surface of food. The higher the surrounding temperature and the dryer the surrounding fluid—and, strange as it may seem, oil is a very dry cooking environment—the faster evaporation occurs. Because it takes energy to change water from liquid to steam, the evaporation of moisture from deep-frying food cools the food as it dries it. Indeed, this same effect is why we are cooled by sweating. So at the same time that hot oil in the deep fryer is adding heat to the food, evaporation is sucking heat out of the food. The two processes are in competition. Which one will win?

The surprising answer is that, for most of the deep-frying time, evaporation wins in nearly all foods. Until the surface is almost completely dried, evaporation holds the exterior temperature of the food at the boiling point of water. The oil in the deep fryer may be 400°F, but as far at the food is concerned, the effective cooking temperature is almost 200°F cooler than that.

What’s happening is that all of the additional heat energy arriving at the surface of the food is being used to vaporize water rather than to increase the temperature of the food. Simply put, none of the heat sinks in. Thus, as long as columns of bubbles are streaming from the food, you can be sure that the surface is wet and, thus, no hotter than boiling water. Raising the temperature of the deep fryer oil doesn’t cook the food any faster; it simply accelerates drying so that a crust will form faster.

Eventually, the surface becomes dry and the boiling zone moves from the surface down into the food. Devoid of water, streams of bubbles start to slow to a mere tickle, and the temperature of the crust rises quickly. This is the point at which the golden color, rich flavor, and wonderful texture of a deep-fried food develops, the raison d’être of deep-frying.

This last stage also presents the principle challenge of deep-frying, which is to cook the food to the center before the crust burns or before the boiling zone moves deep enough into the food to overcook delicate ingredients like meats and seafood.

Sized just right Food is such a poor heat conductor that anything except paper-thin slices of food heat faster at the surface than at the center. This effect can be something to celebrate. In French fries, for example, it accounts for the exquisite contrast between a crispy crust and a light fluffy interior.

But there’s a reason you won’t find French fries much larger than your fingers or, for that matter, that you shouldn’t deep-fry a raw whole turkey. When foods are too large, the surface becomes a bottleneck through which the heat can pass only slowly to the interior, and uneven cooking is inevitable. It takes so long to raise the temperature at the center of the food that the intense heat of deep-frying overcooks the outside or, worse, burns it.

When in doubt, cut food into smaller pieces for deep-frying. Small objects have a greater ratio of surface area to volume than larger ones; as a result, they heat through faster and more evenly from surface to center. If you can cut the size of the food in half, you cut the cooking time by as much as a factor of four. What’s more, you’ll avoid the unhappy experience of eating deep-fried food that is charred on the outside and raw in the center.

In situations where cutting the size of the food down isn’t an option, a two-step deep-frying technique is a good strategy. For vegetables, such as potatoes and other foods that aren’t easily overcooked at near-boiling temperatures, both steps can be done in the deep fryer. The first step cooks the food all the way through at a relatively low deep-frying temperature, typically between 250°F and 285°F. Once fully cooked, the food is removed and the oil temperature is usually raised to between 350°F and 400°F, at which point the food is plunged back into the oil just long enough to create a golden brown crust.

Unfortunately, cooking delicate meats and seafood this way will still overcook a large fraction of the interior. A two-stage cooking process is still a good strategy, but the cooking temperature for the first step should be much lower. Thus, for large delicate foods, the precooking step is best done in a water bath, combi oven, or CVap oven, and then quickly finished in a deep fryer.

Clothe your food For delicate foods that aren’t too large, there is another approach to deep-frying in a single step without overcooking: clothe them with a wet batter. As the water in the batter steadily boils, it will keep the effective cooking temperature beneath the surface cooler. Depending on the batter used, this can be a slight cooling effect (thin tempura-like batters) or it can be substantial (thick beer-batters).

If greater protection of a delicate food is necessary, but a thin crust is still desired, then a good strategy is to foam the batter slightly. As the foam dries and hardens, it further insulates the food from the intense heat of the frying oil. Foaming a batter can be as simple as incorporating a carbonated beverage into it. For greater insulation and a crisper crust, include a leavener into the recipe. Baking powder does an excellent job of aeration; when heated, the acids and alkaline ingredients it contains react to fill the batter with bubbles of carbon dioxide. Finally, for a more direct approach, you can go high-tech with a whipping siphon. This approach has the advantage that the batter is foamed to order and thus can be held throughout a service without losing its bubbles.

Another useful trick is to replace some of the water in a batter with alcohol. Neutral spirits like vodka work well and can replace as much as 40 percent of the water in a typical batter recipe. Although the alcohol will destabilize a foamy batter, it also effectively dries out the batter so that it cooks quickly. And because alcohol boils at a lower temperature than water, it has a stronger cooling effect. This means that the food inside the batter is exposed to a lower effective cooking temperature and the batter itself becomes dry and crisp sooner. Thus, alcohol-imbued batters are particularly useful for the most delicate foods, such as fish, that are easily overcooked in a deep fryer.

Sometimes a delicate, crisp coating isn’t ideal, and a hefty, crunchy coating is more suitable. Breading is the solution. It involves taking solid particles, such as bread crumbs, panko, even flaked or puffed cereals, and affixing them to the food using a binder like eggs, dissolved gelatin, or other modernist hydrocolloids. As with batters, breading insulates the food it clothes, but because breading recipes tend to be fairly dry, they provide much less of a cooling effect. Thus, we usually use breading for the texture it imparts. Breading can also become sturdy enough to form a crunchy shell that holds its shape, even as delicate centers become molten and soft. The cromesqui, or croquette, is a particularly excellent example of this technique in action.

A crispy batter or crunchy breading is the hallmark of great deep-frying. But when poorly executed, deep-fried foods clothed with a crust become unpleasantly greasy. The surprising fact, however, is that greasiness is mostly the result of what a chef does after the food has been fried.

What makes deep-fried food greasy, and when? We may think of deep-fried food as being submerged in oil, but that is not exactly true. If the frying oil is in peak condition, the food will directly contact the oil for no more than half the frying time. The rest of the time, the streaming bubbles shove the oil aside. Because of this, and contrary to conventional wisdom, very little oil is absorbed into food while it’s being deep-fried. Oil is trapped and absorbed by the surface after the food is removed from the fryer and begins to cool. Indeed, this is why blotting excess oil off the surface of the food immediately after it’s removed from the deep fryer helps to make the food a lot less greasy.

Researchers have shown that the cracks, fissures, and holes created at the surface of deep-frying food create capillary forces that wick oil once the food begins to cool. While frying, escaping steam mostly keeps oil out of these nooks and crannies, but when the crust cools, this steam condenses, which creates a slight vacuum that helps to draw the oil in.

This turns out to be the reason that lower frying temperatures produce such a greasy crust. Deep-frying a crusty food in oil at 340°F, rather than 360°F or hotter, can increase the oil absorption by 40 percent. Cooler frying oil is more viscous and sticky and, thus, doesn’t easily drain from the food. Hence, it’s always best to deep-fry at the highest practical temperature because this tends to produce a thin, delicate crust that’s less greasy. But be reasonable with the oil temperature. It’s wasted effort if the food gets scorched and burned.

A coating of oil isn’t necessarily bad, and the goal shouldn’t be to get rid of all the oil on the surface. Deep-fried food simply wouldn’t be deep-fried without some of the flavorful oil coating the surface. The oil provides a lot of the flavor, texture, and mouthfeel of deep-fried food. Interestingly, these effects are entirely superficial. When we chew food, the surface is the first thing that our tongue, cheeks, and palate sense. The first bite leaves them coated with warm, aromatic oil that is mixed into the food as we keep chewing. So the goal of deep-frying should be to leave food anointed with just the right amount of oil, neither too little nor too much.

Oil changes Oil is the most important ingredient in deep-frying. It affects the flavor of the food, how well it browns, how quickly it cooks, how greasy it becomes, and how healthful it is to eat. Yet cooks rarely give much thought to the oil once it’s poured into a deep fryer. They neglect to consider that, like any other ingredient, heating it to deep-frying temperatures cooks the oil!

Some of the complex chemical changes that happen to the oil improve it: oxidation that results from air being churned into the bubbling oil creates a chain reaction that generates the volatile aromas that impart deep-fried flavor to food. Water vapor escaping through the oil also creates a different set of reactions that involve splitting fat molecules into new compounds such as emulsifiers and gums. As oil is “broken in,” the emulsifiers allow oil and water to mix, so that the food spends more frying time in contact with the oil. This delivers the heat more rapidly and evenly, resulting in faster cooking and a more even golden-brown color.

Unfortunately, oil gets old from use. The reactions that improve it ultimately ruin it. Eventually the oil develops a rancid, fishy aroma, the result of oxidation reactions gone too far. Another sure sign that your frying oil is shot is when it foams too much, from steam bubbles getting stuck in the oily goo thickened by accumulated emulsifiers and gums.

Old frying oil isn’t just sticky and smelly—it’s downright dangerous. The breakdown of the oil lowers the smoking and flash points, increasing the risk of a dangerous grease fire.

Some oils survive the rigors of deep-frying longer than others. Saturated oils are more stable than unsaturated oils and, surprisingly, may be no less healthful than unsaturated oils when used for deep-frying. But the key to keeping any frying oil in peak condition is to frequently strain out food debris that accumulates in the oil. When these bits of food start to accumulate and burn, the oil is quickly ruined.

Choosing a deep fryer A good deep fryer should be the right size for your needs, accurately maintain the set temperature, recover to this temperature quickly during use, and preserve the condition of the oil. It should also be energy efficient and easy to clean and maintain. Although deep fryers that meet these basic requirements are available in a myriad of configurations, they all have one feature in common: a vat of oil large enough to hold a load of submerged food. How large this vat should be for your needs is the first factor to consider when choosing among the bewildering number of deep fryers available.

A small commercial grade deep fryer will hold as little as 14 pounds of oil—about 2 gallons—while the largest versions can hold 10 times this amount. A general rule of thumb is that you can deep-fry twice the oil’s weight in food every hour. Therefore, a small deep fryer can prepare about 25 pounds of food per hour.

Your mileage may vary, however, depending on many factors, including how quickly your fryer recovers the cooking temperature after plunging in a basket of cold food. Fast recovery increases the hourly capacity of your fryer and also improves the quality of the food. Recall that allowing the oil’s temperature to drop by as little as 20°F yields greasier food with a poor quality crust.

Thus, a second factor to consider when selecting a deep fryer is how quickly it recovers the desired frying temperature. Although the particulars of a design impact this, it is mostly determined by the available power supplied to the deep fryer (quoted in kilowatts or BTU/hrs for electric or gas powered units, respectively).

All things being equal, electric deep fryers preheat and recover temperature faster than gas powered ones, but things are rarely equal. For example, the ubiquitous “pro-grade” countertop electric deep fryer might seem large enough for your needs, and a cost effective solution, but these fryers are designed to plug into a standard 120-V, 15-amp North American outlet. Under the best of circumstance it can only pull 1,800 watts of power before blowing a fuse. For anything more substantial than a few items of food infrequently prepared, such a low-wattage fryer simply cannot recover the frying temperature fast enough. Chefs tend to overcome this shortcoming by setting the temperature of the oil as hot as possible. While this helps minimize the effects of a plummeting deep-frying temperature that is slow to recover, it also dramatically shortens the lifespan of the oil.

True commercial electric deep fryers require dedicated electric service of 14 kW to 22 kW or more for a standard 14-inch wide frying unit. This is a substantial amount of power, and not every restaurant can supply it. A comparable amount of gas power is available in nearly every restaurant, which is one reason that gas powered deep fryers remain popular. Gas also tends to be a slightly cheaper utility than electricity, a point not lost on those selling gas burning deep fryers.

Whether powered by electricity or gas, any decent deep fryer will feature a cool zone at the bottom of the oil vat. This is why electric elements or gas heating pipes are raised above the floor of the frying tank. Doing this creates a zone of stagnant oil that remains cooler than the circulating hot oil mere inches above it. This layer of cooler oil acts as a crumb collector at the bottom of the fryer, holding bits of crackling and batter that fall off the food at low temperatures so they do not burn, ruining the oil and the flavor of the food. Older deep fryers that heat the oil directly from the bottom should be avoided. Although cheaper, the oil will need frequent changing in these fryers, making any cost savings an illusion.

Given the importance of the oil, modern fryers that incorporate an automatic filtration system that removes accumulating food debris from the oil are worth serious consideration. Not only does this feature maintain frying oil at peak condition for longer—a savings that goes straight to the bottom line—this is also a much safer system than manually draining hot oil and straining out the debris.

Finally, energy efficiency is a factor to consider carefully when buying a deep fryer. Even in a busy kitchen, a typical deep fryer idles 75 percent of the time. Energy consumption at idle varies significantly, but, broadly speaking, electric fryers are more efficient than those that consume gas, although some costly, but high efficiency, gas powered deep fryers do exist. Because electric fryers can reduce their power consumption to a trickle at idle helps offset the lower price of gas as a utility, too, is a fact that those selling electric fryers like to point out.

Deep fryer roundup For the purposes of comparing some of the current deep fryer options available, we’ll look at the most popular size for a commercial deep fryer, the standard 14-inch wide, open deep fryer. This list is by no means exhaustive; rather, it is a selection of popular, or otherwise noteworthy, models that should be used as benchmarks other deep fryers are judged against.

Electric powered deep fryers Frymaster’s current flagship is the Frymaster E4 Series. The 14-inch model is available in 14 kW, 17 kW, and 22 kW capacities and offers energy efficiency around 85 percent under a heavy frying load—surprisingly, frying full batches of food is more energy efficient than lighter loads, which can reduce the energy efficiency by 20 percent or more. At idle, these fryers sip less than 1,000 watts of power. One rather unique option available is the FootPrint Pro filtration system. Most automated filtration systems are only available on larger sized fryers.

Hobart 1HF50D This high efficiency fryer offers 17 kW of power that gives it the stamina to be a workhorse with fast temperature recovery between batches. Its solid-state temperature controller provides excellent temperature regulation with high efficiency (tested at nearly 84 percent under a full load). Moreover, this fryer idles at mere 630 watts of power, making it one of the most frugal deep fryers currently available.

Pitco Solstice SE14 Series These fryers are available with either 17 kW or 22 kW of power and offer efficiency and features comparable to other brands listed. One unique option, however, is the SpinFresh module. This attachment sits on top of the fryer and raises and lowers a circular wire basket into the frying vat. After frying, it removes and spins the basket at a programmable combination of speed and time. Through centrifugal force it easily spins off surface moisture and oil. Recall from earlier that removing the oil after frying makes the food less greasy, while removing moisture maintains the crispy or crunchy texture of the deep-fried food. SpinFresh technology is a clever way to keep fried food from tasting greasy, and it also cuts down on oil use by throwing the excess oil back into the frying vat.

Gas powered deep fryers

Frymaster FPH55 The Frymaster FPH55 gas fryer lives up to the brand’s proven reputation for quality and performance. It preheats and recovers oil temperature nearly as fast as electric fryers. Independent testing also demonstrates energy efficiency comparable to other leading gas powered fryers: around 55 percent under a full load. Idle power consumption is slightly more than 5,600 BTU/hr. This is excellent performance for a gas fryer, made possible in part by the use of infrared burner technology. It should be pointed out, however, that comparable electrical fryers generally idle at one-third this level of power consumption.

Pitco Solstice Supreme SSH55 This series sports solid energy efficiency performance numbers and fast preheat and recovery times. Compared to other gas powered deep fryers, these fryers use a fairly simple burner technology, which Pitco claims makes their fryers more reliable. A more compelling selling point is the option of adding the SpinFresh technology described earlier.

Ultrafryer F-P30-14-CE Perhaps the most energy efficient gas fryer on the market, as more than 60 percent of the energy goes into cooking rather than up the hood. But its efficiency doesn’t take away from performance. It recovers the oil’s temperature remarkably fast, less than 10 seconds after a load of cold fold is plunged into the oil. It accomplishes this with a patented heat exchanger design marketed as PAR-3 technology.