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Back to Basics: The Science of Frying

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Brett Jordan, Flickr Creative Commons
Brett Jordan, Flickr Creative Commons
French FriesFrench Fries. Source: Brett Jordan, Flickr Creative Commons, Jan 9, 2012

Frying is a cooking technique that has been used for centuries. Sautéing, stir-frying, pan frying and deep frying all operate on the same principles -- what differentiates them is how much fat is used in cooking. It can range from a very thin layer (sautéing and stir-frying) to enough to fully submerge an entire piece of food (deep frying). The biggest benefit to frying is speed. Fat is much better at transferring heat than either air or water, so frying cooks considerably faster than baking or boiling. Plus, the fat imparts a crispy crust and a richness and depth of flavor that is, to many folks, irresistible. But there's one major drawback to frying: health concerns that come from eating fatty foods. Regardless of how healthy the food was before it went into the deep fryer (in case you're thinking of deep frying spinach), it's going to come out with five to 40 percent absorbed oil by weight.1 That's a major drawback for anyone who's watching their fat or calorie intake. For healthy people who don't need to worry as much, however, the occasional fried treat is not a problem. To better understand these often defamed snacks and treats (and sometimes whole turkeys), let's dive into the science of how simple hot oil transforms everyday foods into sinful delights!

When Food First Hits Oil

Fried foods are typically cooked in oil that has been heated to 350 to 425 degrees F. If you've ever seen food dropped into a hot fryer, then you know that it immediately starts sizzling and bubbling. It looks as if the oil itself is boiling, but those bubbles are actually caused by hot steam shooting out of the food. This sudden expulsion of steam is caused when water, which normally boils at 212 degrees F, hits oil that has been heated as much as twice that temperature. In fact, this vaporization occurs so rapidly and violently that, if you were able to watch it at a microscopic level, it would look like thousands of explosions all over the surface of the food! This mass exodus of steam is important for several reasons: (1) the water vapor repels the oil and keeps it from penetrating beyond the surface, (2) the steam cools the oil surrounding the food, which buys time for flavors to develop and heat to make its way to the center of the food without burning around the edges.

Plantains frying in canola oil

When Food Is Done Cooking

As more and more steam escapes from the food, its surface dehydrates, which leaves behind a crispy crust. Once most of the moisture is gone from the outer layer, heat is able to travel more quickly to the center of the food. Steam will continue to escape, but you'll see fewer, less urgent bubbles. This is a signal that your food is almost done, and the crust is about to truly dry out and start crisping up. As soon your food reaches the golden-brown color you like, pull it out, drain, season and serve it!

Preventing Greasiness and Sogginess

Remember how I mentioned that the steam released from food cools the oil around it? If you're frying at home, you'll always get some temperature loss after adding your food, but most recipes account for a normal drop in temperature. For example, if a recipe tells you to heat your oil to 375 degrees F, it understands that once the food goes in you'll actually be cooking at 350F, and that's okay. If you overcrowd the pan, however, you may end up dropping the temperature far below what the recipe anticipated, and your food may come out unpleasantly greasy. That's why most recipes say things like "don't overcrowd the pan" or "cook in batches." Why does too-cool oil cause greasiness? Because of steam, or in this case, a lack thereof. When the oil is too cool, the explosions of steam you would normally get when adding food to hot oil are less extreme, or worse, nonexistent. Steam exits the food slower than it should and you lose the oil-repelling power of the steam jets. That's why most recipes tell you to preheat your oil before adding any food. Good hot oil creates a situation akin to trying to cram your way down a flight of stairs in a subway/metro station just moments after a full train lets out at rush hour. You're the oil; the commuters are the steam, and you're not getting through until they stop coming.
Korean Fried Chicken. Source: Taiwai Yun, Flickr Creative Commons, March 28, 2007Korean Fried Chicken. Source: Taiwai Yun, Flickr Creative Commons, March 28, 2007

Excessive greasiness can also be caused by poor drainage or by sitting for too long before serving. Ironically, batch frying, which is supposed to alleviate greasiness, can be the very reason fried foods have to sit too long before serving. The solution once again is to keep that steam flowing! That usually means immediately transferring your food to a warm (200 degrees F) oven until you're ready to serve. Keeping your food steaming hot slows the oil's migration into your food while at the same time preventing another unpleasant end: sogginess. Sogginess is a particularly common problem with fried foods that have been coated with a batter or breading. When food starts to cool, the moisture in the space between the crust and the food turns into water droplets instead of steam. This can make the crust soggy from the inside out and ruin your once crispy crust.

A Quick Look at Batters and Breadings

Fried foods are often dipped in batters or breadings before being cooked. The purpose of these coatings is to protect food from the violent surface reactions of frying, retain moisture and provide a pleasant flavor and texture.2
Fish and Chips. Source: Calgary Reviews, Flickr Creative Commons, Jun 22, 2011Fish and Chips. Source: Calgary Reviews, Flickr Creative Commons, Jun 22, 2011

Batters result in a smooth, crispy and often delicate crust. Batter recipes vary widely, so results differ depending on ingredients. For example, high gluten flours result in a chewy (some might say tough) crust, whereas gluten-free flours (like rice flour) result in a paper-thin ultra-crispy crust (think: Korean fried chicken). Adding eggs or sugar to a batter will result in a darker crust, which may or may not be desirable. Batter coatings are smoother and have less surface area than most breaded coatings, so they tend to absorb less cooking oil. They also tend to offer the most protection for delicate foods, which is why fish are commonly battered before frying (think: fish and chips).

Extra Crispy Fried Chicken. Steven Depolo, Flickr Creative Commons, July 7, 2008Extra Crispy Fried Chicken. Source: Steven Depolo, Flickr Creative Commons, July 7, 2008

Breadings result in a crispy, crunchy, textured crust. Breaded coatings can range from fine breadcrumbs (think: Chick-fil-A), to large, extra-crispy breadings (think: KFC, Popeye's, Japanese tempura). Fine breadcrumbs tend to absorb less oil then the extra-crispy style, since they provide less surface area for oil to soak into, but they are prone to sogginess. Extra-crispy style breadings are usually achieved by incorporating large, already crispy particles, such as Panko-style breadcrumbs or cereal, such as cornflakes.

Although batters and breadings are delicious, some foods don't require a coating at all. Starchy foods, like potatoes, plantains and yucca root will form their own natural protective skin once they're dunked in the fryer. This makes prep easy and accessible for the home cook. Take a look at the simple fried plantain salad I made while on vacation a few weeks ago. It was quick and easy to throw together, and it's a great starter recipe for frying novices!

Fried Plantain Salad

Fried Plantain Salad

Ingredients
1 plantain, mostly brown
Soybean, vegetable, corn, peanut, or canola oil, enough to cover
Salt and pepper to taste
1 tomato, diced
Lime juice, to taste

Directions
1. Heat oil to 375F in a skillet, or, if you don't have a thermometer that will go that high, check to see if your oil is sizzling hot by sticking the end of a wooden spoon in the oil. If it bubbles, so will your plantains!

2. Peel plantain and slice into coin shapes, about ¼ inch thick. Add them to the hot oil. They'll float, so you may need to flip them after a few minutes. Fry until the bubbling has slowed and they have a nice rich, golden brown color.

3. Drain onto a paper towel and season with salt and pepper immediately. Toss in a bowl with diced tomato and a bit of lime juice, to taste. Add additional salt and pepper if necessary. Serve immediately.

Serves 2-4.

 

Notes

1. Owen R. Fennema, editor, Food Chemistry, 2nd Edition (New York: Marcel Dekker, Inc, 1985), 210-211.

2. Amy Brown, Understanding Food: Principles and Preparation, 2nd Edition (Belmont: Wadsworth/Thomson Learning, 2004), 131.