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The Science of Frying

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By David Joachim and Andrew Schloss
from Fine Cooking #121, pp. 38-39

From chips and doughnuts to chicken, french fries, and onion rings, fried food is hard to resist. And it’s no wonder: No other cooking method delivers such crisp, delicious browning while keeping food moist and tender on the inside. But frying can be tricky, and doing it well requires not only good technique but also an understanding of the science behind it. Here’s what you need to know to fry right.

How does frying work?
Any food cooked in hot fat is fried. The method of heat transference is the same whether there’s just a little fat in the pan (sautéing), the fat comes partway up the sides of the food (shallow-frying), or the fat completely envelops the food (deep-frying). When food is added to hot oil (usually 350°F to 375°F), its surface dehydrates. Meanwhile, through a series of Maillard reactions (named after the chemist Louis Camille Maillard), its sugars and proteins break down to create complex flavor and golden-brown color.

Browning is quick and thorough because the hot liquid fat delivers heat to even the smallest crevices on a food’s surface. In the initial moments of frying, as the surface dehydrates, it forms a crust that inhibits further oil absorption, while continuing to conduct heat to the interior of the food, where the heat causes starches to gelatinize (as in french fries), proteins to denature (in fried chicken), and fibers to soften (in fried zucchini).

Maintaining the correct oil temperature is key to frying. If the temperature drops too low, the crust forms slowly, allowing the food to absorb more fat and become greasy. If the oil gets too hot, the food burns on the surface before it cooks through.

Put the science to work
Classic Fried Chicken Fish and Chips with Tartar Sauce Fried Chocolate-Hazelnut Wontons with Orange Dipping Sauce
Classic Fried Chicken   Fish and Chips with Tartar Sauce   Fried Chocolate-Hazelnut Wontons with Orange Dipping Sauce
Yuca Fries  Bite-Size Latkes  Buttermilk-Battered Onion Rings
Yuca Fries with Garlic Mojo   Bite-Size Latkes   Buttermilk-Battered Onion Rings

What is the best fat for frying?
It depends whether you care most about flavor or health or using a long-lasting frying oil. Different fats deliver better results in each instance.

A fat’s level of hydrogen saturation is the most important variable. Highly saturated fats, such as lard and shortening, are solid at room temperature, while polyunsaturated fats, like vegetable and canola oils, remain liquid at room temperature. Food fried in highly saturated fat has a more pleasant, less-oily-tasting surface than food fried in unsaturated fat, because the saturated fat re-solidifies as it cools. Plus, many saturated fats, like duck and beef fat, are more flavorful than refined liquid oils. So from a flavor and texture standpoint, solid fats make a great choice for frying.

However, saturated fats are considered less healthful than liquid polyunsaturated fats because they may be associated with a higher risk of cardiovascular disease. Also, saturation causes these fats to degrade sooner during frying. They begin to break down at typical frying temperatures (350°F to 375°F), releasing free fatty acids that burn and create visible smoke. Once the fat has reached this smoke point, it will create off-flavors and noxious aromas.

For a longer-lasting fat that’s more healthful, choose a refined polyunsaturated oil with a high smoke point. Almost all vegetable oils make good choices for frying, since their smoke points are higher than common frying temperatures, ranging from 410°F (corn oil and olive oil) to 435°F (canola oil) to 445°F (grapeseed oil) to 450°F (peanut, soybean, and safflower oils).

Top Ten Tips for Frying

  1. Cut food into similar-size pieces so they fry at the same rate.
  2. To help breadcrumb coatings dry and adhere, let raw breaded food sit on a rack for up to 30 minutes before frying.
  3. Always deep-fry in a deep pot and fill it only a third or half full with oil. Tall, narrow pots help extend the oil’s frying life, since less of the oil’s surface is exposed to oxygen.
  4. Use a deep-fry thermometer and adjust the heat while frying to maintain a steady temperature.
  5. Fry in small batches to prevent the oil temperature from dropping too low, which can lead to greasy food.
  6. Briefly drain fried food on a rack or absorbent paper.
  7. Season food immediately after frying so the seasoning adheres to the hot food.
  8. If not eating right away, transfer each batch of fried food to a 200°F oven to keep it crisp.
  9. Let the oil come back up to frying temperature between batches.
  10. Never leave oil unattended on the stove-it can overheat and become a fire risk.


What creates a crisp crust on fried food?
When food is plunged into hot oil, the water in the food starts to boil and percolate toward the surface. In order for a crisp, dry crust to develop, there must be a barrier between the hot oil and the migrating water. This barrier is typically something starchy. As the starch fries in the hot oil, it dries into a pleasantly crisp shell and protects the moisture beneath. The food inside steams while the coating browns and crisps.

After frying, the food will continue to steam, but as long as you can see steam rising from the surface of your fried chicken or tempura, the moisture is escaping and the coating will stay crisp. As soon as the food cools and the steam stops, any remaining moisture in the interior is absorbed into the coating, making it soggy; this is why fried foods are best eaten soon after frying.

Starch is naturally present in certain foods, like potatoes and fritters, but for frying some things, especially meats and fish, a starchy coating like batter or breadcrumbs needs to be added. Both create crisp crusts during frying, and there are pros and cons to each.

Batters form a more solid surface than breadcrumbs do. This makes them better for frying delicate ingredients like fish fillets, which tend to fall apart during cooking. The downside to batter coatings is that they don’t allow as much evaporation as breadcrumbs do, so battered foods tend to get soggy faster. For a crisp batter crust, use low-protein wheat flour such as cake flour, which doesn’t develop as much gluten as higher-protein flours, and therefore doesn’t become as chewy. For an even crisper crust, use corn flour (finely ground cornmeal) or gluten-free rice flour.

Breadcrumb coatings stay crisp longer than batter coatings, but the process of breading can be more involved: Typically, the food needs to be dusted with flour to dry its surface, and then dipped in egg to help the crumbs adhere and form a crust. For the crispest breaded crust, use panko breadcrumbs. Shaped in long slivers rather than rounded bits, these crumbs have more surface area exposed to the hot oil, so they crisp more thoroughly. Panko crumbs also don’t compact the way regular breadcrumbs do, which allows steam to evaporate faster and helps prevent the crust from getting soggy.

What to do with used frying oil

Reuse it Do this only if the oil still appears clear and light in color. To extend its life, keep it clean by regularly straining out crumbs.

Discard it Don’t pour used frying oil down the drain. Instead, let it cool, pour it into a biodegradable container, such as a paper milk carton, and throw it out with your regular trash. Some cities also have collection centers for recycling used cooking oil.


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