The Science of French Fries

America’s Test Kitchen TV
Season 10, Episode 22: Best Burger and Fries
Snippet: The Science of French Fries

Christopher Kimball: Here are two things we know absolutely for sure—the earth is flat [smiles slyly], and the other thing is if you put raw French fries into room temperature oil, they’re immediately going to suck up that oil and get greasy. Well, guess what. The earth is not flat, and it’s also not true that French fries in oil suck up oil right away. And I’m here at the Science Desk with our Science Editor, Guy Crosby [motions to GC, who’s standing in front of a chalkboard], who’s going to explain the whole new science of French fries.

Guy Crosby: Well, Chris, the reason we start cooking the French fries in room temperature oil is because it gives the center of the fry time to cook and become creamy and soft without letting the outside become really tough and leathery [CPK nods in anticipation]. But there’s one other factor we’ve found that’s very important in cooking and in low-temperature oil, and that is the fries actually absorb less fat. So let’s look at why that is, and why we use lower temperature, less fat, [or] higher temperature, more fat. Water in food like a potato occurs in three forms [turns to chalkboard]. The first one we call free water [writes “free” on board]. And this is water you can literally squeeze out of a food. The next one is called adsorbed water [writes “adsorbed water” on board, then turns back to CPK]. And this is the water that is literally bound onto the surface of all the proteins and starch and other things in the food. And the last one is called bound water [writes “bound water” on board]. This is water that occurs within the crystal structure. Each one of these takes more and more energy, or more heat, to drive them off. So the hotter we cook the fry, the more of this type of water we’re evaporating, and the more water we evaporate, the more oil we absorb. Because studies have shown that the amount of oil that’s absorbed is directly proportional to the amount of water we evaporate from the food. So let’s look at this in a little bit more detail, if I draw a graph. And here we have time across the bottom axis [writes “time” at bottom of graph], increasing this way [motions to the right], and we have the percent oil here [writes “% oil” on left side of graph]. Take a fry…

CPK: [Pointing to “% oil”] That’s the oil absorbed?

GC: [Nodding] This is oil absorbed.

CPK: Okay.

GC: That’s the oil absorbed, right. So if we take a fry and we heat it up in the low temperature oil [draws line that goes up from apex of graph, then peaks and stays steady about halfway up the graph], it’s going to absorb a certain amount of fat, because we’re only driving off some of the free water. If we heat it even higher—and this [pointing on graph to where first line stays steady] I should say is maybe around 300 degrees or less…

CPK: That’s the final temperature we get to.

GC: Correct, the frying oil temperature. If we heat it up in a classic method [draws line that goes up from apex of graph, past first line, and peaks at a higher oil percentage, then steadies], the temperature gets much hotter, like 375 degrees, we actually drive off not only some of the surface free water, but some of the adsorbed water. So the amount of oil we absorb is proportional to the water, and that’s directly dependent on the temperature, but not really the time. So the higher we heat it, the more water evaporates, the more oil we absorb.

CPK: [Turns to camera] So our recipe is surprising, because those fries sit in the oil a very long time. But as Guy just described, it ain’t time [flailing arms in enthusiasm], it’s temperature. And we end up at 300 degrees, not 370 [GC nodding in agreement], which means less moisture loss, less oil intake, and therefore we have a much less greasy, therefore a much better French fry.

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Captioned by Media Access Group at WGBH

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