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The Burger Lab: Why Double Fry French Fries?

Posted by J. Kenji Lopez-Alt, January 15, 2010 at 7:30 AM

It's time for another round of The Burger Lab. Got a suggestion for an upcoming topic? Email Kenji here, and he'll do his best to answer your queries in a future post.

Clockwise from top left: Perfectly cooked double-fried french fries; using calipers to measure the crust on fries prepared by different methods; a cross section of the fries shows the difference in those crusts. [Photographs: J. Kenji Lopez-Alt]

If Heidi Montag and Spencer Pratt have taught us anything at all, it's that sometimes, when you combine two extraordinarily simple things, the results can get a little complicated.

Case in point: french fries.

The intricacies involved with taking potatoes and oil, applying science, heat, and a bit of blind faith are so complex that I will not even attempt to cover it in a single blog post. But I'd like to start with this week's Burger Lab—a departure from beef, and the beginning of a long, and hopefully interesting road toward french fry perfection—by attempting to answer a question that has plagued my subconscious thoughts since the very first day that I started working in a decent restaurant.

But first, let's quickly run through french fry basics. To make a standard french fry, you first start by carefully and uniformly cutting a high-starch (i.e., a russet) potato into precise 3/8-inch sticks and rinsing them in water to remove excess starch. After carefully drying them, you give them a 5 to 6 minute bath in oil at a relatively cool 325°F (163°C). Next, you remove the potatoes, increase the oil temperature to the standard frying range of 375 to 400°F (196 to 204°C), and fry them a second time, this time crisping up the exterior to a beautiful golden brown. Add a bit of salt, and what you've got is something that looks like this:

...and you're one step closer to heaven.

The thing that's always bugged me with french fries is: Why the double fry?

I've heard three reasons that have a bit of sense to them:

• Theory #1: "The first fry gets the outside to create a water tight barrier. That way, when you fry them the second time, they stay moist."
• Or, Theory #2: "The first fry cooks them through to the center—if you skipped that step, your fries would brown on the outside, but still be raw in the middle."
• Or, my personal favorite, and the very first response I ever received when I was still a lowly line cook: Theory #3: "I don't know Kenji, and it doesn't matter. Just keep cutting them. There is such thing as stupid questions."

Well, It's been 10 years, and it's time to put the theorizing behind me, and get some real answers so that I can once again sleep soundly at night.

Theory 1: The Water Tight Barrier

This is easily put to rest with a single observation:

These are french fries that are in the midst of their second fry. They have already been fried once, so any sort of watertight barrier-formation should have already occurred.

Any time somebody uses the phrase "dipped in boiling oil" or "boiled in oil," what they really mean to say is "dipped in oil hot enough that the object's internal moisture evaporates and bubbles to the surface of the oil, giving the illusion that the oil itself is boiling, when in reality it is the water in the object being fried that is boiling."* Except in very strange, laboratory conditions, oil itself catches on fire and burns far before it even approaches boiling temperature.

*The only people who are allowed to use the inaccurate phrase "dipped in boiling oil" are medieval torturers, if only for the sake of poetic license and brevity.

The point of all this is that the bubbles streaming out of the side of the fries indicate that internal moisture is escaping, and escaping steadily during the second frying phrase.

Theory #1: Debunked.

Theory 2: the First Fry Cooks Through, the Second Fry Crisps

This is the more common theory, and the one that really got me thinking. If the only purpose of the first fry is to cook the potatoes through, then does it need to be a fry? Could I boil the potatoes first in water, then fry them, for instance? After all, my go-to method for producing crisp roasted potatoes involves first boiling the potatoes, then roasting them at high temperature. Would the same work for fries?

I decided to put it to the test. I cut a single potato into perfect 3/8-inch square matchsticks, soaked them in water, then divided them in half. The first half, I fried once at 325°F, according to traditional french fry technique. The second half I placed in a pot of boiling water until they were just cooked through, but not to the stage that they were falling apart.

As you can see, there are already immediate differences. The fried potato on the left is sheathed in a robust, skin-like pellicle, while the boiled potato on the right is starchy—almost crumbly—in appearance. I still held out hope that this crumbly texture would give the boiled potatoes the edge. Since rougher texture = more surface area, and more surface area = more bits to crisp up, perhaps the boiled potatoes would crisp up nicely after all?

I carefully lowered the potatoes into my hot wok full of 375°F oil (which I had set up with perforated aluminum foil divider so that I could fry both batches simultaneously without mixing them together—one of my more clever moments), and fried them for exactly two minutes before removing, draining, and tasting.

The boiled french fry on the right was certainly crisp, but delicate to the point that I could barely pick it up without shattering its surface—the layer of crispness was paper thin, and quickly softened within minutes after emerging from the oil. The double-fried french fry, on the other hand, had a thick layer of golden brown, cracker-like crispness that stayed crisp even when it was almost fully cooled. The ideal french fry.

But wait—perhaps the problem is that I chose a moist cooking method. If the purpose of frying, and the way things get crisp, is through dehydration of the surface, boiling the potatoes in water is probably not the best par-cooking method. Next, I turned to the microwave. With the power of radiation, I should be able to cook the potatoes through, driving off some of their excess moisture in the process. But the results were nearly identical: The microwaved-then-fried potatoes had an extremely thin layer of crispness.

The best way to calm the nerves after a fevered round of testing is often to try and put a number to what your mouth is feeling. Eight years ago my mother sent me a fancy set of calipers in a vain attempt to draw me out of the dark depths of the restaurant kitchens and back to a much more sensible career like mechanical engineering or gunsmithing. Who knew that their inaugural run would be on a couple of potatoes?

Well mom, your gift has finally paid off.

Using the thoughtful (and rather presumptuous) gift, I confirmed that the crust formed on a traditional french fry, at 39/1000ths-of-an-inch, is over twice as thick as the 17/1000ths-of-an-inch crust formed on a fry that is only fried once:

Measuring the thickness of the respective crusts shows the difference double-frying makes.

Theory #2: Debunked.

The Real Solution

Turns out that the true reason double frying is necessary has nothing to do with the very center of the fry—only with its outermost layers.

The key lies in the fact that not all water in a potato is equal. Some of it is bound within the potato's structure more tightly then the rest, requiring more energy to expel it. During the first fry, some of the water present in the potato evaporates and exits, allowing the oil to enter the space it was taking, and come in direct contact with the potato's cells. Meanwhile, water that is more tightly bound in the potato's structure remains.

Soon, individual molecules of starch break free from larger granules with the help of the energy provided by hot oil. These starch molecules then come in contact with the water still present in the potatoes, hydrating and forming a gel that acts as a kind of glue, reinforcing the structure of the cells around it. Over time, as the oil works its way into the potato, a relatively thick layer of starch-reinforced cells can build up around the exterior of the potato. This is the paper-like sheath you see around a potato that has been fried once at low temperature.

Since water maxes out at at 212°F (100°C), boiling a potato does not have the same effect—there is not enough energy to expel excess moisture and gelatinize the starch. In order to confirm this, I tried par-cooking a potato in oil that I kept at exactly 212°F. It completely failed to form the requisite sheath, producing fries that were just as frail and thin-walled as the boiled potatoes.

Only after this thick outer layer has been sufficiently built up can the potato be fried a second time at a sufficiently high temperature to drive off the last remnants water, leaving the crisp, desiccated structure behind.

And what about Theory #3 you ask? Well it's true. There are such things as stupid questions, but in the words of Nigel Tuffnel, those are the ones that are "best left un-solved, really."

Now, close your eyes...

No wait—don't close your eyes yet. Look at this first:

When I say so, close your eyes, imagine the smell of freshly fried potatoes, the crisp, salty crackle between your teeth and the hot, saltiness on your tongue, sigh deeply and contentedly, shut off your computer, and take a 15 minute nap, letting yourself float through a golden sky of french fries with sea salt stars and fluffy white clouds of mayonnaise...

And.... now.

Continue here for French Fries: Iteration 1 Recipe »

About the author: After graduating from MIT, J. Kenji Lopez-Alt spent many years as a chef, recipe developer, writer, and editor in Boston. He now lives in New York with his wife, where he runs a private chef business, KA Cuisine, and co-writes the blog GoodEater.org about sustainable food enjoyment.

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Tags: french fries, recommended, The Burger Lab