How do you revive soggy French fries?

In Food Science, Dave McCowan from the University Of Chicago’s Department Of Physics answers our confounding questions about the mysterious world of food.

Those fries you picked up last night on the way home from the bar hit the spot—golden, crispy outsides and fluffy, soft insides. But the leftovers heaped in a pile on your kitchen table this morning? Soggy, damp, sad, and destined for the trash bin.

What happened? Is it possible to revive the dreamy taste of yesterday and perk up those limp has-beens?

Thankfully, yes. But to understand the dead, we must first understand the living. How do these perfectly cooked taters come to be, and how can we use that same know-how to resuscitate and rescue?

Deep-frying is a type of cooking that differs in several important ways from other common kitchen methods. First, submerging food in oil means the heating is uniform and cooking happens on all sides at once. (Compare this, for example, to the one-directional heating achieved in a hot pan or on the grill.) Second, oil has an extremely large volumetric specific heat (meaning it contains more heat energy per unit volume per degree of temperature) compared to other cooking media like the air in an oven or a hot cast-iron pot. This high specific heat means that heat transfer is quick and efficient, cutting down on cook times and keeping the oil hotter for longer when cold food is added.

Finally, if the food has been battered, then deep-frying gives another unique advantage: a sturdy crust. Moisture in the batter quickly boils off (water evaporates at 212 degrees Fahrenheit) leaving big air pockets in the gooey protein mesh of the batter as the steam bubbles pop. Heating continues, and the Maillard reaction—which sets in at about 300 degrees Fahrenheit—hardens the batter, browns the exterior, and performs its chemical reaction magic to make everything extra tasty. Since this outer crust now forms a barrier (no oil gets in and no water gets out) the water remaining inside is trapped, and our food steams itself from the inside, leaving it moist and fluffy.

This explains juicy fried chicken and tender fried cod, but French fries hold an even more exalted place in the fried-food pantheon. Whereas a piece of meat (or a Twinkie) gets its crust from an added batter, potatoes are naturally high in starches that can do the same job. When warmed, starch clumps into blobs called granules that pull in water, and these swollen granules grow large enough at high heat to burst. When that happens, the gelatinous goop that explodes outward ends up airy and sticky like batter, and can crisp up in hot oil to form the crust.

Perfecting the ratio of golden crust to soft center has been the subject of many culinary studies, with Heston Blumenthal’s triple-cooked chip and J. Kenji Lopez-Alt’s perfect thin-and-crispy fries being the acknowledged paragons. In both methods, potatoes are first boiled, then double-fried (once at low temperature, then a second time at a higher temperature). The rationales and details are a little different in each recipe, but the processes both serve to build up rough, starch-craggy surfaces that fry into crisp crusts protecting soft interiors.

Whether it’s one of these recipes or the local hot dog stand’s take, fries are best when they’re fresh. As they sit, that glorious crust begins to take on moisture from the air and soften. It’s natural and unpreventable, but it leads us back to the original question. Once they’ve gone soggy, can we save them?

Microwave? It will be a disaster. The water inside boils and heats the fry, but since there’s no protective layer anymore, the result is simply a steamed spud.

Oven? Better, but with caveats. By slowly heating from the outside in, there’s hope that moisture will boil out of the surface—reestablishing a crust—before the water on the inside heats up the fluff. Unfortunately, though, this same temperature gradient tends to burn the potato shell before the inside’s ready, risking an overcooked matchstick.

Cast-iron pan? Getting warmer! Laying your tots out (well-spaced) in a cast-iron pan set to high heat on the stovetop can actually approximate many of the features of a deep fryer. The pan will heat faster than the oven yielding a quick-hot surface, and the residual oil that seeps out of the fries as they warm should be enough to draw out the moisture to re-dehydrate the surface. Keep the fries moving to prevent overcooking, and make sure they stay spread out to avoid steaming one fry with the moisture of another; the oil needs room to crisp the crust back up without letting the escaping water seep back into adjacent fries.

A waffle iron? It sounds odd, but yes. Placing cold potatoes between the sides of a panini press or waffle iron will mimic the hot cast-iron pan with the added perk of multi-directional (and therefore, more uniform) cooking. As before, oil will leak out as the fries heat, but this time the nooks and crannies (or at least parallel sides) trap the oil and better surround the chip as it cooks. Additionally, both the press and the iron are designed to vent some of the steam out as you cook, keeping the moisture away and keeping things dry. Because the fries have already been cooked, there isn’t much sticky (raw) starch left on the surface, so fries will develop only minimal cling to each other, remaining separate even when packed tight.

This brings us to the final suggestion…

Deep fryer? Bingo. Refrying your French fries—though messy and inconvenient unless you’ve got two quarts of canola ready—is by far the best method, especially if the fry was a little underdone to start with. Remember that the best crust comes from pushing out water and busting up starch granules, so the most efficient method is still the original method. If your fry is shoestring thin, though, be warned. A vigorous dunk in the oil might burn the fry and dry the insides out, meaning the pan or the waffle iron is safer. However, if we’re talking thick-cut potatoes here, repeated frying can be carried to extremes. In his book, The Food Lab, Lopez-Alt even gives a steak-frites recipe that calls for five rounds of crust building!

Leftover fries may seem sad, but don’t let a soggy spud scare you. Whether refried, waffled, or simply sautéed, crisp crust rejuvenation is within your reach.