In the end, Lopez-Alt found that the oil of the first frying changes the structure of the outer part of the chip. The water that's easy to boil away leaves, and the starches – released by the heat of the oil – combine with what's left to form a gel. That gel seeps around and stiffens the edges of the potato strip. When the hot oil of the second frying hits, the whole thing is primed to form that thicker crust and to have what remains of the moisture become steam. Boiling water cannot get hot enough to cause the gel to form.
But are twice-cooked chips the pinnacle? Heston Blumenthal of the Fat Duck thinks not. He famously concocted a recipe for cooking chips three times. The first time is a light boiling, followed by a stint in a vacuum chamber to remove the traces of water. The second is deep-frying at a relatively low temperature, and the third is the high-heat, crust-forming extravaganza. The chips you get at the end are so desiccated they have a glassy texture, with a fluffy interior – making them arguably the finest chips on the planet.
Nathan Myhrvold, former Microsoft CTO and author of the Modernist Cuisine cookbook has gone several levels of difficulty further with his recipe for “ultrasonic French fries”. It’s not exactly something you can try at home, but the result is apparently a “hugely satisfying crunch when you bite through the exterior” before yielding to “a centre of incredibly smooth mashed-potato consistency”, and a process he says could be automated by a food manufacturer.
The recipe vacuum seals chips in 2% brine before hitting them with ultrasound waves from the same device dentists and jewellers use, which cracks and blisters each chip with tiny bubbles and fissures. Next, the chips are vacuum-dried, to adjust the water content of the exterior, and then are briefly blanched in oil at 170C (340F) to tighten the network of starch molecules. After cooling, they are fried for a few minutes in hot oil at 190C (375F). The idea here is that the water turns to steam inside each minuscule bubble on a chip’s surface, forcing the bubbles to puff up.