Buzz Blog

A Watched Spaghetti Still Curls: Why the Pasta Won’t Lie Flat

Monday, January 13, 2020

Ah, spaghetti. There’s just something about the loveable dish that’s captured popular attention for decades, from the BBC’s spaghetti tree hoax to that famous kiss scene in Lady and the Tramp. But it’s also kept researchers busy with projects like feeding pasta through an MRI machine to see how it cooks, explaining why you just can’t keep sauce from covering your shirt, and building the ultimate spaghetti-snapping machine.

Third-year graduate student Nathaniel Goldberg is one of the latest researchers to try his hand at the floppy dish. He works in Dr. Oliver O’Reilly’s lab at the University of California Berkeley, which models all kinds of things—from plant stems to shoelaces—as slender rods, a one-dimensional mathematical construction. Maybe it’s a case of having a hammer and only seeing nails, but Goldberg can’t help but think about rods anywhere he looks. “The other day, I was looking out the window at all the palm trees and started thinking, Wow, that would be fun to model,” he says—so perhaps it was inevitable that his dinner would eventually turn into a scientific paper, recently published in Physical Review E.

Curious about why perfectly straight stands of dry spaghetti transform into a messy knot on the plate, Goldberg decided to tackle the problem as an intellectual exercise. “It was just curiosity,” he admits. “I just saw something and thought, Hey, can I model that?” A dive into the scientific literature and several head-scratchers later, he finally has his answer.

“There are really two things to look at as the spaghetti cooks,” he says. “You can think about what’s happening to a given cross-section of spaghetti, and you can think about what’s happening to the piece of spaghetti as a whole.” As Goldberg discovered, food scientists have already described the process through which a spaghetti strand absorbs water from the outside in and undergoes a chemical change called starch gelatinization—that’s when the starch molecules in the pasta start disconnecting from each other, resulting in a more gel-like texture.

What Goldberg didn’t find was a description of why the noodle changes shape as it swells, transforming from straight-line pitched against the side of the pot to a curled heap on the bottom in nine-to-eleven minutes. “People have worked on totally cooked spaghetti, and they’ve worked on uncooked spaghetti,” he says, “but no one has ever studied how spaghetti deforms as it cooks.” Take a look at the images below:



As this (real) strand of spaghetti shows, when the pasta is cooked in a tall pot it goes through three stages. The first, shown to the left, is a slight sagging, caused by the pull of gravity but resisted by the still-rigid noodle. However, as the pasta absorbs water its elasticity changes, allowing it to settle along the bottom of the pan (middle image). Finally, right as the pasta finishes cooking it curls over, as shown to the right.

According to Goldberg, these cooking stages result in a permanent—if slight—curvature in the pasta, which he was able to model mathematically, though it wasn’t easy. To begin with, the length and diameter of a noodle increase as it absorbs water, changing the strand’s moment of inertia. Water absorption also affects the elastic modulus, which is responsible for spaghetti’s change from stiff to floppy. “From a problem-solving perspective, this is a really challenging thing to formulate numerically,” Goldberg explains. He had to rely on some mathematical tricks, like expressing segments of the strand as fractions, to get a functional model: