Want to know if spaghetti is al dente? Check how much it curls in the pot

Place a strand of spaghetti in a pot of boiling water and it will start to sag as it softens, before sinking slowly to the bottom of the pot, where it will curl back on itself to form a U shape. A cursory explanation might be that as the spaghetti softens during cooking, it deforms more easily, and gravity causes the saggy strand to sink. But what accounts for the curling behavior? Physicists at the University of California, Berkeley, provide a much more thorough explanation in a new paper in Physical Review E.

There have been a surprisingly large number of scientific papers seeking to understand the various properties of spaghetti, both cooking and eating it—the mechanics of slurping the pasta into one’s mouth, for instance, or spitting it out (aka the “reverse spaghetti problem”). The most well-known is the question of how to get dry spaghetti strands to break neatly in two, rather than three or more scattered pieces.

The late Richard Feynman famously puzzled over the dilemma, conducting informal experiments in his home kitchen. French physicists successfully explained the dynamics at work in 2006. They found that, counterintuitively, a dry spaghetti strand produces a “kick back” traveling wave as it breaks. This wave temporarily increases the curvature in other sections, leading to many more breaks. Basile Audoly and Sébastien Neukirch won the 2006 Ig Nobel Prize for their insight.

In 2015, two MIT students in search of a final project set out to discover if there was any way to control those natural forces to achieve a neat, clean break. They found that twisting the spaghetti and bringing the ends together would cause the strand to break in half—but it required a pretty strong twisting motion. 

Finally, in 2018, Ars reported on work by two MIT mathematicians who figured out the trick: twist the spaghetti at 270 degrees before slowly bringing the two ends together to snap the spaghetti in two. The twist weakens the snap-back effect discovered in 2006. As the strand twists back and unwinds to its original straightness, it will release pent-up energy in the strand so there aren’t any additional breaks.

With that mystery solved, Berkeley scientists have turned their attention to another pressing pasta question: devising an accurate model to predict how a single strand of spaghetti will change shape as it cooks. Spaghetti, like most pasta, is made of semolina flour, which is mixed with water to form a paste and then extruded to create a desired shape (in this case, a thin straight rod). The commercial products are then dried—another active area of research, since it’s easy for the strands to crack during the process.

So what happens to the dried spaghetti when it is submerged in boiling water? Only a few seconds are needed for the strands to reach the same temperature as the water, but it takes a bit longer for water to work its way through the starch matrix of the pasta. As this happens, the spaghetti swells, and small amounts of a starch called amylose leach into the water. Finally, starch gelatinization occurs, a chemical process that governs textural changes, so one’s well-prepared spaghetti is al dente.

Berkeley physicists Nathanial Goldberg and Oliver O’Reilly focused on modeling the mechanical behavior of spaghetti as it cooks, drawing on prior research on the mechanics of growing rods, as well as Leonhard Euler’s elastica theory. They made a few assumptions for simplicity’s sake, most notably that the strand would not stick to the pot and that the thickness of the spaghetti doesn’t matter.  The model accounts for changes in the length, diameter, density, and elastic modulus of the spaghetti as it hydrates during cooking.

Goldberg and O’Reilly next soaked spaghetti noodles—selected randomly from a package of Trader Joe’s brand—in room-temperature water to test their model. They took snapshots every 15 seconds and tracked the position of the right end point with software. The authors acknowledge that this is not how one would (hopefully) cook spaghetti in one’s own kitchen. At room temperature, gelatinization does not occur. The strands still absorb the water, swelling and softening, but the texture is noticeably different—”as the reader can experience by attempting to enjoy a plate of spaghetti that has been soaked for several hours in room-temperature water,” the authors wrote. But the room-temperature soaking made for a much simpler experimental set-up.

They found that the experimental results matched the predictions of their model for how the spaghetti strands would deform. As Katherine Wright wrote at APS Physics, “It’s the change from rigid to viscoelastic behavior—and the strand’s resulting ability to develop curvature and permanently deform without breaking—that drives the shape change.”

No doubt this work will trigger a bit of the usual snark about scientists working on trivial questions when there are more important issues to be addressed. The paper should be a strong contender for anther Ig Nobel Prize. But the Ig Nobels mostly honor research that first makes you laugh and then makes you think, and Goldberg and O’Reilly’s work is far from trivial. For instance, it’s likely to be of interest to the commercial food production industry, since the model could make it possible to determine how well cooked noodles are just by looking at the deformed shape.

DOI: Physical Review E, 2020. 10.1103/PhysRevE.101.013001  (About DOIs).

https://arstechnica.com/?p=1638861