Math explains a chef’s intuition: When cutting soft stuff, slice, don’t dice

L. Mahadevan, an applied mathematician at Harvard University, delights in unraveling the principles that underlie seemingly simple phenomena, ranging from how cucumber tendrils coil around supports to help plants climb, to the optimal way to balance on a tightrope. His latest work is something he’s been pondering for years, nearly every time he’s cooked a meal: the physics of cutting soft stuff.

Mahadevan, a vegetarian, found it interesting that when cutting stiff vegetables, such as a pumpkin, or a potato, people push down with the blade. They very rarely “slice,” drawing the blade toward them while they push down. Soft fruits and vegetables, such as a tomato, or a persimmon, on the other hand, require more slicing to make an effective cut. As he mulled the problem, his mind ranged all over, and he recalled that when he met his wife, he had a paper cut, bringing up the everyday mystery of how paper can slice flesh. What was the explanation for the two techniques, he wondered.

In a careful experiment published this month in the journal Physical Review Letters, Mahadevan and colleagues measured the forces involved in slicing, using a stripped down experimental apparatus—a soft gel and a fishing line as the blade. They mimicked both dicing and slicing with the apparatus—“dicing” defined as just pushing down with the blade, and “slicing,” the action of drawing the blade at an angle as one pushes down.

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What he found was that the force required to cut is much less when one slices a soft object, rather than dicing it. Slicing generates “microtears” in the soft object, and those coalesce and eventually allow the thread to break through. The force required, in comparison to dicing, is less by a factor of five, he found.

“The simple reason for that is when I dice, I have to squeeze down, I’m deforming the soft object over a large scale, which means I have to have a lot of force,” Mahadevan said. “If I was slicing, I don’t have to push down. We show mathematically, the forces required to reach the same nominal stress to break it will happen earlier.”

The experiment was driven by curiosity, but as with a lot of everyday phenomena, Mahadevan thinks understanding the science of cutting could lead in new and unexpected directions—perhaps in food processing, or other industrial applications. It’s also spurred the scientist to think about the evolution of cutting instruments, both in the range of tools human beings have used to cut, and in teeth.

One area Mahadevan’s gotten interested in studying is different types of knives—bread knives, tomato knives, the knives our ancestors used, scalpels—to see if each of those instruments are ideally designed for the tasks for which they have been set aside.

“If you were an alien and you came and saw an array of cutting instruments, surgeons using different cutting instruments for different surgeries, would you be able to infer what they are for?” Mahadevan said.

At a time when many scientific questions can seem dauntingly difficult, requiring a certain amount of knowledge to even understand what is being asked, Mahadevan sees profound discoveries waiting to be made all around us.

“It’s one thing to experience it; it’s another to understand it,” Mahadevan said. Familiarity, he says, does not mean comprehension.