Knife Sharpening Class

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This electron micrograph of a sharp knife reveals that the cutting edge is crisp, with peaks and valleys along its length that generate a lot of friction as it’s pulled across the surface of food and tears open a crack. Once the crack is started, gentle downward pressure is enough for the blade to plunge through the food with ease.

Special thanks to Michael McDonnell at Midwood High School at Brooklyn College for providing ChefSteps with these two SEM photos.

Sharp Blade

Seen under an electron microscope, the cutting edge of a dull knife is actually round and smooth. When drawn over food, it glides along with very little friction. Because of this, a dull blade cannot get enough traction to rip open the crack in the food that begins the cut. Instead, you’ll have to push down with more force—squishing the food—to get a crack started and cut through the food.

Dull Blade

Have you ever wondered why a sharp knife slices through food with ease, whereas a dull knife does not? Or why slicing cuts cleanly, whereas chopping can mutilate delicate ingredients? The answers to these questions even explain why a sharp knife is safer than a dull knife. It all has to do with the physics of cutting.It may surprise you to learn that cutting anything always starts with a crack. In the kitchen, we use a knife to open and expand a crack in the food we’re cutting—whether we’re gently slicing or aggressively chopping it.But anyone who has done much cutting knows that slicing is fundamentally different from chopping. Why? Chopping is all about pressing the blade of your knife downward. When your knife strikes the surface of the ingredient, there is a split second during which the food beneath the blade is compressed into a V-shaped valley. A moment later, a threshold is crossed, and the food ruptures under compressive force. Now the blade plunges downward, propagating this crack through the food. This is not unlike how a log is split under the sharp blow of an ax.Slicing is different from chopping because the knife is pressed down with only modest force, so there is only a small amount of compression. Instead, slicing stretches the food as the blade is drawn across its surface. Like most materials, food is strong under compression but weak when stretched.And this is why slicing cuts cleanly. The tearing force easily rips open a crack without the need to press down hard. This results in a much cleaner cut.Creating a strong tearing force when slicing is also why it’s essential that your knife be sharp. The stretching force arises through sideways friction between the blade and the surface of the food; if there’s not enough friction, then the knife cannot get traction to rip open a crack. Instead, you’ll need to push down harder with your blade—squishing the food—to get a crack started.Seen under a microscope, the cutting edge of a dull knife is actually round like a wire and very smooth. When drawn over food, it glides along because there is so little friction. In fact, there is so little friction that it’s hard to control the blade, and it can suddenly slip off the food and plunge into your fingers—the main reason a dull knife is dangerous.A sharp knife, by comparison, has a crisp edge, with microscopic peaks and valleys along its length that generate a lot of friction as you drag it across the surface of food. This sharp edge easily grabs hold of a surface and tears open a crack. Once the crack is started, a gentle downward pressure allows the blade to plunge through the food, opening the crack as it goes.For those who are interested in the physics of cutting, Professor Mahadevan, an applied physics researcher at Harvard University, has published a wonderful paper on this subject, and you can find it <a href="http://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.109.244301">here</a>.

How Knives Cut