The Science of Knife Sharpening
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Anyone with cooking experience knows the pleasure of working with a good, sharp knife for food preparation. With the proper knife, slicing vegetables, cutting up a chicken, or cleaning a fish goes smoothly, much more smoothly with a sharp edge to work with than a dull one. A knife at the most basic level is a wedge that splits things in two. However, the science of cutting with a knife appears a bit more complicated than that, which will explain the importance of proper sharpening. In fact, a group of Harvard physicists and engineers led by Lakshminarayanan Mahadevan looked much more closely at how knives cut in a research paper titled “Slicing Softly with Shear.” The article appeared in the journal Physical Review Letters in 2012. The researchers used instruments to measure the force needed to cut through agar, which is a firm substance used in laboratories to grow bacteria and, in this experiment, was similar to the firmness and texture of a roast. Their analysis compared the force needed to cut or, as they called it, crack the agar (it would be best to think of the agar in this experiment as an eye of round roast). To understand how cutting works, they varied the angle of the knife to see which types of cutting required less force. They varied the angle from pressing straight down to pulling the blade at different angles across the ‘roast.’ Interestingly, they found that pressing down required the greatest force to crack the agar ‘roast’ because the force of the knife gets distributed over all the agar. In other words, the agar dimples in under the blade and absorbs the force of the knife. However, the study showed that drawing the edge at an angle across the agar focused the energy more locally and required much less force to cut the agar ‘roast.’ Looking more closely under a microscope at the knife edge, the researchers found that knives with little teeth or peaks and valleys in the metal produced friction while cutting across the agar ‘roast’ and that friction helped the blade cut with ease (note: both serrated and non-serrated knives have these microscopic teeth). A smooth or dull blade causes no friction, so it slides across the material without cutting or cracking it.
A good knife edge needs tiny peaks and valleys to provide the teeth that create the friction to cut materials with little force. A good knife consists of a hard material such as steel or ceramic that can be ground to make the tiny teeth on the edge of the blade. The Mohs scale was invented in 1812 by the German geologist Friedrich Mohs to classify the relative hardness of minerals. The scale ranks hardness of substances from one to ten with diamond at ten being the hardest. The Mohs scale puts steel near the middle of hardness at 4-4.5. So, in order to sharpen steel the material in the sharpening stone needs to be harder than steel. Many sharpening stones contain silicon carbide or carborundum, which ranks at nine on the Mohs scale making it almost as hard as diamond.
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Sharpening a knife requires a series of steps to produce a sharp edge that will not dull quickly. The sharpening process begins with a coarse sharpening stone to create a new edge or the tiny peaks and valleys. To sharpen your knife on a sharpening stone, first lay the knife blade flat on its side. Then, lift the knife up about an eighth of an inch keeping the edge on the stone facing away from you. Next push the knife away from you working from hilt to tip. Once the edge forms after a few strokes, flip the knife and do the other side. Then proceeding to finer sharpening stones with smaller grains of carborundum, the tiny teeth on the blade edge become shorter and more evenly lined up (most commercial sharpening stones have a coarse side and a fine side). The shorter teeth will keep the knife sharp for longer because the short teeth do not bend over as easily as long ones. Bent over teeth or ground down teeth lead to a dull blade (remember, however, these ‘teeth’ cannot be seen with the naked eye). Often one sees chefs quickly running a knife up and down a steel rod to sharpen their knife. The steel rod or honing steel plays an important part in keeping a knife sharp. The honing steel does not add a new edge rather it straightens out the microscopic teeth, restoring the edge to peak performance. Personally, before I use my knives for cooking I run them down a honing steel three or four times per side. I use my sharpening stone every four months or so to restore the edge to my most heavily used knives. Most knives for cooking come in different grades of steel.
Working with a sharp knife can make the difference between an enjoyable or frustrating experience. Science has shown that an edge with microscopic teeth produces the friction necessary to cut food with much less energy than a dull blade that just slides across the food. Most knives have a steel blade. Sharpening steel requires sharpening stones made of fine grained material that is harder than steel such as carborundum. Working from coarse to fine stones produces tiny peaks and valleys on the edge of the blade that create the friction necessary for smooth cutting. Using a honing steel keeps the edges microscopic teeth straight and sharp. Because a sharp blade requires less force than a dull one, many believe that a sharp knife is safer than a dull one. Dull knives need more pressure which can cause one to lose control of the blade and cause an accident. Moreover, knife sharpening is fun, and it makes kitchen work more enjoyable.
Dr. Smith’s career in scientific and information research spans the areas of bioinformatics, artificial intelligence, toxicology, and chemistry. He has published a number of peer-reviewed scientific papers. He has worked over the past seventeen years developing advanced analytics, machine learning, and knowledge management tools to enable research and support high level decision making. Tim completed his Ph.D. in Toxicology at Cornell University and a Bachelor of Science in chemistry from the University of Washington.
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