Each hole in a grater is a small opening that faces one way; if a sponge or cloth is pushed against the front of the opening, it can tear, and your hand can slip directly onto the edge. The correct direction is the reverse: move a brush from the back of the opening toward the front, in the direction the food comes out. The second rule is timing—dried cheese and peel harden inside the holes and permanently dull the grater.
A grater hole is not a straight hole. It is a small opening cut and lifted from the sheet metal, with one sharp edge and a single facing direction; food enters at the front of the opening and exits through the curved space at the back. That is why a grater cuts in only one direction and does nothing when turned the other way. Cleaning must follow the same geometry: anything coming from the front catches on the edge, while nothing coming from the back catches.
Cleaning with a sponge can cause two different problems. When the sponge is pushed against the front of the opening, it tears and pieces can remain inside the holes; the second and more serious danger is that a finger can slip onto the edge where the sponge is thin. Some kitchen cuts that go unnoticed come from graters rather than knives, because people do not think of a grater as a sharp tool. Do not rub a grater by hand under running water either. If you hold it under the faucet, turn it around so the back faces the water; water flowing over the back pushes residue out, while water flowing over the front pushes it in.
The right tool is a dry brush or a soft-bristled brush. Brush from the side where the food exits—that is, from the back of the grater—toward the openings; residue is already free to move in that direction and falls away in one pass. If you do the same from the front, the residue is pushed into the hole and gets stuck inside. On a box grater, work each side in its own direction; use an even lighter touch on a fine grater.
Timing matters more than the method. When the fat in Kashar cheese, the essential oil in citrus peel, and the sugar in carrots cool and dry, they leave a glassy film in the holes; a grater left for a few hours can no longer be cleared with a brush. If you clean it right after use, while the food is still moist, the job takes only a few seconds. This single habit does more than anything else to extend the grater’s cutting life. Fibrous foods such as garlic and ginger are especially likely to lodge right in the opening; make immediate cleaning a rule after grating these 2 foods.
A dishwasher does not solve the problem; it delays it and comes at a cost. The machine’s abrasive detergent and prolonged contact with hot water dull the edge of the openings; after a few years, the grater starts crushing food instead of cutting it. Drying is also incomplete, and moisture left in the holes can rust an uncoated grater. Clean the grater by hand with a brush from the back; dry it immediately and store it so the edges do not rub against other dishes, preferably in its own box. When stacking box graters, position them so their edges do not face one another, because metal rubbing against metal dulls the edge at least as quickly as use.