An onion peeler is a mechanical device that is designed to remove the skin from an onion without damaging the underlying onion or bruising it in a way that will cause it to quickly rot. There are very few models designed for use in a home, because the mechanisms most commonly employed are fairly large, although the machines are a staple in food preparation, manufacturing and some high-volume kitchens. The two most common onion peeler designs both operate on the principle of forcing the onions to touch sharp blades or teeth that, over time, completely remove the loose outer skin of the onion. One design uses a barrel-like chamber to hold the onions, while the second type uses a screw-like auger or conveyor belt to move a line of onions across a series of blades. More rarely, an individual onion peeler can hold a single onion and, with a blade, remove the skin in much the same way that an apple peeler functions.
The basic concept behind most designs for an industrial-grade onion peeler is that the combination of friction, the mass of the onion and a set of small teeth or blades will mesh to eventually remove the outer, loose skin of an onion. The process requires that the onion be moved and rotated so each side of the onion will pass by the teeth or blades. This method is quite effective and, when care is taken, does almost no damage to an onion, meaning the peeled alliums can be stored for a short time after being peeled.
The problem of moving the onions across the teeth or blades in an onion peeler is solved in one design by using a barrel-like hopper. The hopper is lined with sharp teeth and the onions are loaded inside. When activated, the hopper begins to rotate, causing the onions to tumble inside and come into contact with the teeth at different angles. These machines can be prone to clogging as the skins are removed.
Another solution is to use a long, horizontal screw that constantly turns. The onions are placed on the screw and moved down the line as if they were on a conveyor belt. Plates of sharp teeth are positioned at different points so they cut away the skin of the onions as they move and rotate, driven by friction with the turning screw. This type of onion peeler tends to be very large, however, and usually works in conjunction with some type of loader.
A single onion peeler can be used but normally requires that individual onions be specifically loaded into a special holding vice or brace. This peeler operates by rotating the onion in place while a blade moves from one end to the other, stripping away the skin. This model functions much like an apple peeler but is not as efficient for use on large batches of onions.