Koshering is the process of making food or equipment fit for use under Jewish dietary law (kashrut). It most commonly refers to two things: preparing meat so it’s permissible to eat, and purging cookware or kitchen equipment of non-kosher residues. Both rely on the same core principle, that absorbed substances can be drawn out through salt, heat, or water, but the specific steps differ depending on what you’re koshering and why.
Koshering Meat: Soak, Salt, Rinse
The Torah prohibits consuming blood, so after an animal is slaughtered according to kosher rules, the meat must go through a salting process called melihah to draw out residual blood. This is what most people mean when they say “koshering.” The steps are straightforward but precise.
First, the meat is soaked in cold water for thirty minutes, then rinsed fully with cold water. Next, coarse salt is applied to all surfaces of the meat and left in place for one hour. The salt draws blood to the surface through osmosis. After the hour, the meat is rinsed thoroughly in cold water three times to remove all remaining salt and extracted blood.
The type of salt matters. Standard table salt is too fine; it dissolves on contact and absorbs into the meat rather than sitting on the surface long enough to pull blood out. Koshering salt (the origin of what grocery stores sell as “kosher salt”) has large, coarse, uneven grains that cling to the meat’s surface without dissolving immediately. It also contains no additives, no iodine, and no anti-caking agents, which could affect color or flavor during the process.
Why Liver Requires Broiling Instead
Liver is the one major exception to the soak-and-salt method. Because liver contains such a high concentration of blood, salting alone can’t extract enough of it. Instead, liver must be broiled over an open flame or on a grate that allows juices to drip away freely.
The liver is broiled until the outer juices stop flowing and the surface is dry. Color is the key visual indicator: properly broiled liver changes from its raw deep brown to a lighter shade. If any section remains that deep brown color, it hasn’t been broiled thoroughly enough and needs to go back on the flame or be cut away. Salt can be sprinkled on before broiling as a custom, but it isn’t required the way it is for other cuts of meat.
Checking Eggs for Blood Spots
Koshering principles extend to eggs as well, though the process here is inspection rather than treatment. A blood spot in an egg may indicate the early formation of an embryo, which makes it forbidden. In fertilized eggs, a blood spot means the entire egg must be discarded.
The common practice (particularly among Ashkenazi Jews) is to crack each egg into a separate bowl and check it before use. Any egg with a visible blood spot is discarded regardless of where the spot appears in the egg. That said, not every dark mark is actually blood. Brown spots with an irregular, non-round shape are typically tissue fragments, not blood, and are permitted. The distinction matters in a busy kitchen: a true blood spot looks red and round, while a harmless deposit tends to be brown and shapeless.
Koshering Kitchen Equipment
When cookware, utensils, or appliances have absorbed non-kosher food (or when preparing a kitchen for Passover, which has its own dietary restrictions), the equipment itself needs to be koshered. Jewish law treats different materials and cooking methods differently, based on the idea that flavors are absorbed and released through the same process that introduced them.
Boiling (Hagalah)
Utensils that were used to cook with liquid, like pots, silverware, or ladles, are koshered by immersion in boiling water. The item must first be scrubbed clean and then left unused for 24 hours. After that waiting period, every surface of the utensil is submerged in a pot of rolling boiling water. A large spoon, for example, can be dipped halfway for about 10 seconds, then flipped to immerse the other half. Items with narrow cracks, deep scratches, or crevices that can’t be fully cleaned cannot be koshered this way, since trapped residue won’t be reached by the water.
Burning (Libun)
Equipment used directly over fire, like barbecue grates or baking pans, requires more intense heat. Full burning (libun gamur) means heating the metal until it glows, which burns away any absorbed flavors entirely. Running a self-cleaning oven cycle at roughly 850°F also qualifies. A lighter version called libun kal heats items to 550°F in a dry oven for at least one hour, and is used for things like stovetop grates where the need for full burning is less strict.
Materials That Can and Can’t Be Koshered
Metal, wood, stone, natural rubber, and fabric can generally be koshered. Ceramic and enamel-coated items cannot, because they’re considered too porous to fully release what they’ve absorbed. Glass is a point of disagreement: Ashkenazi tradition holds that glass cannot be koshered, while other communities permit it. Plastic and synthetic materials fall into a gray area where opinions vary, so individual guidance from a rabbi is typical.
How It Works in Commercial Food Production
The same principles scale up to factories and industrial kitchens. When a production line needs to switch from non-kosher to kosher output (or between dairy and meat), the equipment goes through a process called kashering-in. This typically involves applying intense heat, often process steam or even a blowtorch, to every surface that contacts food. A supervising rabbi must be physically present to oversee the process and confirm that every piece of equipment receives sufficient heat for an adequate duration.
Only metal and glass equipment can reliably be koshered at the industrial level, since plastic and ceramic surfaces are considered too absorbent. Steam systems introduce another layer of complexity: most factories recirculate water through a shared boiler. Because steam absorbs the character of whatever it heated, steam that was used in non-kosher production can’t simply be redirected to kosher lines. Facilities that run both kosher and non-kosher products often need separate steam systems or careful scheduling to avoid cross-contamination. Where dedicated kosher equipment isn’t feasible, the kashering-in process becomes a regular part of the production cycle, repeated each time the line switches over.