Kosher salt, also sold as rock salt, is a type of coarse salt that is usually made without additives. The salt itself is not necessarily kosher, but takes its name from the curing process used on kosher meats. It is ideal for certain cooking projects and is preferred to table salt by many professional chefs because it has a more mild flavor, and the flaky crystalline structure of the salt helps it adhere to a variety of surfaces, from fish to margarita glasses.
Like all salts, this variety is a form of sodium chloride. It can be extracted from seawater by a direct evaporation process, or it can be mined from salt deposits under the Earth's crust. Table salt is heavily refined so that it has a precise square shape, and iodine is usually added during the refining process. Kosher salt is allowed to remain a more coarse-grained salt, meaning that the structure under a microscope looks like a series of cubes stacked on top of each other, rather than a single grain.
This type of salt is ideal for curing meats, because the many facets of the salt help to draw the liquid out of fresh meat. According to Jewish dietary laws, called kashrut, blood cannot be consumed, even in small amounts. Meat with blood in it will not be approved for Jewish consumption by a rabbi and cannot carry a kosher label. Kosher slaughterhouses use salt to extract all of the blood from their meat to ensure that the meat will comply with dietary laws. Many non-kosher slaughterhouses use it in their curing process as well, because the presence of blood can adversely affect the flavor of meat.
Because kosher salt is not heavily refined or treated with iodine, it has a flavor that many chefs consider to be more pure. For this reason, it is favored for seasoning in professional kitchens. The coarse grain also allows chefs to measure out pinches with ease. Because of the large grain, this salt is not well suited to baking or table service, when fine grained salts are more appropriate.
In addition to being used for seasoning, kosher salt is used to create salt crusts on baked fish, to create a salty rim on margarita glasses, and to rub meats along with other spices before cooking. It is also used in pickling because the lack of iodine reduces the risk of discoloration and cloudiness. Most professional kitchens keep a stock of this salt in small dishes around so that it can be quickly and easily added to foods.
Kosher salt is readily available in most stores and restaurant supply houses. Generally, the salt is comparable in cost to other varieties. A number of websites have information on how to use it for curious cooks interested in experimenting with this seasoning option.