Kashering is the way in which utensils, dishes, cutting boards, glasses and any other kitchen instruments are made kosher, whether they are brand new or if they have come into contact with a non-kosher food. While it isn't limited to the holy day of Pesach, or Passover, many Jewish families kasher right before Pesach to remove chametz, crumbs and leavening sources that are forbidden food during Pesach. Another definition of kashering refers to the process by which meat is drained of blood so that flesh and blood are not consumed together — a very important kosher dietary law.
Meat
Kashrut, the Jewish dietary law, governs the process by which meat is made kosher. It forbids the blood of an animal from being consumed because doing so is equated with eating a living animal, another Jewish dietary prohibition. While most blood is drained during the slaughtering process, the remaining blood is dealt with through kashering.
This involves a process of soaking the meat in water, salting it, and then rinsing it. This process pulls the excess blood out of the meat and makes it kosher for eating. With certain meats, like liver, it involves making cuts in the meat to help extraction of the blood, salting it, and then grilling or broiling it.
Cleaning
People often speak of kashering in reference to making all things kosher by cleaning. There is some debate on whether things used in violation of kosher standards can later be made kosher. For instance, a person who converts to Judaism, or begins to observe Jewish dietary laws when he or she did not do so in the past might need to replace certain kitchen objects, like dishes. Ultimately, those who have specific questions as to whether it's possible can make something “old” kosher should consult a qualified rabbi.
Kashering is a two step process. First, a person must clean the objects. After cleaning, the person must wait at least 24 hours. The 24-hour break allows the objects to lose the unkosher "flavor" they may have had. The second step is to kasher the objects. There are two basic ways to do this: boiling or heating. There are different forms of heating, including heating in the oven and heating by blow torch, and the necessary form depends on the way the item was made non-kosher.
Pesach
While Jews are obligated to keep kosher always, some less observant Jews focus on kashering only for the most observed holiday, Pesach. The process is focused on removing all chametz, or leavening, from the kitchen. Because crumbs can get pretty much anywhere, this is often a long process.
During this process, it's important to note that certain things can typically not be made kosher. These include any items with cracks, narrow necks (like baby bottles), items with slots, like slotted spoons, and certain things like toothbrushes, sponges and covers for kitchen sink drains. These either need to be replaced or placed elsewhere, away from the home during Pesach.
People may also make sure to cover surfaces they haven’t cleaned, especially tables, in case they’ve missed a few minuscule pieces of chametz. The level to which cleaning occurs before Pesach tends to be determined by the type of Jewish sect to which a person belongs. Some take the dietary strictures extremely seriously, and others do not.
During the cleaning process, visible chametz is removed from various items, especially anything that will be used on the Pesach table. This is often a time to perform various tasks, anyway, like polishing silverware. In the purging process, items are placed in very hot water. Kashering a cloth tablecloth for example might mean making sure it is free of any crumbs, and then washing it on a hot cycle in the washing machine.
Kitchen appliances must also be thoroughly cleaned, particularly ovens, refrigerators, stoves, microwaves and the like. This helps prepare the home for the very important celebration of Pesach. Moreover, it obeys the biblical directions of God that Jews keep no source of leavening or leavened material in their home so they will be recognized as the people of God.
