What is Kashering?

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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.


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.



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.


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.


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Post 5

I'm quite interested in the chemical makeup of the red meat and red juices - are there any components of blood in them? What about the red "veins" in the chicken?

Is there any concept of being more strict in the salting in order to take out even *more* blood?


Post 4

To sgbfishman, You raise an extraordinarily interesting point. Please let us know how your research in this area goes, as this is a fascinating topic and your questions are right on point from a scientific view! Tricia Ellis-Christensen

Post 3

To elfi64 - Thanks for your thoughts. You are absolutely right about the red liquid under cooked meat not being considered blood. I asked a rabbi about that, and was informed that, just as you said, it is just the juices of the meat.

As for the question about actual contents of that red liquid, it truly amazes me that no one has carried out this investigation, or at least no one has published results. I'm actually planning to do the investigation myself in the near future, and if I have anything to report, I'll let this discussion group know.

Post 2

Thought I'd share something interesting about kashering. Kosher meat has the blood extracted through kashering. The red liquid in a under cooked piece of kosher meat (say a steak cooked medium-rare) is still kosher because that red liquid is not considered blood, rather it is the meat's juices.

Sgbfishman - I'm by no means an expert, but I thought I'd give it a bit of a go... While blood gets it's color from the iron in hemoglobin (a protein in red blood cells, blood is comprised of red blood cells, white blood cells, and platelets. So since kashering is the process of removing blood, not just the red color, it is removing all the components of blood, not just the hemoglobin. Regardless, I think both the salt and the soak are crucial. If it was only the soaking that mattered why bother with the salt? Salt absorbs liquid too so I assume that's why it is useful in the process.

Post 1

The question I really was hoping to get an answer for was, when kashering meat by soaking in water and then in salt, the reddish color of the liquid removed from the meat is due to whole red blood cells or just the hemoglobin. If the latter, then perhaps the crucial step is not the salting, but the soaking, as that would cause the cause cells to burst through osmosis, thereby freeing their contents to be removed by diffusion into the (subsequent) salt layer. In other words, I'm trying to nail down the actual scientific mechanism of kashering meat. If anyone has done any research on this, I'd love to know. I have my own speculations,(which I'd be happy to share) but have not found definitive proof for any of this.

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