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Kashering Knives and Grinders

Rabbi Dovid Cohen
Administrative Rabbinical Coordinator of the cRc

January 2008 – Updated December 2022

Typically, if a utensil is used with cold, non-kosher food, the utensil may be used for kosher food after it is thoroughly cleaned. However, when a knife or other utensil is used to cut, chop, grate, or grind a non-kosher food, a residue of that non-kosher food remains on the knife even after the knife is “cleaned”.1 This principle, known as duchka d’sakinah, is discussed in the Gemara and Shulchan Aruch,2 where it states that the knife may only be used for kosher food if it either undergoes a process known as נעיצה or is filed down (i.e. sharpened). נעיצה involves thrusting the knife into semi-solid earth 10 times, and it is generally understood that this process is effective, because the earth serves as an abrasive cleaner for the knife’s blade.

If we take these halachos at face value, any grinder, chopper, grater or blades used for cutting non-kosher food in a factory would require נעיצה before it could be used for kosher food. However, in just about every factory situation, נעיצה is impossible for halachic or practical reasons. A grinder and cheese-cutting wires are good examples of this issue; even if the grinder’s blades were physically able to be removed from the machinery, they would likely not be suitable for נעיצה on halachic grounds (see Shulchan Aruch 121:7), and the wires are not firm enough to be “thrust” into dirt.

Is there some other way to clean a non-kosher knife instead of נעיצה (or sharpening) that accomplishes the same goals? [The simple reading of Shulchan Aruch OC 451:3 implies that hag’alah may be done in place of נעיצה , and the question here is whether there is some form of heat-free cleaning which can replace נעיצה ]. There are seveal indications that other forms of cleaning are not sufficient including (a) the manner in which the Gemara gives such fine details as to how the נעיצה must be done, (b) a subsequent halacha in Shulchan Aruch (10:3 as per Taz 10:15) which notes a few other seemingly-thorough methods of cleaning knives which are only acceptable in special cases, and (c) Acharonim cited in Darchei Teshuvah3 who discuss whether other thorough cleanings are acceptable b’dieved.

Nonetheless, it is generally accepted in the kashrus world that modern methods of cleaning industrial plant equipment via detergents, solvents, and similar methods, are so effective at removing residue from equipment that they may be used in cases where נעיצה is required. Therefore, the grinder and cheese wire used in a plant for ambient-temperature cheese do not require a hot kashering and may be used for kosher after an industrial cleaning.


1 See, for example, Toras Chattas 23:7.

2 See Gemara, Avodah Zara 75b & 76b, and Shulchan Aruch 10:1, 96:1 & 121:7.

3 The fact that Shulchan Aruch 96:1 uses the words “אינו מקונה ” implies that after a thorough cleaning one may assume a knife is free or residue. Those same words are found in Toras Chattas (Rema) 61:4. Darchei Teshuvah 96:14 and Rebbi Akiva Eiger to Shach 96:3 cite Minchas Yaakov (in his commentary to Toras Chattas, 61:15) and Tevuos Shor (10:18) who (a) give details of what such cleanings include, (b) agree that they are not acceptable l’chatchilah, and (c) disagree as to whether they may be relied upon b’dieved.