What to Do If a Kosher Knife Was Used on Non-Kosher Cold Food
Question
I was staying with family members (who are not all religious) in an Airbnb when I accidentally used my kosher knife (which I brought with me) to cut a cold food item from the fridge that belongs to another family member who does not keep kosher. What should I do with the knife? Must I kasher it, and if so, how?
Answer
Since the knife was used to cut cold, non-kosher food, it does not require kashering through boiling water (known as hagalah). However, it must be thoroughly cleaned:
- Wash the knife well with soap and a sponge.
- Pay special attention to any grooves or serrations if it is a grooved knife, ensuring all residue is removed.
Once cleaned properly, the knife can continue to be used for kosher purposes.
Explanation
When a kosher knife is accidentally used on non-kosher food, the cleaning requirement depends on the food’s texture: cutting soft items like regular cold meat or soft cheese only requires a simple rinse, as these foods do not penetrate deeply into the knife. In contrast, hard foods such as hard cheese or dry meat can leave residue in the knife’s grooves, and ideally halacha recommends performing Neitza—stabbing the knife into a hard surface ten times—to remove any trapped particles. Nevertheless, many Poskim rule that thoroughly washing the knife with soap and water, ensuring all grooves are cleaned, suffices even in place of Neitza, making this the practical approach in most cases. Another effective solution for a non-grooved knife is to sharpen it using a knife sharpener, which is considered the best option if available. However, for a quick and practical solution, thoroughly washing the knife with soap and water, suffices even in place of Neitza as we explained.
Sources:
See regarding Kashering a Treif knife that was purchased from a Gentile: Michaber Y.D. 121:7
See regarding doing Neitza to a meat knife which was accidentally used to cut cold hard cheese: Shach 94:31; Issur Viheter 58:4; Kreisi 94:23; Halacha Pesuka 94:7; Erech Hashulchan 94:15; Chochmas Adam 47:4; Beis Yitzchak 3:18; Zivcheiy Tzedek 94:53; Kaf Hachaim 94:76; Hakashrus 10:88
See regarding that it suffices to simply wash the knife well in place of Neitza: Sefer Hayashar of Rabbeinu Tam 790 “Lav Davka Neitza”; Re’eh 4a; Semak 213; Shulchan Gavoa 1; Zivcheiy Tzedek 89:45; Kaf Hachaim 89:69 “One cleans the knife and returns it to its designated use”; See Hakashrus 10 footnote 225; See Shach and Taz 10:6 regarding the function of Neitza, although clearly in this case its function is simply for cleanliness; See Rabbeinu Shimshon brought in Beis Yosef 89 Others suggest that as an alternative to Neitza one can clean the knife well and then pour on it hot water from a Keli Rishon.
See regarding that if it was used to cut regular cold meat [which is soft, or soft cheese], then a mere rinse suffices: Implication of Shach and Poskim ibid; Erech Hashulchan ibid; Zivcheiy Tzedek 94:53; Kaf Hachaim 94:76; However, See Rama 94:7 regarding meat of a Keli Sheiyni that was cut with a milk knife that the knife requires Neitza, however, seemingly this is because the meat was hot, while if cold the Poskim ibid explain that a mere rinse suffices; See Rashal Kol Habasar 8, brought in Shach 89:22 that Neitza is always required
