Rabbi Eli Gersten – Rabbinical Coordinator At The Orthodox Union – On Kashrus – Duchka D’Sakina- ‘The Pressure Of A Knife’

Although ordinarily heat is required to cause a transfer of ta’am from a food into a utensil or from a utensil into food, however one exception is in regards to a davar charif (a sharp or spicy food). If a spicy food is cut with a knife there will be a transfer of ta’am even thoug both the knife and the foods are cold. There are two Gemaras that explain extra stringencies that relate to cutting a davar charif with a knife.

The Gemara Chulin (111b) teaches that if a radish was cut with a fleishig knife, it may not be eaten with milk. But if a gourd was cut with a fleishig knife, one only needs to scrape away the edge where the cut was made. The Gemara explains the difference. A radish is a davar charif, so when it is cut with a knife it absorbs. However, a gourd is sweet. When one cuts a gourd, the only concern is the fat that was on the surface of the knife might wipe off on the gourd. Therefore, it is enough to scrape away the sides of the cut.

Why is the radish forbidden to be eaten with meat? The knife absorbed fleishig ta’am and that ta’am was then transferred to the radish. Isn’t this a standard case of nat bar nat? Rashi (Chulin 112a Kishus) offers two explainations as to why nat bar nat does not apply.

  • A knife is assumed to be greasy. The grease that is on the knife is absorbed into the radish.
  • Because a radish is a davar charif, it absorbs more taste. Therefore, it is considered like one nesinas ta’am.

Shulchan Aruch (Y.D. 96:1) paskens l’chumra like both reasons of Rashi.

The Gemara (Avoda Zara 39a) teaches us another chumra that relates to davar charif. Ordinarily, an aino-ben-yomo utensil will not affect the kashrus of a food. Since the ta’am is pagum (bad tasting), it is always batel. However, the Gemara rules that if a chiltis (a certain extremely sharp vegetable) was cut with a non-kosher knife, even if the knife was aino ben yomo, the chiltis will become non-kosher. This is because the strong spicy taste of the chiltis has the ability to revive a ta’am pagum. While all agree that this halacha applies to a chiltis, Shulchan Aruch cites two opinions as to whether this halacha also applies to a radish, or other vegetables that are less spicy than a chiltis.

Rema (Y.D. 95:2 and 96:3) however paskens that even a radish has the ability to be mechalya l’shvach (to revive stale bliyos from an aino ben yomo). Shach (Y.D. 96:6) writes that the accepted ruling is to extend this halacha to all d’varim charifim (e.g. even onions).

It should therefore follow that if a milchig aino-ben-yomo knife was used to cut an onion and the onion was subsequently cooked with meat, the food should be assur even bidi’eved. However, the Beis Meir argues that the Rema was only machmir regarding radishes (or items that are more charif than a radish). However, regarding onions or items that are less charif, we could be maikel bidi’eved. This is because there are several doubts.

  • Perhaps only a radish is charif enough to prevent it from becoming a nat bar nat, but not an onion.
  • Perhaps only a chiltis is charif enough to be michalya l’shvach and not an onion.
  • It is uncommon to use a milchig knife with hot milchig foods (kli rishon). Rov tashmisho of such a knife is b’tzonen.

Since this case is a machlokes Shach and Beis Meir, one should only be lenient if it is a hefsed mirubah or there are other mitigating factors (see Darchei Teshuva 96:62).

How much of the radish becomes fleishig?

Shulchan Aruch (Y.D. 96:1)  rules that if one cut a radish with a fleishig or a non-kosher knife, the bliya enters the radish the thickness of a k’dei netila. The Mishnah Berurah (462:25) writes that this is the thickness of one’s thumb (approx. one inch). Rema writes that lichatchila we are machmir that the bliya will spread b’kulo. However, bidieved if the piece was already cooked with other foods, then we are maikel to evaluate b’kdei netila.

How much ta’am is absorbed into the radish?

Shulchan Aruch (Y.D. 120:6) rules that a non-kosher knife cannot be used to cut a davar charif, until it undergoes ne’itza. Ne’itza involves scraping the sides of the blade by pushing it into the ground ten times. This process is considered an affective method for kashering the outer k’dei klipa (thinnest layer) of the knife. After ne’itza the “non-kosher” knife may be used to cut a davar charif. Rebbi Akiva Eiger (96:8) initially suggests that since we only require kashering the outer layer of the knife, this should prove that a davar charif is only mavliya from the k’dei klipa of the knife. However, if so, Rebbi Akiva Eiger asks that since this is a tiny amount, it should be enough to just rinse off the piece. How can such a small bliya give ta’am into the entire k’dei netila? Yet we know that the knife gives ta’am into a k’dei netila of the food. This should prove that it is maflit from the entire thickness of the blade. Rebbi Akiva Eiger leaves this question unresolved. However, the Pri Migadim M.Z. (Y.D. 10:6) explains that the blade does indeed give ta’am from the entire thickness. Still ne’itza helps, since it is mivalbel (mixes around) the ta’am that remains in the knife and prevents it from exiting. Therefore, if ne’itza was done, the knife may be used to cut a davar charif. But if ne’itza was not done, then the entire thickness of the blade will be absorbed into the davar charif.

Although we see that the entire thickness of the blade gives ta’am, however the Pri Migadim M.Z. (Y.D. 94:12 ) writes that regarding duchka d’sakina on a davar charif, we do not need to be concerned with the section of the knife that did not cut through the radish. Although when food is cooked in a pot, we must cheshbon all the bliyos in the pot, even from the top of the pot that is not touching the food that is because we say cham miktzaso cham kulo (the entire pot is hot). Pri Migadim explains that this obviously does not apply to a cold knife. Although the pressure of the knife can combine with a davar charif to draw out a bliya, but that is only where there is pressure. It certainly does not warm up the blade and cause ta’am to emit from the part of the blade that was not used. Therefore, if only part of the blade cuts through a davar charif, we only need to cheshbon the bliya that was in that part of the blade.

This is why Shulchan Aruch writes that the amount of bliya in the radish cannot be more than the volume of the part of the blade that came in contact with the radish. So for example even if the blade is 8 inches long, but if only half the blade was used to cut the radish, only ta’am from 4 inches of the blade potentially entered the radish. Similarly, if the height of the blade is 3 inches, yet only two inches of the blade were needed to slice through the radish (i.e. the radish was only 2 inches tall and 1 inch of blade remains above the radish) then we only need to deal with the two inches of blade that actually cut through the radish. However, as noted above, we need to cheshbon the entire thickness of the blade. So if in this example, the blade was 1/8th of an inch thick, we would have 4*2*(1/8th) which equals one cubic inch of ta’am. Even if 20 radishes were cut with this knife, and cooked in a soup with meat, so long as there is 60 cubic inches among all the water, vegetables and meat in the pot, the dairy from the knife would be batel and the soup would be mutar. In this case, even the radishes may be eaten.

If one shaved a block of salt with a milchig knife, the salt is considered milchig. This is because salt is a davar charif and will draw out ta’am from the blade. Still, since only the edge of the blade is used to shave the salt, the amount of milchig bliya in the salt is very small. Therefore, the Maharsham (2:180) writes that if bidieved, the salt was used for melicha, we can assume that the bliya of milchigs was batel. See also Har Tzvi (Y.D. 90) who writes similarly.

Taken with permission from the OU’s Daf HaKashrus.