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 Halachos of Basar B'Cholov      Part 8

In order to transfer taste from one food to another a medium is required [e.g. heat].  Knives are unique in that the pressure caused while slicing can transfer taste.  This is known as duch'ka d'sakina (pressure/pushing of a knife) (Gemara Chullin 8b).  This halachah is not exclusively for knives; there are numerous utensils that create this pressure when pressed against food, which will cause a transfer of taste.  Utensils included are forks, grinders, blenders and food processors (Laws of Kashrus pg. 330-331).

There is an additional, unique halachic aspect in regard to knives. When a serrated there is concern that it was not cleaned well and some residue remains on the knife that may rub off onto the next food being cut (Rashi Chullin 112a s.v. kee'shus, Taz YD 94:13, Shach YD 94:29, Chochmas Adom 47:3).  For example, if one used a knife to cut pizza, even after the knife was rinsed off there may remain some pizza grease on the knife.  If one would then use the knife to cut something fleishig the milchig grease may rub onto the meat product.  Some Poskim explain that this concern is not applicable nowadays, since in modern society knives are generally washed off well, therefore they are like any other utensil which is assumed to be clean of any residue after being washed off (Darkei Teshuva 96:4)  L'halachah, one may rely on this opinion (Rabbi Biberfeld, shlit"a).

As a general rule any utensil that has not been used for more than 24 hours ("aino ben yomo") will transmit an undesirable taste ("no'sain ta'am lif'gam").  If this undesirable taste is transferred into another food, it will not prohibit the food (Shulchan Aruch YD 93).  If, however, the food which was absorbed was at the time of the transfer a good taste ("no'sain ta'am l'shevach") even if it became "stale" [i.e. it was not used for 24 hours] the utensil will require kashuring (Shulchan Aruch YD 93, Shach YD 93:3).  Therefore, if one were to cut a milchig food with a clean fleishig knife (or vice versa) that has not been used [with either hot milchigs or fleishigs] within 24 hours, any taste that was transferred from the knife will not prohibit the food.  However, the knife should be kashured.

The one exception is if one cuts a davar charif (sharp food).  The sharp taste is able to rejuvenate the stale taste in the knife to implant a good taste into the sharp food (Shulchan Aruch YD 96:1, Shach YD 96:6).  Practically speaking, if one cuts an onion with a fleishig knife, even if the knife has not been used within 24 hours, the fleishig taste transfers into the onion, rendering at least part of the onion fleishig.  Therefore, the minhag is to have 2 separate knives for both milchig and fleishig (Rama YD 89:4).  Practically, in order to avoid many potential kashrus problems in the kitchen, it is essential to also have pareve knives. (Magen Avraham 451:34, Aruch Hashulchan YD 89:16).

Which food is considered "sharp" is a dispute amongst the Poskim.  The Gemara [Chullin 111b], when describing a case of davar charif uses the example of a radish.  Most Poskim hold that the Gemara was not teaching that exclusively radishes, rather any food that is sharp like radishes (Aruch Hashulchan 96:13).  Possible examples of  sharp foods are: radishes, garlic, leek, onions, horseradish, olives, lemons, esrogim, sour apples, sour plums, sour pickles, sauerkraut, salted herring, and spices (See Laws of Kashrus pg. 322- 323, Kashrus in the Kitchen Q & A pg. 106).  L'maaseh, there are Poskim who hold that any food that can be eaten by itself without cringing is not a davar charif  (Rabbi Biberfeld, shlit"a quoting Rabbi Shmuel Felder, shlit"a).  According to this opinion many of the foods mentioned would not be considered sharp.

There is a machlokes haposkim whether a davar charif which absorbed taste can import that taste into something else.  For example, if someone cut an onion with a fleishig knife [rendering the onion fleishig] and then grated the onion in a food processor [or used a pareve knife], can the onion now impart the fleishig taste into the blade of the food processor or knife?  Another common scenario is, if someone cuts a davar charif with a fleishig knife [rendering the onion fleishig], on a pareve cutting board, does the fleishig taste transfer in the cutting board, making it flieshig?  The Magen Avraham [451:31] and Chochmas Adom [49:10] hold the davar charif will impart the fleishig taste.  However, the Even Ha'ozer [YD 96:3] argues that it cannot.  L'halachah, one should ideally follow the stringent opinion, however b'dieved (ex post facto) one may rely on the lenient opinion (Rabbi Biberfeld, shlit"a quoting Rabbi Shmuel Felder, shlit"a).

If one cooked a davar charif, it can be assumed that it lost its sharpness and would be considered a non- sharp food if cut afterwards (Pischei Teshuva YD 96:4).