When the food that one is dealing with is sharp or pungent there are three big changes in the general halachot of the kitchen. 1: Sharp food can sometimes cause a transfer of taste without heat. 2: Sharp food can make taste from a eino ben yomo have the rules of ben yomo. 3: As previously discussed, a sharp food does not have the rules of nat bar nat, rather the taste absorbed from a pot has the rules of taam rishon.
Sharp food can cause taste to transfer without heat. This would occur when one cuts a sharp food with a non-kosher knife. The sharp food absorbs the taste of issur from the knife, even though neither the knife nor the food was hot. Similarly, if a knife had traces of non-kosher food on it and then was used to cut a sharp food, the sharpness in the food would cause the knife to absorb the non-kosher food, thus rendering the knife itself non-kosher. It should be noted that if a knife has kosher meat or dairy on it and is used to cut something sharp, there is a dispute if the knife acquires the meat or dairy status.
If a sharp food was cooked in a pot that was an eino ben yomo or was cut with a knife that was an eino ben yomo, the food still absorbs taste as if the pot or knife was a ben yomo. The source for this is the Gemara (Avoda Zara 39a) which says that a sharp food can imbue taste in an eino ben yomo pot with new strength. There is a dispute if this is true concerning all sharp foods or only the ones specifically mentioned in the Gemara. The Rama (יו”ד צה:ב) and Shach (יו”ד צו ס”ק ו,יט) are stringent concerning all sharp foods to treat them as if they can make an eino ben yomo into a ben yomo. (In specific cases there is room to be lenient. These cases are beyond the scope of this article.) That which a sharp food makes bad taste be considered good again is only by eino ben yomo, but if the taste was always a bad taste (such as non-kosher soap), the sharpness has no effect on it and will not be considered to turn the bad taste into good taste.
The definition of sharp is a food which is extremely sharp or pungent. The Gemara brings down radishes and chiltit (what exactly chiltit is, remains unclear). Most rishonim (Rambam, Sefer Hatrumot, Tosafot, Rosh, and others) add onions, garlic, and leek. The Shulchan Aruch (יו”ד צו:ב) adds that any very sharp or sour food such as pickled fruits or vegetables would also be considered sharp, as well as very salty fish (meaning so salty that it is inedible without prior soaking in water to remove some of the salt). Once a sharp food was cooked (פנים מאירות ח”א סי’ סד) or soaked in water (ערוך השלחן יו”ד צו:כ) it loses its sharpness and is like a regular food. (Since it is not clear exactly what is considered cooked enough to make the food lose its status of sharp, one should consult a rabbi if the case arises.) If a food which wasn’t very sharp (meaning it is sharp but is eaten on its own) was cut with an eino ben yomo meat knife, one can be lenient and eat the food with dairy (עיין ט”ז יו”ד צו ס”ק ט, יד יהודה שם ס”ק כב).
When sharp food absorbs taste from a knife, how far into the food does the taste travel? The Shulchan Aruch (יו”ד צו:א) rules that the taste goes a netila into the food. Therefore, if one cuts a sharp food with a meat knife and wants to eat the sharp food with dairy, they must first remove a netila from the sharp food, and the rest would be permitted to be eaten with dairy. The Rama argues and rules that initially we treat the sharp food as if the taste travelled through the whole food, and only after the fact can we be lenient and just remove a netila. The Shach and Rabbi Akiva Eiger disagree as to what would be considered after the fact. The Shach (יו”ד צו ס”ק ו) is of the opinion that it means after the food was already cooked with dairy. Rabbi Akiva Eiger says that once a sharp food was cut with a meat or dairy knife it is already considered to be after the fact, since it was already cut. The Chochmas Adam says that when there is a small loss one may rely on Rabbi Akiva Eiger.
The Shulchan Aruch (יו”ד צו:ג) writes that not just a knife can cause taste to go into a sharp food, rather anything that is pushed against the food with pressure will transfer taste into the food. Therefore, a grinder that was non-kosher would impart non-kosher taste into pepper that was subsequently ground inside it. Similarly, one can infer, that if someone cut through something sharp with a utensil other than a knife (such as a spoon or fork) that would also impart taste to the food. The Chochmas Adam (56, 2) writes that a cutting board or plate also can transfer flavor, since when one cuts on them there is equal pressure from the knife and from the cutting board or plate against the food. Therefore, when cutting food on a board, any taste absorbed in the board can get transferred into the food.
If one cut a sharp food with a dairy knife and then a meat knife, or vice versa, one should consult a rabbi as to the status of the second knife. The food itself is forbidden because it absorbed taste from both meat and dairy. If the food was then cooked, the taste of meat and dairy was cooked together, thus creating the biblically prohibited mixture of meat and dairy. The pot in which this was cooked would need to be koshered.
Practically, one should not cut onions or garlic with a dairy knife and fry or cook them in a meat pot nor eat them with meat (or vice versa). If one did, they should consult a rabbi as to the status of the pot. Therefore, it is worthwhile to have a pareve knife and cutting board for vegetables, so as to avoid getting into the questions of sharp foods.