The Gemara recognizes a number of ways in which taste can transfer from one food to another without the presence of heat. First, there can be surface transfer when one or both of the foods are moist, such as when cheese touches a piece of meat. In most cases, this requires a mere rinsing off of the items, although in some cases where rinsing is not sufficient to remove the adulterating film on the surface, a more vigorous scrubbing or, when that would be counter-productive, a scraping would be required (Shulkhan Arukh, YD, 91:1, and the Pitchei Teshuva, ad. loc.). This issue comes up frequently in cases such as buying cut, kosher fish from the supermarket. The best way to handle such cases is to bring in one's own knife, or to work with the supermarket to ensure that they use a special knife for kosher customers who request it. However, when this is not an option, and the fish is cut with a knife that was used for cutting non-kosher fish, the fish must be cleaned to remove any possible transfer. Because of the adhesive surface of the fish, washing may not suffice, and the best would be to lightly scrape all cut surfaces of the fish. However, if the establishment cleans their knives and surfaces between fish, the possibility of any real transfer decreases, and a thorough rinsing may suffice. It should also be note, that the ability to buy cut fish from a non-kosher establishment assumes that the fish can be accurately identified as kosher. This is the case when the skin, with scales, is still on it, or with fish that is immediately identifiable, such as salmon or tuna. When it does not have its skin on, and it is not immediately identifiable - as with most white-fleshed fish - one cannot assume the accuracy of the labeling, as was made clear by a New York Times article in 2008.
In addition to surface transfer through touch, there can be a concern of absorbency. Soft, porous foods, such as cooked flanken or a piece of bread will immediately absorb the liquid they are sitting in, even if everything is cold. If my flanken falls into a bowl of milk, or my bread falls into cold chicken soup, I must acknowledge that the liquid has been absorbed and deal with the food appropriately. (Rema, Shulkhan Arukh, YD 91:7).
The previous two categories deal with cold taste transfer that is acknowledged as such. Beyond these cases, however, the Gemara states that there are two types of cold taste-transfer that operate through a "pseudo-heat" principle: "Says Shmuel: Salting (maliach) is like grilling, pickling (kavush) is like cooking." (Hullin 97b, 111b, 112a, 113a). While the Gemara applies this statement of Shmuel to a number of actual cases, all the cases are those of salting. Thus, when it comes ot pickling, the Gemara never states what the parameters of such pickling are. Does it matter what type of liquid something is being pickled in? Does it matter what is being pickled? How much time does an item need to be pickled before we say that taste transfer occurs? The field was left wide open for the Rishonim to assess the parameters of this principle of pickling. As an extreme example of this, we may turn to Rabbenu Gershon, who states (Hullin 111b) that all the Shmuel intended to say was that if a non-Jew pickles a food, it is like he cooked it, and forbidden as bishulei nakhrim, the rabbinic prohibition of food cooked by non-Jews.
According to Rabbenu Gershon there is no principle at all of taste transfer due to pickling, and he is free to say this because - while not consistent with the context in the Gemara -there is no Gemara that applies or discusses the rule of pickling, and thus no Gemara that precludes this interpretation. Now, even given this, we may ask what compelled Rabbenu Gershon to explain this statement of Shmuel's so out of context, and to deny that pickling matters for taste transfer. The answer emerges when we look at a number of mishnayot that address situations of pickling. The mishna in Shvi'it (7:7) states:
If a new rose [from the Sabbatical year] has been preserved in old oil, the rose may be taken out [and the oil is unaffected]; but if an old [Sabbatical] rose was preserved in new oil, the oil is subject to the law of destroying [Sabbatical produce after the time for eating has ended]... This is the general principle: if one kind is mixed with a different kind and it imparts flavor [to the other], they are subject to the law of destroying.
Here we have a case of preserving - of immersing an object in a liquid that can absorb its taste - and we are told that we do not have to assume that taste will transfer. Only certain cases (and old rose in new oil) will cause a transfer of taste, and the general rule is that we have to look at each case individually - based on factors such as what is being preserved, what liquid it is being preserved in, and how long it is being preserved - to determine if taste is transferred.
This case-by-case approach also emerges from a series of mishnayot in Terumot (10:7-10), which give a number of special cases when taste transfers through pickling, and then states at the end that "Whatever vegetables are pickled together are permitted, save [when pickled] with leeks" (Trumot 10:10). Bland vegetables never transfer taste through pickling, and thus are never a problem. In line with these mishnayot, Rambam rules that pickling is only a problem in the special cases that are mentioned in the above mishnayot, or - presumably by analogy - when a kosher fish in pickled with a non-kosher fish (Forbidden Foods 15:34, Trumot 15:9-10). We now understand why Rabbenu Gershon reinterpreted Shmuel's statement. Shmuel seems to be saying that the general rule is that pickling transfers taste, whereas all the mishnayot state that the general rule is that it does not. Rabbenu Shimshon (Trumot 10:8) recognizes this contradiction and leaves the problem unresolved. Rabbenu Gershon resolved it by reinterpreting and neutralizing Shmuel's statement.
The other Rishonim, outside of Rambam, ignore the above mishnayot, and apply Shmuel's statement as a general rule: taste transfers when pickling occurs. The question remains, however, when does pickling occur? Rashi states a number of times that Shmuel's pickling refers to a case where "it was pickled in vinegar and spices" (Rashi, Hullin 97b, s.v. Kavush). While Rashi would probably apply this to all foods which were pickled, including, say, vegetables, he does insist that the liquid being used must be a true pickling liquid. In contrast, and at the opposite extreme, Rosh (Avoda Zara 5:11 and Hullin 8:49) states that pickling occurs in all liquids, with all foods, after 24 hours. Thus, according to Rosh, if one put a piece of meat and a piece of cheese in a container of water, we would say that they were pickled together and that the taste transfers from the cheese to the meat and vice-versa. Rosh derives this from the Gemara is Pesachim (44b) which states, implicitly, that if one were to leave meat in milk "a full day" that the meat would absorb the taste of the milk. Thus, says Rosh, we can see that "pickling" occurs with any liquid after 24 hours. Most other Rishonim, however, ignore or implicitly reject the relevance of this Gemara, presumably because - as we stated above -this is a case of absorbency (the milk seeps into the porous meat), and is irrelevant to the discussion of pickling.
Rosh, by using the Gemara about soaking meat in milk, transforms the concept of "pickling" to one of "soaking." While this seems to take us far afield from the original statement, this opinion is adopted by Shulkhan Arukh and Rema (YD 105:1) and becomes the established halakha. Nevertheless, as with many halakhic concepts, earlier or alternative positions do not fully disappear, and resurface in different ways. Thus, Shulkhan Arukh while ruling that all liquids effect "pickling" after 24 hours, states that brine and vinegar effect pickling "after the time that it takes to heat up the liquid and begin the process of cooking." This would seem to acknowledge cases of real pickling, and to be more strict with them. However, Shakh points out that the original sources on which Shulkhan Arukh is drawing do not mention vinegar, and only mention brine. The reason that brine is more strict, according to these sources, is not because it effects pickling, but because its saltiness is the basis for taste transfer ("salting is like roasting"). Shakh concludes the vinegar is like any other liquid and only effects pickling after 24 hours. While Shakh is undoubtedly correct regarding the earlier sources, I would contend that "vinegar" appears in Shulkhan Arukh because the original concept of true pickling has not been totally effaced. Thus, Shulkhan Arukh acknowledges that there is pseudo-pickling, in all liquids, and real pickling, in vinegar, and the real case demands greater stringency than the "pseudo" case.
The posek that most explicitly states that two categories exist - real pickling and pseudo-pickling - is the Nodah BiYehudah. Nodah BiYehudah (Kama, YD 26) investigates in great depth the statement of Shmuel that "pickling is like cooking" and notes that firstly, R. Yochanan rejects this principle and, secondly, that it runs in contradiction to the mishnayot in Terumot and Shvi'it which we quoted above. Nodah BiYehudah 's conclusion is that our ruling which applies "pickling" to all liquids after 24 hours is a rabbinic extension of the concept, and that Biblically the only pickling that transfers taste is with sharp or pungent foods in a true pickling liquid. Thus, most cases, which are not true pickling, are only rabbinically forbidden, and in cases of doubt - for example, whether the food had actually been in the liquid for 24 hours - one can be lenient.
This dialectic, or yo-yoing of first extending the concept and then pulling it back to somewhat closer to its original meaning, also occurs in the other, more common, scenario of "pickling" and that is with storing liquids in vessels. If I store my cold chicken soup in a metal or Tupperware container for 24 hours, does this container now become fleishig? The general case of vessels absorbing through "pickling" is never discussed in the Gemara, but the Gemara does talk about wine casks that were used for long-term storage of wine. These casks, according to the Gemara (Avoda Zara 33a), are considered to have absorbed the taste of wine, and are forbidden to be used without kashering. While most Rishonim understand this to be a special case of wine and of long-term storage, Rosh (Avoda Zara 2:20), once again, generalizes this case and states that it is talking about any 24 hours of storage and would apply to all liquids. Just as - according to him - "pickling" occurs with all liquids and after 24 hours, so is the case with liquid in vessels - the will be "pickled" and absorbed into the walls after 24 hours.
The case of vessels is not mentioned in the Shulkhan Arukh in Yoreh Deah (see, however, Orah Hayim 447:5). Nevertheless, Taz and Shakh (YD 105) both assume that it takes place, in line with the ruling of Rosh. However, they both state that if one uses this Tupperware, now - after 24 with chicken soup - fleishig to store milk in for 24 hours, that the milk would still be kosher, since after those 24 hours any meat taste absorbed in the walls of the Tupperware would be considered to have gone bad. Nevertheless, it would still be forbidden to use this Tupperware at the outset for milk, since one cannot use even old non-kosher or wrong-gendered vessels, at the outset, with kosher or other-gendered food.
However, just as in the case with food, there is some pull-back from this very general ruling by vessels. A number of Achronim believe that the rule would not apply to hard, non-porous materials, such as metal, stone, plastic, or glass vessels. (Some poskim are lenient with glass and strict with metal and stone). Others say that it would not apply to kosher foods -such as the chicken soup example above - and only apply to non-kosher foods. (See Darkhei Teshuva, YD 105). Although normally poskim do not make such case-based distinctions and, indeed, Taz states that he can see no formal halakhic basis to be more lenient by metal vessels, it is not surprising that they do so here. When the rule has been abstracted and extended so far from its original meanings and moorings, it is not surprising that there is a pushback to bring it closer to its more native meaning and more manageable scope.