Wine

"An Unintended Result" - From R. Dorff teshuvah on Wine, page 210
The principle of no prior nullification however, has certain exceptions. According to many authorities, one exception is those situations in which the person adding the forbidden substance does so not to nullify it but rather for some other reason. So, for example, if worms fall into honey, you may heat the mixture until it melts and then strain it because your intention is not to nullify the taste of the worms in the honey but rather to fix the honey. This, in fact, is the reason that we may purge vessels in order to make them fit for kosher use, even if they contain forbidden food less than a day old: when the forbidden food is extracted from the vessel and absorbed into the water, it is nullified there because the person does not intend to effect that nullification but rather to make the vessel kosher by extracting the absorbed, forbidden food. Those cases, though, are both situations in which the intention is to remove the forbidden substance in order to make something else kosher. In the manufacture of wine, however, it is the specific intent of the wine maker to insert the fining agent (although not that it remain in the wine).

"Permissive Precedents" - From R. Dorff teshuvah on Wine, pages 210-211

There is one case in the sources in which a similar action is permitted. Specifically, one may intentionally cook a non-Jew's butter in order to eliminate the particles of milk in it (since a non-Jew's milk is forbidden), and if a little milk remains, it is null and void because the person's intention is not to nullify the little that remains but rather to remove the particles of milk. In this case one is intentionally doing something which will leave the milk in the butter (albeit it in a different state), and yet one may eat the butter. The strength of this line of argumentation goes even further. In what clearly became a landmark decision, Ezekiel ben Judah Landau (the Noda B'Yehudah, 1713-1793) ruled permissively in the very case before us. The way he phrases the question indicates that fining wine with an unkosher fish had been done by the Jews of Poland for twenty years and had become common practice among the Jews of Germany too by the time he wrote his responsum.:

(The question revolves around) Krok, which some call Heusen Bleusen, which is the bladder of the fish called Heusen, which is an unkosher fish. People dry the bladder of that fish and insert it into the drink which is called med in Poland, or honey juice. Its nature is to precipitate the lees and to clarify the drink. In Germany they are already used to acting like this, i.e., to put it into barrels of wine for this reason, and it is now about twenty years that they began doing this also in Poland in the drink of honey juice called med. And the great scholars of the generation were aroused by this to forbid it on the grounds that it remains in the drink, and "that which is preserved is treated legally as if it were cooked" (i.e., it is as if the juice and the unkosher fish were cooked together). And if one argues that it (the unkosher fish) is nullified by being less than one part in sixty, we do not nullify a forbidden substance ab initio. And there are those who want to permit the practice on the grounds that it is dried out, and it is therefore like wood which has no taste whatsoever, and they see it as being analogous to the inner lining of a stomach. And there are those who want to permit the practice on the grounds that we only restrict prior nullification when it is one's intention to nullify, but here the intention is only to clarify the wine and not to give it a taste. My honored cousin, the rabbi, the great luminary, Rabbi Joseph, the head of the court and the academy of the city Hadesh in the region Cracow, ruled to forbid the practice.

But since the custom has already spread to permit the practice in the regions of Germany and Poland, I have decided to write according to my humble opinion.

After a long responsum, he ultimately says this:For all the reasons mentioned above, it seems that it is permitted to put Heusen Bleusen into the wine or the drink which they call med in Poland because the intention is not to nullify but only to clarify the drink and to precipitate the lees. And "it is good for Israel, for if they are not prophets, they are children of prophets." (Pesal;im 66a) According to my humble opinion it is completely permissible. And what seemed right to me I have written