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Q: Is it true that beer is filtered with isinglass which is made from non-kosher fish?
A: Classically, isinglass – a product made from the swim bladder of non-kosher sturgeon fish – was used to filter the protein particles out of beer. The particles are attracted to the isinglass, and then collectively fall to the bottom of the beer where they can be filtered out leaving the beer without any cloudiness. Nodah B’yedhudah YD 1:26 rules that this does not pose a kashrus concern, and one of his reasons is that such a miniscule amount of isinglass remains in the beer that it is batel (nullified). [In this case there is no concern of bitul issur l’chatchilah – intentionally diluting non-kosher into kosher – because the isinglass is put in with the specific intention of removing it.] Some hashgachos rely on this line of reasoning and others hold that items which are certified as kosher should meet a higher standard and not contain any non-kosher components.
Nowadays, most beer companies do not use isinglass and instead use other methods to filter the beer.