Answer: Fish Swim Bladders

Finings are a class of substances added to wines, beers, and non-alcoholic fruit juices to aide in the clarification of the liquid by removing various organic compounds like sulfides, proteins, polyphenols, benzenoids, and copper ions. Finings are also sometimes used to adjust the flavor or aroma of a particular beverage.

Before the advent of modern finings, there was a long history of brewers using whatever was available to help clarify what they were brewing, including egg whites, Irish moss, and, popular to this day among British breweries, fish swim bladders.

“Isinglass”, a substance consisting of dried fish swim bladders ground up and added to the beer, is a handy clarifier as it easily binds with the compounds clouding the beer. The beer is further refined and the isinglass (along with the bound organic compounds) is almost entirely removed.

Isinglass is very similar to modern gelatin (and, in fact, was historically used in many of the same confectionery ways we presently use gelatin) and with the rise of mass produced gelatin, it fell largely out of favor as a clarifying agent.