The Oxford Companion to Beer definition of
is a traditional finings, a substance that causes yeast to precipitate out of suspension, leaving beer clear. Isinglass is derived from the swim bladders of certain tropical and subtropical fish. When macerated and dissolved for several weeks in dilute food-grade acids, they form a turbid, colorless, viscous solution largely made up of the protein collagen. This material is known to brewers as isinglass finings.
There is much speculation as to the first use of this unlikely substance in beer making. The most logical conjecture is that at some point in history a resourceful fisherman used the swim bladder of a large specimen to store his beer, akin to keeping wine in a skin. It is likely that the beer of the day was somewhat acidic, perhaps from lactobacillus or acetobacter contamination. The acidity would have certainly caused some collagen to dissolve and clarify the beer. The observant individual might have noticed a bright, clear beer when pouring it into a drinking vessel.
Traditionally, isinglass for brewing purposes was derived from sturgeon, although modern commercial isinglass is more typically derived from tropical estuarine dwellers, such as the Nile Perch Lates niloticus from Lake Victoria, where it is considered an invasive species. The best quality finings originate in the South China Sea and are identified as Round Saigon or Long Saigon finings. The swim bladder is sun dried at the catch site and then packed for export to markets in China, where it is used to make fish maw soup, or to the UK to make isinglass finings. With the advances in centrifugation and filtration technologies, the use of isinglass has declined and today it is largely confined to cask-conditioned ales, although some American craft brewers also use it to clarify beer without the use of filtration.