Isinglass is a substance that is derived from the swim bladders of certain fish. It comes from the Dutch word “huisenblas”, which translates directly to “sturgeon’s bladder”- sturgeon is the type of fish traditionally used for brewing purposes. Isinglass is a traditional finings – a substance that is used for clarifying liquid by causing yeast to precipitate out of suspension, leaving beer clear.
When isinglass is used as a fining agent for beer, it has the ability to settle yeast and beer proteins very quickly and can do so repeatedly. This is helpful when brewers make cask-conditioned ales – a process when draught beer retains yeast to enable a secondary fermentation to take place in a cask in the pub cellar. Cask-conditioned beer is the traditional drink of the British pub. This beer is neither filtered nor pasteurized.