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Isinglass (collagen) should not be confused with Isinglass (mineral) which is made from sheets of mica and was once commonly used as a heat-resistant substitute for glass.

Isinglass is a substance obtained from the swimbladders of fish (especially Beluga sturgeon), used mainly for the clarification of wine and beer. It is a form of collagen. Prior to inexpensive gelatin production and other competitive products, isinglass was used in confectionery and desserts such as Fruit Jelly and blancmange. Isinglass was originally made exclusively from sturgeon until the 1795 invention of a cheap substitute using cod by William Murdoch. This was extensively used in Britain in place of Russian isinglass. The bladders, once removed from the fish and processed, are formed into various shapes for use.

Isinglass finings are used extensively as a processing aid in the British brewing industry to accelerate the fining, or clarification, of beer. They are used particularly in the production of cask-conditioned beers, known as real ale, although there are a few cask ales available which are not fined using isinglass. The finings, a pure form of gelatin, flocculate the live yeast in the beer into a spongy mass, which settles to the bottom of the cask. Left to itself, beer will clear naturally; however, the use of isinglass finings accelerates the process. Isinglass is sometimes used with an auxiliary fining, which further accelerates the process of sedimentation.

Non-cask beers which are destined for kegs, cans or bottles are often pasteurized and filtered. The yeast in these beers tends to settle to the base of the storage tank naturally, so the sediment from these beers can often be filtered without using isinglass. However, some breweries still use isinglass finings for non-cask beers, especially when attempting to repair bad batches.

Although very little isinglass remains in the beer which is drunk, many vegetarians consider beers which are processed with these finings to be unsuitable for vegetarian diets (although it is suitable for pescetarians). An alternative to isinglass which is suitable for vegetarians is Irish moss, a type of red alga. The process differs, however: isinglass is added at the end of the brewing process, before bottling, whereas Irish moss is added to the hot wort while it is being boiled, and primarily reduces hazes caused by proteins. Since the two fining agents act differently, some beers will make use of both.

Isinglass finings are also used in the production of kosher wines, although for reasons of Kashrut they are not derived from the sturgeon, as this fish is considered not Kosher.

• Davidson, Alan (1999). ""Isinglass"", Oxford Companion to Food, p. 407. ISBN 0-19-211579-0. 

• A list of vegetarian beers
• Beer and cider suitable for the Vegetarian