Isinglass is a gelatin-like substance developed from the bladders in fish that hold air. The bladders are shredded, freeze-dried and ground into a powder. The powder is then dissolved in liquid, and sold as such.
It’s used as a “fining” — to clarify liquids such as beer, cider, wine and juice by removing the particles that make them hazy otherwise. It’s not as powerful as other finings, but very gentle on the taste of what you are clarifying.
When used, it means essentially that that product is not vegetarian.
It is also used in making some glues and cements, and used to be used in making gelatin.
Fish that Isinglass is typically produced from include hake, cod and sturgeon.
Use 1 ml of Isinglass per 250 ml of beer or wine being clarified. Let stand for two weeks, then rack.
Store in a cool place, tightly sealed.