Cookinglass

Some old recipes call for a mysterious ingredient called isinglass. It was the inner membrane of the sturgeon’s swim bladder.

So what’s a swim bladder? It’s the sack of air inside a fish’s body that serves as a float so the fish can hang around at a particular depth in the water. Fish that don’t have swim bladders have to work to stay above the bottom, so they’re either bottom-dwellers like rays or ceaseless swimmers like sharks.

In some fish, the swim bladder serves as a primitive lung. Other fish use it as a resonating chamber for making noise. If you ever wondered how drumfish got their name, here’s the answer: It was by booming on their swim bladders.

For cooks, the attraction of isinglass was that it is the purest source of gelatin, making sparkling clear, finely molded aspics and gelatin desserts. Unfortunately, it’s expensive--from 300 pounds of sturgeon you get only one pound of isinglass--which is why commercial gelatin made from animal skin and bones has basically replaced it.

For kitchen use, the isinglass membrane was processed through rollers into translucent silk-like sheets. The peculiar name “isinglass” comes from the Dutch word huizenblas, which literally means sturgeon bladder. The blas part became “glass” in English because the sheets looked like glass.

As for the “isinglass windows you can roll right down,” boasted of in the song “Surrey With the Fringe on Top” (in “Oklahoma”), they were actually muscovite, a mineral that comes in thin, translucent sheets that reminded people of isinglass.