Guinness recently announced that they were making a change to their two century-old recipe, one that wouldn’t make any use at all of fish bladders. Wait, said many people. Guinness has fish in it?

“There’s fish in Guinness’ beer” has been a popular food rumor for a long time now—and it’s sort of true, sort of not true. According to Guinness, their recipe is a four-ingredient mix of barley, water, hops, and yeast. So just where does fish come in? Through the filtration process.

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Just-brewed beer can be pretty cloudy. One very common way brewers clear it out is with a collagen called isinglass, which is made from sturgeon bladders. Isinglass is added to the cloudy beer, where it reacts with yeast. When it is filtered back out it takes yeast with it, leaving the beer clear. It’s a process called fining—and you’re just as likely to find it in juice and wine as in beer.

So why after all these years is Guinness swapping it out? The problem, according to the company’s statement to the New York Times, appears to have been not one of problems with the recipe, but one of marketing to a growing vegetarian market:

“Because of its use we could not label Guinness as suitable for vegetarians and have been looking for an alternative solution for some time.”

The substance is removed from the beer after it has fulfilled its filtration role. Zsoka McDonald, a spokeswoman for the company, said that only “trace amounts” ever make it into the final product. That has been enough, however, to keep most vegan drinkers away.

Of course, that begs the question, if not isinglass, what does Guinness plan to use next to keep beer nice and clear? Various studies have tested out other filtration additives—from avian collagen to pea proteins—with fairly good results, though certainly avian collagen would be unlikely to make it anymore vegetarian friendly.

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Though Guinness hasn’t revealed their plan for the switch, it actually sounds like they may intend to move to a mechanical filtration system. (In that same interview, Guinness’ spokesperson was also talking up a new filtration system they had bought.) We’ll have to wait till the end of next year when the new recipe starts rolling out to see if we can taste the difference.

Image: VanderWolf Images / shutterstock