A PINT of fish guts doesn't sound like the most appetising thing you could order in a pub.

But bits of fish are a little-known ingredient in most real ales – and that makes those beverages forbidden for vegetarians and vegans.

Now, a local beer festival aims to serve up veggie-friendly ale – and to educate punters on the fishy things that go into some pints. Nottingham's first vegan beer festival will be on Saturday and Sunday at the Sumac Centre, Forest Fields.

Festival organiser Tracy Shaw became a vegan several years ago. Then she found out the truth about her favourite tipple.

"I realised that if I wanted to have bitter – my favourite drink – it was going to be a struggle," she said. "That's where it started."

The main culprit here is isinglass. While it might sound like a business you'd contact regarding double glazing, isinglass is in fact a substance from fish swim bladders. It doesn't affect ale's taste, it's not really present by the time the ale gets to the glass and it's not used in draught or bottled beers.

"Keg beer is vegan," Tracy said. "Unfortunately, it's only real beer that's not vegan."

It's used in real ale for one reason – to make it less cloudy.

But there's nothing wrong with cloudy beer, she said.

"It's perfectly drinkable. There's no reason except for the way it looks. And it looks perfectly natural to me.

"If the beer tastes all right, it's all right. It's nothing to do with how cloudy it is or how clear it is."

The Nottingham branch of the Campaign for Real Ale has helped source and set up the beer for the festival. Branch chairman Spyke Golding agreed that cloudy beer has a silver lining.

"On the continent, homebrew pubs are expected to serve cloudy beer," he said. "People would think something's wrong if it's not cloudy."

The use of isinglass and other animal products in British ale has its roots in the days when pewter tankards gave way to pint glasses, he said. All of a sudden, people could see what they were drinking. They noticed that sometimes it was clearer, sometimes it wasn't.

"They associated the clarity with superiority even though there's no discernable difference in the taste," Spyke said.

So brewers set about finding ways to make it clearer all the time.

"Quite how the brewers discovered that a product made from fish bladders came to make it clear is anybody's guess," he said.

The festival will feature one-off brews from regular breweries as well as beers from several of the country's small number of vegetarian breweries. There will also be entertainment on both days, including a set from Martin Taylor, aka Martin the Livewire.

"I'm a vegan hip-hop artist, and a lot of my content is quite relevant about animal rights, human rights and that sort of thing," he said. "I've been vegan over eight years now. At first, I had no idea (about ale and isinglass). It was a bit surprising – not a lot of people know it."

After this weekend, event organisers are hoping that a few more people will know it. For Tracy, it's a simple choice.

"If it comes down to having it with an animal being killed to produce it or having a cloudy beer," she said, "I know which one I'm going to pick."

The vegan beer festival runs Saturday and Sunday, noon until late, at the Sumac Centre, 245 Gladstone Street, Forest Fields. Admission is free.