A group of ale aficionados has organized a festival devoted to the succulent sapors of flat, warm beer. And they want you to come try it.

Real ale — as partisans refer to it, brushing off all other beers as somehow inauthentic — is a special brew. And the die-hard enthusiasts to be found at the fifth annual New England Real Ale Exhibition (NERAX) see themselves as a chosen few.

That’s terrific, you say. But I like my beer frothy and frosty. What the hell is this stuff?

As is the case with all ales (but not all beers), real ale is born when malted barley is fermented by yeast. Real ale is fermented twice, then served directly from the vessel in which it was secondarily fermented, which is usually a cask (sometimes called a firkin). Eschewing sissy treatments like pasteurization or filtration, cask-conditioned ale is literally alive with yeast; when sugar is added to the cask, it’s eaten by the hungry little buggers, producing mild carbonation and a panoply of complex tastes. Therein lies the stuff’s appeal.

Because there’s no pasteurization, and because the method of serving (usually with a hand pump, called a " beer engine " ) means that oxygen comes into contact with the beer, the ale must be consumed within a few days before it spoils. But this also means that as it ages, its flavors change from day to day, even from hour to hour.

And taste, of course, is the thing. " The advantage of cask-conditioned ale is that different flavors in the beer come out, " says Geoffry Larsen, a member of the Cask-conditioned Ale Support Campaign (CASC), an organization devoted to raising awareness and consumption of real ale in New England. " In other beers, big carbon-dioxide concentrations tend to mask the flavors, but the really low carbon dioxide in real ale lets you taste the malt character of the beer, and the flavors don’t get so hidden. "

Traditionally, real ale has found most of its adherents in the musty pubs of the British Isles and has received a frosty stateside reception. But thanks in part to the efforts of Larsen, his cohorts at CASC, and the organizers of NERAX, cask-conditioned ale is gaining something of a foothold in the US, where beer consumption is still dominated by watery, mass-produced suds and a glutted microbrew market. With any luck, says Larsen, NERAX will " get people out there to try real ale, find out where it is, learn the different styles available, learn the difference between British and American real-ale brewers. The real push is to say, ‘Here’s what’s out there, and here are all the wonderful flavors.’  "

So come on down to NERAX and try an Inveralmond Thrappledouser or a Harviestoun Bitter & Twisted or an Orkney Dragonhead. They’ll be flat, they’ll be warm, and they’ll be delicious. And more likely than not, the real-ale enthusiasts will welcome you into their rarefied ranks with a flat but warm embrace, singing " Ale, ale, the gang’s all here! "

The New England Real Ale Exhibition takes place May 1 through 5 at the George Dilboy VFW Hall, 371 Summer Street, in Davis Square, Somerville. Call (617) 625-5224, or visit www.nerax.org