AS IF AMERICA'S 1,500 breweries don't already produce an unimaginable variety of ales and lagers, now some are expanding their portfolios with series of one-offs that are testing the limits of our palates . . . and pocketbooks.

The highly popular specials - often called "reserve" beers - typically feature unusual or extreme flavors geared toward the breweries' most passionate followers. Devotees closely track the beers' yearly or monthly release dates on the Internet, hoping to pounce on the limited-edition brews before they're sold out.

When the Bullfrog Brewery in Williamsport announced it would release a special bottling of raspberry-flavored Frambozen earlier this month, for example, fans lined up outside before opening time. Its supply of $20 corked bottles was gone in 10 minutes.

Beer companies love that kind of excitement.

Amanda Johnson of Odell Brewing in Colorado said her brewery's Woodcut Series of oak-aged beers is good publicity for all of its brands. The one-offs, she said, show that "if we can make beers this spectacular, our everyday beers must be top-of-the-line as well."

Brewers love them because they're a change of pace.

"It's basically a way for us to entertain ourselves," said Chris Wilson, head brewer at Weyerbacher in Easton, which launched its Brewers' Select series on draft and 12-ounce bottles last year.

Weyerbacher's employees contribute ideas for new styles, hashing out recipes with each other and tinkering with ingredients. Each release is named after letters in the old ham-radio phonetic spelling alphabet. Alpha was a Belgian pale ale, Bravo a red wheat ale. Last month it released Foxtrot, a dark biere de garde.

Dozens of breweries are doing something similar.

Magic Hat (Vermont) brews an experimental style every season and calls it Odd Notion. Harpoon (Massachusetts) releases a new variety every couple months under its 100 Barrel Series. Cigar City (Florida) ages its Humidor Series on cigar box cedar.

The Sly Fox (Phoenixville, Royersford) Hop Project is a draft-only series that features a pale ale spiced with a different hop variety each month. (This month: the unusual Marynka hop from Poland.)

Full Sail (Oregon) lets its employees take turns hand-crafting a single batch in its Brewers Share series. The Midnight Brewing Project at Terrapin (North Carolina) is a collaborative series with Left Hand Brewing of Colorado.

Flying Fish names its one-offs after exits along the New Jersey Turnpike. Its first, Exit 4, was a hoppy Belgian tripel; coming next, Exit 11, a hoppy American wheat, a la Three Floyds' cult favorite, Gumballhead.

Some are exceedingly difficult to track down.

Rogue (Oregon) releases kegs of its monthly John's Locker Stock series to just 100 locations across the country. You have to drive to Harrisburg to get a taste of Troegs' Scratch Series; the brewery doesn't sell it to retailers.

Brewer Chris Trogner said Troegs started the series to feature experimental recipes that never made it into full production. And that, he said, "morphed into a great way to become more knowledgeable brewers."

Like other one-offs, the Troegs' series often employs unusual ingredients, including star anis, yarrow flower, espresso beans, ginger root. Coming next: Scratch #20, a hoppy double imperial amber ale that will be unveiled tomorrow at the Harrisburg Brewers Fest.

One of the original one-off series comes from Stone Brewing in Escondido, Calif., which produces the wildly popular Vertical Epic Ale. Brewed annually since 2002 and named for their corresponding release dates (this year's comes out on Sept. 9 and will be called 09.09.09), they're intended to be cellared and tasted as a group after the final one is packaged on Dec. 12, 2012.

I confess, I haven't had the patience to wait. But Stone's brewers sample them periodically and share tasting notes on its Web site. So far they're holding up.

Why go to all the trouble?

"Think of it as a great excuse to play around in the Belgian-influenced realm of beer," said Stone CEO Greg Koch.

But it's more than just play. Experimenting with the series' unusual yeast strains eventually led to Stone's successful production of Cali-Belgiqué, a hybrid California/Belgian India pale ale.

Likewise for beer-drinkers, one-off series are a fun way to experiment with unusual flavors. And even if they don't enjoy the taste, they can always try trading or reselling unopened bottles. Koch said he's seen 22-ounce bottles of Vertical Epic that originally sold for $4.99 going for as much as $650 on eBay. *

"Joe Sixpack" by Don Russell appears weekly in Big Fat Friday. For more on the beer scene in Philly and beyond, visit www.joesixpack.net. Send e-mail to joesixpack@phillynews.com.