BRUSSELS, Nov 10 (Reuters Life!) - A five-month brewing window, three years in storage and a sour end product with a farmyard bouquet.

Lambic beer risked dying out a decade ago, but its survival and growth now seem assured despite being among the most awkward beers to make, store and market.

“Ten years ago I might have said it was set to disappear. Now there’s great expansion of traditional brewing and blending,” said Tim Webb, author of the Good Beer Guide Belgium.

Beer can be divided into three basic forms: bottom-fermented lager and top-fermented ale, both of which have cultivated yeast introduced, and a third form, spontaneous fermentation, using only wild yeasts in the air.

Lambic beer is produced in this third way.

The umbrella group for brewers and blenders Horal says it is the oldest surviving brewed beer in the world.

Jean Van Roy, head of the Cantillon brewery in Brussels, has been glued to the weather forecasts since late October.

Spontaneously fermented lambic beer is not a 365-day a year product and can only be made when the night temperature drops to around 5 degrees Celsius (41 Fahrenheit) or below.

That is because the boiled wheat and barley juice known as wort must sit overnight in a large copper vessel to cool and absorb airborne yeasts that waft in through vents.

Van Roy’s brewery, which doubles as a museum welcoming about 30,000 visitors per year, boasts equipment dating back to the 19th century and spiders as natural pesticides.

Most beers are consumed within weeks of production, but gueuze, the most commonly drunk form of lambic beer, sits in barrels for an average of two years and then in a bottle for as much as a year before being sold.

Pure lambic, essentially flat, can be consumed as it is, but is generally drunk as gueuze, a sparkling blend of lambics of different ages that ferment again in the bottle, or as kriek, gueuze with sour cherries. Other fruit can also be added.

The very dry and sour taste is a shock to anyone used to standard lager or ale.

“If you approach it as a wine or a traditional cider it’s easier to appreciate... The only reason it’s not celebrated by people with a taste for excellence is that they have not heard of it,” said Webb.

CHAMPAGNE OF BEERS?

European law decrees that the terms lambic, gueuze and kriek can only be used for acid beer made by spontaneous fermentation with specific airborne yeasts.

However, unlike champagne, it need not be made in a specific region -- in lambic’s case the traditional area is in the Senne valley in and to the southwest of Brussels.

Armand Debelder, a brewer and blender, established an umbrella group, formalised in 2004, to secure the artisanal industry’s future.

“You can brew spontaneously fermented beer all over the world, but lambic only here. The wild yeasts live in this region,” he said.

The problem for Lambic brewers is that it is far more expensive to make than average beers and they have struggled to push through price hikes at bars mostly controlled by big breweries.

Unlike Trappist ales, you will struggle to find a gueuze of kriek in a Belgian bar, with the exception of the Belle-Vue and Mort Subite brands of brewing giants Anheuser-Busch InBe and Heineken.

“With hygiene norms of the 1990s, all the small breweries could have closed,” said Debelder. “People also only had a taste for sweet things.”

“Now at least we are respected in the Belgian brewing industry. Growing interest in slow foods has also helped.”

Van Roy agreed that the appeal of products such as organic wines and traditional ciders had also extended to lambic beer.

“The Internet has clearly played a role here. More and more people also know what is an authentic lambic,” he said.

The word has indeed spread. Cantillon exports some 55 percent of its beers.

North American, Scandinavian, Japanese and increasingly Italian drinkers are buying up gueuze. Bottles on sale for some 5 euros ($7.49) in a Belgian brewery could fetch at least $50 in a Manhattan bar.

Visiting Carlos Rodriguez, of Ales Agullons near Barcelona, is hoping to adjust Spanish palates with mixes of his own brews and lambic. Californian speciality brewer Russian River has also turned its hand to spontaneous fermented beer.

“Twenty five years ago, we had to beat against doors to sell our beer. Now people beat on our doors to buy,” said Van Roy. Beer author Tim Webb believes the lambic brewers and blenders are at a crossroads, facing a decision of whether to survive as folksy craft beer producers or to establish themselves as makers of a growing premium product.

“You have premium white wines from champagne. Why can’t that happen to gueuze and lambic. That would justify the money they need to invest,” he said.