As spring melts away a long winter deep in Latvia's vast forests, the stillness is almost imperceptibly broken by a rhythmic drip, drip, drip.

A small black tube protrudes from the trunk of a leafless tree growing among spruces, birches and pines. Trickling from it, into a plastic bag suspended below, is a clear, sweet, watery sap which has been one of this country's most popular drinks for centuries.

Here, late March to mid-April is "berzu sula", or "birch juice" season. Stalls groaning with bottles full of the sap have popped up by roadsides, while top chefs tout it as an essential ingredient in Latvian nouvelle cuisine and scientists, a health wonder.

Linards Liberts, the country's foremost birch juice expert who has revamped its rustic image, is especially enthusiastic after this year's long and bitterly cold winter. For him, even the snowiest cloud has a silver lining.

"The colder the winter, the sweeter the juice," says the 34-year-old. "That's why our birch juice is so special and why you can't get it in France or Italy - it simply doesn't get cold enough there for long enough. We're lucky to have such harsh winters!"

As soon as the temperature hits zero, he and throngs of other birch juice fans flock to forests, or their own back gardens, to tap Latvia's millions of birches, distinguished by their brilliant white bark.

For Liberts, this delicately sweet fluid has become the life-blood of his business.

"I deal with birch juice all year round, but for these two to three weeks, I am totally obsessed!" Liberts chuckles.

At his small organic farm in the central Latvian town of Ikskile, his cellar would make any French winery proud.

But instead of fermenting grape juice, it is stacked full of his birch juice products: still and sparkling wines, syrup, lemonade and schnapps, all elegantly-bottled and premium-priced.

"I have only around 200 trees. Compared to maple syrup production in Canada where even the smallest farms have thousands of trees, we're Lilliput," he smiles.

Still, Liberts is attracting an international reputation. Following an appearance at the World Organic Food fair in Germany in February, he received so many orders he had to turn most away as he was short on sap.

"People were amazed how fresh and pure the taste is, especially if they have only previously encountered the pasteurised, sweetened versions of birch juice that are popular in Belarus and Russia," he says.

Liberts is also doing his best to ensure birches are tapped in a way that causes the least possible damage.

"The old-fashioned way is to drill a large hole right into the heart of the tree, but we prefer to do something more like modern keyhole surgery. Seven millimetres is the optimum width of the hole and you should not go into the tree more than three to four centimetres," he warns, explaining that larger holes that damage trees only increase the flow by five to seven percent.

A powerful anti-oxidant

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The sap is also prized by eco-cosmetics maker Madara, one of Latvia's most succesful businesses with outlets in 28 countries.

Research into the sap's anti-ageing properties at the University of Latvia, prompted the company to launch a new line of products last year promising a youthful glow.

"Birch juice both stimulates the growth of dermal and epidermal cells, and delays cell ageing," Madara founder Lotte Tisenkopfa-Iltnere told AFP.

The studies by researcher Dr. Janis Ancans show the organic sap's array of benefits as a powerful anti-oxidant.

"Birch juice not only rejuvenates but also protects skin cells from oxidative stress, including Ultra Violet rays, environmental pollution and consequences caused by inflammation," his recent study found.

A new generation of chefs are also turning birch juice -- long regarded as a humble drink for peasants -- into a must-taste ingredient on the menus of Riga's trendiest restaurants.

"It is especially good for poaching fish, making syrups and for sauces," says Chef Martins Sirmais, who co-owns several of the Latvian capital's most fashionable eateries.

"We have been promoting the use of birch juice for the last five or six years and it has definitely gained popularity, especially among foreigners who have never tasted it before," adds Latvia's top TV chef.

Even visiting Turkish President Abdullah Gul and his wife were treated to birch juice at a recent state banquet in Riga, Elvira Stepanova from the office of Latvian President Andris Berzins confirmed.

Given that the president's name actually derives from "berzs" -- Latvian for "birch" -- it seems appropriate that even the leader of the country can't resist the sap: "Yes, President Berzins taps birch trees in the springtime for their sap, and he does drink birch juice," Stepanova reveals.