MOSCOW - Outside the Savelovskaya Metro station, Galina Krivonosova performs one of the city's rites of summer, pouring draughts of the national semisoft drink from a tanker-trailer that looks like it could be used to haul toxic chemicals.
"I don't like Pepsi or Coke at all," says Krivonosova, 41. "I prefer kvass."
The reason for her preference for the ancient beverage? Russians are practically raised on it, she says. And what's not to like, she asks, about a drink traditionally made of fermented stale rye bread? "Bread is very healthy and tasty."
With the first warm weather every year, the green-and-yellow kvass tanks, mounted on chassis with truck tires, swarm near bus stops and subway stations in Moscow's middle-class neighborhoods.
For about 60 cents a quart, peddlers fill plastic cups with the murky, fizzing liquid. Muscovites on their way to work or school toss it back and babushkas trundle home with a liter or two in empty soda bottles.
Kvass - the name comes from the Russian for "sour" - is zesty, piquant and, in its traditional form, somewhat alcoholic. (One Russian boasted that it takes "gumption" for a faint-hearted Westerner to try the brew.)
Like Russia, kvass is making a big comeback.
The stout-colored liquid once seemed headed for oblivion, threatened by a glasnost-era infatuation with Western soft drinks and a post-Soviet preference for Western products in general. According to one survey, per capita kvass consumption fell from 16 gallons annually before the 1917 Bolshevik Revolution, to two gallons by 1989 to just a half-pint by 2003.
Although Russians are drinking a lot less kvass than they once were, experts say consumption is increasing. For nostalgic and patriotic reasons, all things Russian are suddenly stylish. Kvass - like the Orthodox faith and Communist-era symbols - is once again a source of pride.
The brew has traditionally been produced at home or in small-scale commercial plants. But now, some of the Moscow area's largest beer breweries, including the mammoth Kostroma Brewing Plant, are engaged in commercial kvass production. The city government has even launched a string of kiosks called Moscow Kvass, dispensing a traditional form of the beverage from taps like beer.
There also are plans to start commercial-scale production elsewhere in European Russia, including Rostov-on-Don and Krasnodar near the Black Sea.
But commercially brewed kvass doesn't always taste like the traditional beverage, gourmets say. The reason is that kvass is like homemade bread: It's only good when its fresh, and it doesn't stay fresh long.
Normally, the beverage spoils after 48 hours, about the time it takes a Moscow peddler to sell an 80-gallon tanker load.
To extend kvass' shelf life, manufacturers are forced to pasteurize it, filter it or - worst of all, in the eyes of purists - make a kvasslike beverage using syrup, sugar and artificial carbonation.
So the Russian government has launched a crash program to figure out how to keep traditional-style kvass fresh without ruining its flavor.
Leading the effort is Konstantin V. Kobelev, a scientist with the All-Russian Scientific and Research Institute of Beverages, who works in a lab about a mile west of the Kremlin.
Like the best beer, Kobelev says, the best kvass is unfiltered, unheated and unadulterated.
"As soon as we begin to process kvass, so it can be stored, that changes the content," he says. "And that changes the flavor."
Russians are finicky about the taste of their kvass. Almost every Russian cook has his or her own secret recipe for the beverage, used as the basis for hundreds of kinds of summer soups.
One Moscow brewery has succeeded in producing a traditional-style kvass that can be stored for up to 60 days, Kobelev says. But that's still impractical for large-scale brewing and distribution.
Kobelev is working on perfecting a method to extend the shelf life of kvass up to six months. He predicts that his new method will be ready for testing next spring.
When it comes to competing with Western soft drinks, kvass faces another handicap.
Drinking kvass is a strictly seasonal activity.
So the kvass breweries are planning to use advertising to persuade Russians that it's a drink for every season.
Changing the drinking habits also means changing the traditional use of a food that dates back more than 1,000 years to the creation of the Russian state.
When Prince Vladimir, the Grand Duke of Kiev, set about converting Russians to Christianity in 988, kvass was a staple. As an incentive to stubborn pagans, Vladimir reportedly ordered the distribution of barrels of honey and kvass to converts.
Grain-based kvass also was used as the basis for a variety of okroshka, or Russian cold soups.
The Russian diet leans heavily on meals made of grain and vegetables, particularly during the 180 days of fasting on the Russian Orthodox calendar when the faithful are barred from eating meat or poultry.
Kvass has always been a village drink and Russian women once gathered to brew a batch every few days, storing what they could in cellars in jugs stacked among pieces of winter ice.
"The making of kvass from bread is as widespread as bread making," one physician wrote in 1898.
Kobelev keeps a list of the names of some of the 150 kvass recipes he's collected: "Northern," "Spring," "Ancient," "Dark," "Monastery," "Cossack's," and, of course, "Moscow." In addition to bread kvass, he says, there are berry, honey and fruit kvasses.
Kobelev says he frequently gets calls from entrepreneurs who want to start a commercial kvasovarnya or kvass brewery. He says the cost would be at least $100,000 for the equipment needed to meet health standards.
Few people wind up launching a brewery, he says.
"Quite many say we can't afford such sums," he says.
Traditionally, kvass is quaffed by everyone from babushkas to babies, despite its alcoholic content - now limited by law to 1.2 percent.
No one flinches at giving low-alcohol drinks to children in Russia, where teen-agers drink beer on street corners and vodka is considered a health tonic.
"By many, kvass is considered nonalcoholic," Kobelev says.
As to the taste: "Many people don't like beer the first time they drink it," he says. "Kvass grows on you."