CAN SAINSBURY'S be serious? Its latest own-label beer is a peasant brew with the strangest sweet- and-sour taste, reminiscent of a fino sherry and a dry vermouth, with the sparkle of a champagne and a bottle to match (though the price of pounds 2.15 denies the association). This, the most winey of beers - known in its native town of Lembeek, near Brussels, as Gueuze - is presented to us this month as Sainsbury's Belgian Ale.

The area to the west of Brussels is full of place-names ending in a beke or beek which, like 'beck' in northern England, mean a creek. Around Lembeek, Vlezenbeek and Itterbeek, Pieter Brueghel the Elder found the characters to people his paintings The Peasant Dance and The Wedding Feast; near Itterbeek is the church of St Anna-Pede, which Brueghel depicted in The Parable of the Blind.

Each of these beeks flows into the small river Zenne. This stretch of the Zenne valley contains an area known as Payottenland, where the small-grained, reddish wheat is grown which is one of the principal raw materials of a basic and non-fizzy beer called Lambic (possibly a corruption of Lembeek) and the blended, sparkling version known as Gueuze (perhaps deriving from geyser, as in a gushing spring).

Farmhouse breweries used to make these beers. Eight or nine breweries remain in the region, though several have shut their doors in recent years. One of the oldest brewery businesses, dating from the 1600s, has been given a new life in recent years by a young devotee called Frank Boon (pronounced bone).

His brewery is in Lembeek itself, and he is the supplier of the Sainsbury's product. Purists, who love the truly dry Lambic and Gueuze, will criticise Boon for clearly sweetening his beer for Sainsbury's. Others will praise his sacrifice as an attempt to make the beer more accessible.

In the great brewing nations, no style of beer is older than the Lambic and Gueuze family. The use of wheat in brewing is a very old practice, and is having something of a revival.

In Lambic, the wheat is unmalted, an ancient method scarcely seen outside Belgium. Wheat makes for tartness in beer, and the lack of malting perhaps contributes a firmness of body.

The hops play an old role, too, acting as a preservative. They are aged for two or three years to diminish their aroma and flavour: wheaty tartness does not combine well with hoppiness. (Traditional brewers, on the other hand, like their hops to be as fresh as possible.)

Finally, the defining characteristic of Lambic comes from the use of wild yeasts instead of the usual cultured strains. The brew is cooled overnight in an open vessel, in a loft with vents, open windows, and often the odd missing roof-tile.

Airborne wild yeasts enter the room and have their way with the brew. A cool autumn evening is considered the best time.

There follows a series of multiple fermentations, over a period of between one and three summers, in wooden casks previously used for claret, muscat, port or sherry. Galleries full of these casks are allowed to remain musty, mouldy and cobwebby so the wild yeasts are not disturbed.

More than 80 wild yeasts work on beer in the Zenne valley, some - such as Brettanomyces Lambicus and Bruxelliensis - with obviously local names.

Others are oxidative yeasts and old film-forming cultures from the sherry casks, which impart the sherry-like notes to Lambic beers; yet others create flavours reminiscent of fruits, especially rhubarb (which may explain the hint of vermouth, which often contains Chinese rhubarb).

Such a cocktail of fermentation characteristics astonishes the brewer of more conventional beers, who keeps his premises hospital-clean to maintain the purity of his cultured yeast. They are also bewildered by the blending procedures in making Gueuze.

When it is only a few months old, a Lambic will still contain plenty of residual sugars. One that has seen two or three summers will have developed a powerful complex of yeasts. When the two are blended, the result is the champagne sparkle that characterises a Gueuze.

In another variation on the Lambic theme, these beers are sometimes given yet another fermentation over the small, dark, bitter cherries of the region. This style is known as Kriek, after the Flemish name for this type of cherry. Other Lambic-based beers are also made with raspberries and other fruits.

The basic Lambic is usually found only on draught in the region of production, though the odd bottle from the traditional Cantillon brewery in Brussels has been sighted in specialist beer shops in Britain.

Gueuze has also been difficult to find here, but perhaps Sainsbury's efforts will help more people in Britain to acquire the taste. However, fruit versions from Lambic brewers - such as Timmermans, Lindemans and St Louis (the last mentioned lies outside the traditional region) - have been more readily available in the high street chains.

Lambic and Gueuze make an unrivalled accompaniment to soft, sharp-tasting, cheeses. Visitors to Brussels can find this combination, albeit with some relatively tame Lambic, in the 'Flemish kitchen' ambience of Cafe Becasse, up an alley off rue Tabora, between the Grand Place and the Bourse.

Gueuze and fruit beers are also a speciality at the Twenties cafe A La Mort Subite, in rue Montagne aux Herbes Potageres (also near Grand Place, but in the opposite direction). More tart examples of these beers can be found at Chez Moeder Lambic, 60 rue Savioe, in the St Gillis district.

The Bruxellois make outings to the nearby villages for a Sunday constitutional and a glass or two of Lambic, Gueuze or Kriek. At the end of such strolls around the countryside, I have tucked into mussels poached in Gueuze, then been served the same beer with boudin noir (black pudding), at a pubby place called In De Rare Vos, on the main square of the village of Schepdaal (22 Marktplaats, tel: 02-5692086).

If you have ever enjoyed black pudding with apple sauce, you can imagine how well the sanguine sausage is accompanied by the sharp-tongued beer.

Weekends at In De Rare Vos are variously devoted to black pudding and tripe. The pub also specialises in pigeon and horse. A glass or two of Gueuze has often made me want to eat a horse, and on occasion I have succumbed to the temptation. What other beer has such powers?