The Belgian beer style that spontaneously ferments with wild yeast is lambic. Unlike beers fermented with a deliberately added cultivated yeast strain, lambic is exposed to naturally occurring microorganisms in the air and environment around the brewery.
Traditional lambic production is especially associated with the Pajottenland region and the Senne Valley near Brussels. Brewers commonly use aged hops, which provide preservative qualities without adding intense bitterness, and a grist containing unmalted wheat along with barley malt. After brewing, the wort is cooled in open vessels called coolships, where wild yeasts and bacteria begin the fermentation.
Lambic may mature in wooden barrels for months or years. Brewers can blend younger and older lambics to create gueuze, while fruit versions include kriek, traditionally made with cherries, and framboise, made with raspberries. The style is often tart, dry, earthy, and complex.
Ale, stout, and pilsner are broad beer categories or styles, but they are not synonyms for lambic’s spontaneous-fermentation tradition.