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Introduction to traditional and mixed-culture beer fermentations

Beer is among the oldest fermented beverages and is the product of a fermented sugar extract from grains. Historically, beers were the result of spontaneous fermentations, involving several yeasts and bacteria. Conversely, pure yeast cultures have been commonly used in most modern breweries ever since they were first proposed by Hansen (Claus- sen, 1904). Most commercial beer brands are produced using proprietary Saccharomyces cerevi- siae or Saccharomyces pastorianus and Saccharomyces bayanus strains for the production of ales and lagers, respectively. The resulting beers have a standardized batch-to-batch flavour and are appreciated by beer lovers around the world. The presence of wild-type yeasts and/or bacteria during the fermentation process or in the final beer can alter the flavour and physical characteristics of the beers, turning them unsellable. The economic losses related to these problems are a threat in commercial breweries worldwide, emphasizing the need for quality control of the final beers prior to their shipping to the customers.

In contrast to modern commercial ale and lager brands, some traditional beers do not require axenic yeast cultures to pitch their worts and some breweries do not even inoculate the wort to induce the fermentation of their beers. These products are thought to resemble ancient beers, since they involve both bacteria and yeasts and have a vinous acidic character. Two major types of such beers are brewed in Belgium. For the production of lambic and gueuze beers, no yeast or bacteria are pitched to start the fermentation, whereas red-brown acidic ales re-pitch a lactic acid bacteria (LAB)-harbour- ing yeast suspension, that is harvested after every fermentation. Nowadays, there are traditional beers brewed outside Belgium that also apply mixed fermentations, either by spontaneous inoculation or through the use of mixed starter cultures. Lambic beers in particular and spontaneously fermented beers in general are currently highly appreciated all over the world, since they are trademarks of traditional craftsmanship. Traditionally, Belgian lambic beers were produced in the Senne river valley (south-west of Brussels) and in the southeast of Brussels, but due to their growing popularity lambic-type beers are now produced elsewhere in Belgium and internationally.

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