Traditional sour beer

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Sour beers, traditionally including lambics, oud bruins, Flanders red ales, Goses, Berliner weisse, and more recently American wild ales, represent some of the oldest commercial brewing styles. No active addition of microbes is carried out in traditional sour beer production; rather the wort is exposed to an environment by which it is spontaneously inoculated. Boiled wort is cooled down in open vessels where airborne microorganisms come into contact with the wort, before it is transferred to wooden barrels used in previous fermentations. The involvement of more than 2000 different yeast and bacterial strains in a lambic fermentation has been documented. Species considered important for final beer character include Saccharomyces, Brettanomyces and lactic acid bacteria (LAB) such as Lactobacillus and Pediococcus. The fermentation and maturation of traditional sour beer can take many years.

Regardless of the fruit type, “Fruit Lambic” beers (ADB-I) were the only style that could be clustered. They had the lowest ABV and highest density and ◦Brix. This old beer style is characterized by an up to threeyear-long spontaneous fermentation performed by a mix culture of yeasts and bacteria (De Roos and De Vuyst, 2019; Spitaels et al., 2014; Verachtert et al., 1995). During fermentation, sugars are transformed into ethanol, which is then converted into acetic acid (De Roos and De Vuyst, 2019). Fruit lambics are obtained by mixing old and young lambics with a high content of fermentable sugars and adding up to 25 % fruit resulting in low ABV and high ◦Brix beers (Gatza et al., 2017). Nardini and Garaguso (2020) reported a higher ABV (~5.0 %) for a raspberry lambic beer.