Sour beer is a beer with an intentionally acidic, tart or sour flavour. Once hard to find, they have become increasingly easy to source thanks to the rise of craft breweries. Sour beer is one of the oldest styles in history. Prior to pasteruisation and sterlisation almost all beer would have an element of sourness to it.
Whilst sour beers can come in a range of styles from mouth-puckeringly sour to fruity and light, they all have one thing in common and that is that they are made with wild bacteria and yeast. Most commonly used are Lactobacillus, a bacteria that turns sugar into lactic acid and pediococcus, which adds acidity that increases the longer it is in a beer.The most commonly used wild yeast, brettanomyces adds a layer earthiness to a beer.The addition of fruit during the aging process spurs a secondary fermentation and helps create the tart flavours.
Using wild yeast makes the brewing process extremely unpredictable, taking months to ferment and years to mature.