Wild by birth, sour beers or Sour Ales are one of the oldest types of beer. Unknown to most, they are strongly returning to the limelight thanks to events such as the Arrogant Sour Festival, come to its 7th edition this year. Sour beers are mostly made thanks to the spontaneous fermentation of yeasts: before being put into wooden barrels, the must is left to cool for a whole night in large tanks called "coolship", placed in the brewery attic. Then, yeasts and bacteria naturally present in the air are deposited on the must and in few days will give rise to fermentation. The spontaneous fermentation beer par excellence is the Lambic, it is produced only in winter months in Belgium in the area that extends from Brussels to Pajottenland, along the Senne river valley. However, spontaneous fermentations are not the only existing sour beer style, but there are others that differ in their origin, production method or type of yeasts and bacteria. Thanks to technology, in the last decade it has been possible the in vitro isolation of wild yeasts, used for a controlled inoculum. In this case it is no longer possible to speak of spontaneous fermentation but we will talk about inoculated beers with yeasts or bacteria.