There are four types of fermentation: top, bottom, spontaneous and mixed fermentation. Beer obtained from top fermentation uses yeast that ferments at relatively high temperatures and for a short period of time. This is beer of the ‘Special’, ‘Trappist’, ‘Stout’ and ‘Ale’ type. Bottom fermented beer on the other hand ferments for a longer period at a lower temperature. Lager and Export type beers are bottom fermented. With regard to spontaneously fermented beer: this is beer that ferments without any addition. It has a rather sour flavour. This comes from the fermentation of lactic acid and acetic acid which ferment naturally: ‘Geuze’, ‘Lambic’, ‘Faro’. Mixed fermentation beers (the southern Flanders red beers) finally, are the result of a combination of top fermentation and bacterial flora, that convert organic acids and higher alcohols into esters.