Civilizations have brewed beer since around the sixth millennium B.C. In order to make beer, fermentation must take place. There are three methods of fermentation cool, warm, and spontaneous. Warm fermentation requires temperatures of 59⁰F to 75⁰F. This type of fermentation produces a foam layer on the surface due to the yeast adhering to the CO2 and rising. Ale beer produced by this method is typically ready to drink in three weeks. Cool fermentation occurs at temperatures of 41⁰F to 50⁰F. This method produces lager beers. They are fermented and then set aside to continue aging for thirty days near 32⁰F. Spontaneous fermentation is rarely used and occurs in open air fermenters, mostly in Brussels. This method creates Lambic beer. It uses naturally occurring yeast and microorganisms from the air to naturally ferment the starch materials. It takes several years after it has been barreled to reach maturation.