For the commons beer in the middle ages the usual spirits was. It was more safe than water, because the water in the cities was frequently polluted, whereas beer was filtered at the construction cooked and. Moreover beer was nutritive: the contained grain and it leaven was rich to vitamin B and other necessary substances. Beer consumption was in the middle ages thus very high: in the low countries one drank zo' n 300 litres beer per year. Thereby must be noticed, however, that the beer contained rather little alcohol. Brewing beer was entrusted in the middle ages to convents and abbeys, where it mainly women became gebrouwen. As a rule abbeys had a heavy and light version of the beer, where the heavy version was drunk paters and guests, and the lighter version by the sisters. In the course of the middle ages a professionalisation came on pace and in the present Germany beer breweries was opened there. Already rapidly there also arose in the low countries professional beer breweries. Up to the 14e centuries beer only gebrouwen with spontaneous fermentation became, after the 14e centuries old beer and later leaven are added during brewing. Vestiges of beers of spontaneous fermentation are find the lambik beers, in the Zennevallei, at south west of Brussels.