<P> The sweet wort collected from sparging is put into a kettle, or "copper" (so called because these vessels were traditionally made from copper), and boiled, usually for about one hour . During boiling, water in the wort evaporates, but the sugars and other components of the wort remain; this allows more efficient use of the starch sources in the beer . Boiling also destroys any remaining enzymes left over from the mashing stage . Hops are added during boiling as a source of bitterness, flavour and aroma . Hops may be added at more than one point during the boil . The longer the hops are boiled, the more bitterness they contribute, but the less hop flavour and aroma remains in the beer . </P> <P> After boiling, the hopped wort is now cooled, ready for the yeast . In some breweries, the hopped wort may pass through a hopback, which is a small vat filled with hops, to add aromatic hop flavouring and to act as a filter; but usually the hopped wort is simply cooled for the fermenter, where the yeast is added . During fermentation, the wort becomes beer in a process which requires a week to months depending on the type of yeast and strength of the beer . In addition to producing ethanol, fine particulate matter suspended in the wort settles during fermentation . Once fermentation is complete, the yeast also settles, leaving the beer clear . </P> <P> Fermentation is sometimes carried out in two stages, primary and secondary . Once most of the alcohol has been produced during primary fermentation, the beer is transferred to a new vessel and allowed a period of secondary fermentation . Secondary fermentation is used when the beer requires long storage before packaging or greater clarity . When the beer has fermented, it is packaged either into casks for cask ale or kegs, aluminium cans, or bottles for other sorts of beer . </P> <P> The basic ingredients of beer are water; a starch source, such as malted barley, able to be saccharified (converted to sugars) then fermented (converted into ethanol and carbon dioxide); a brewer's yeast to produce the fermentation; and a flavouring such as hops . A mixture of starch sources may be used, with a secondary starch source, such as maize (corn), rice or sugar, often being termed an adjunct, especially when used as a lower - cost substitute for malted barley . Less widely used starch sources include millet, sorghum and cassava root in Africa, and potato in Brazil, and agave in Mexico, among others . The amount of each starch source in a beer recipe is collectively called the grain bill . </P>

Where do all the calories in beer come from