Fermentation is one of the key parts of brewing. It takes place in fermentation vessels which come in various forms,from enormous cylindroconical vessels,through open stone vessels,to wooden vats. After the wort is cooled and aerated-usually with sterile air-yeast is added to it, and it begins to ferment. It is during this stage that sugars won from the malt are converted into alcohol and carbon dioxide,and the product can be called beer for the first time.