Beer phenols are chemical compounds, similar in structure to alcohols, that are generated by yeast during fermentation. In certain styles of beer, such as Bavarian hefeweizens and wit beers, phenol flavors such as bubblegum, banana and clove are considered desirable; but in other styles they are considered to be an off flavor or flaw. Causes of unwanted phenols include wild yeast or bacteria, chlorine, and excess sanitizer.