Have you ever cracked open a bottle of beer and found that it tasted like butter? In this video, we go over the common beer off flavours: buttery, green apple, salty, yeasty, and rotten eggs. Diacetyl is the compound in microwavable popcorn which gives it the "butter" flavour and mouthfeel. While diacetyl is naturally produced during fermentation, it is also taken up by the yeast during the course of fermentation. An incomplete fermentation or bacterial infection by lactic acid bacteria can both cause high levels of diacetyl in finished beers. Beer that tastes like rotten eggs is caused by sulphur production during fermentation. This off-flavour is usually found in lager beers. Green apple or nail polish flavours are caused by acetaldehyde production during fermentation. Salty beer is caused by adding too many brewing salts, and yeasty beer is caused by beer that has been left to sit on the yeast cake too long. Let's take a closer look into some more common off flavors in beer!