Ever since the turn of the century, however, modern methods have mostly eliminated this catastrophe, placing more of an emphasis on controlling the degradation of beer after it has been brewed. Beer enthusiasts could now focus on the nuances of their beer of choice, and note certain flavors manifesting themselves especially as the beer aged. Some of these flavors were desirable, but not all, and the current craft movement that has injected a multitude of styles into the mainstream market has made the discussion of off-flavors more fervent then ever.
While some beer may be made with only barley, yeast, water and hops, there is a wealth of various proteins and molecules that are present in those ingredients . Those compounds, whether they form new ones during the brewing process or degrade into others, can eventually build up enough of a presence to push past the “taste threshold” . This is done with the help of some outside force, mainly oxygen, and can cause your IPA to taste a little funky. Many chemicals have been identified and categorized based on which undesired flavors they produce. For example, if you find your beer to have a soapy flavor, that can mean it was left in the carboy too long and fatty compounds, from the malted barley, had a little too much time to break down into a substance that is actually soap.
The list is long, and inquiring minds want to know more about how and why we get these flavors. A user on the forum Reddit has supplied a scientific paper that was published a few years ago that nicely describes the various compounds that have been identified and what exactly causes them. As mentioned, oxygen is a big culprit, being necessary to help your yeast population grow but an absolute hazard when present anytime else. Time is another big factor, leading to the degradation of the compounds in hops that make them “hoppy” and that is why most brewers implore their drinkers to imbibe their IPA as close to the brewing date as possible.
A word of caution, the paper that Reddit links to was published for a scientific journal, and can be rather dense. The first couple of pages are a breeze, but once they get into the analysis, you start getting into the really fancy and long compound names that the organic chemist inside me loves. Also below is a link to the section in John Palmer’s “How to Brew” that goes over the various off flavors in more layman’s terms, and can be quite the handy cheat sheet.
Source: John Palmer