Along with Light Struck beer, oxidized beer is a common off flavor beer drinkers experience. Brewers can improve their quality control for problems like acetaldehyde and diacetyl, but it’s hard to fight oxidation because time is a significant contributor.
Trans-2-nonenal is the chemical name, but everyone calls it oxidized beer. Beer drinkers vary in their perceptions, but most describe the oxidized flavors like paper, cardboard, Sherry, honey or wax. Personally, most oxidized beer strikes me as the odor of wet-cardboard or the taste of waxy lipstick. Lighter colored beers often exhibit paper or waxy flavors, while darker beers typically get sherry-like.
Almost all beer oxidizes because it’s difficult to prevent oxygen from getting into the beer during brewing, fermentation and packaging. Oxygen can get into the beer almost anywhere in the brewing process, but packaging is the most likely entry point for oxygen. Most bottling and canning machines attempt to remove oxygen by purging the containers with CO2, but complete expulsion of oxygen is virtually impossible.
Drinking beer fresh is the best defense, a small amount of oxygen can’t hurt the beer if it’s stored correctly. Time and heat intensify oxidation, so proper care and storage by distributors and retailers is critical to keeping your beer fresh. Aging beer is not a good thing because time intensifies the oxidation effect. Also, heat is also the enemy of beer, so cold storage is critical to minimizing oxidation.
Unlike most off flavors, slight oxidation is acceptable in some styles, like Barleywines, Old Ales and barrel-aged beers. The commonality in these beer styles is they are all very malty with high alcohol contents, and typically age well. Those style are usually aged by the brewer prior to packaging, but consumers may age them too (AKA cellaring). The key to appropriateness in these styles is light oxidation, heavy oxidation is undesired.