Stale flavors in beer are perceived as sherry-like, old bread, wet cardboard and paper. Wet Cardboard is the most common descriptor for these off flavors.
Oxidation occurs in brewing when the wort, at temperatures above 80° F (27° C) is introduced to oxygen.
The reaction is commonly called hot side aeration (HSA). During HSA, aldehydes are formed in the wort which will not be degraded by the boil and which remain in the finished beer. These aldehydes will result in the oxidative character described above as the beer is stored.
Oxidation is inevitable in all beers. For beers that are meant to be aged for a long time, such as barleywines, the flavors are akin to sherry or fig notes rather than the cardboard flavors and is not considered a fault.
To avoid oxidation and wet-cardboard flavors in your beers, avoid hot side aeration.
Do not splash your wort while its temperature is above 80° F (27° C) during any stage of brewing.
Stir slowly without splashing when cooling your wort with an immersion chiller.
If you whirlpool your wort, design the system so that very little to no splashing occurs. This may involve adjusting the pump speed or the re-design of the copper pieces attached to your system or immersion chiller.
Just remember that when the wort is warm or hot to the touch, avoid aerating the wort to keep HSA to a minimum.
There are other ways that oxygen can enter your beer. Minimizing the absorption of oxygen into your beers is a worthwhile habit to get into.
Any time there is a headspace above your beer, purge it with CO²:
I have had some very good beers marked down in competitions for being oxidized. Even though all precautions were taken, the judges still noted the typical flavors of oxidation.
I suspect this may have something to do with the journey the beer took once it left my home on the way to the competition. One time only I bottled a few beers with a picnic tap and bottling wand.
This was one of the times that I got gigged for oxidative flavors. I suspect there was oxygen in the headspace which was absorbed as the beer was jostled and agitated.
The heating and cooling cycle the beers encounter while en route to a competition is supposed to speed the aging processes, not to mention the tremendous increase in the speed of chemical reactions that takes place with elevated temperatures.
Combine the two and I believe you can get premature oxidation in your beer and the stale flavors in beer in a matter of days.
Another cause of stale flavors in beer is the cycle of hot and cold and the agitation that are inevitable if you ship beer during the summer months. The only thing you can do is pay more to ship next day or in 2 days.
You can, however, make sure there is CO² in the headspace of the beer before you cap it. Doing so will give your beers the best chance of arriving without the stale flavors in beer associated with oxidation. Remember, control what you can and leave the rest up to the brewing gods.
Although many say it takes a lot of oxygen to oxidize your beer, it is best to err on the side of caution. It's always a good brewing practice to keep oxygen out of your beers.
Just like sanitation, you can't be too anal about the details. This is one of the things that separates really great brewers from the average brewers. Attention to details. Check out the BJCP page on Beer Faults for more information.
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