When oxygen gets to a beer after fermentation then the beer is in trouble. The typical flavours you can expect are cardboard, paper and wood, while extremely oxidised beer can become sherry-like and sour. In a fresh beer these stale are not good characteristics; in an aged beer, particularly vintage ales and old ales, these can be good things. It’s a matter of taste and of relevance to the style.
While the brewers need oxygen to stay alive, if it gets into the beer it can have the opposite effect. Oxygen can get to beer at many different stages during beer production, some when brewing and some with filtering and packaging, but it becomes most relevant after fermentation. In fresh beer oxidisation is bad. I find it one of the least enjoyable flavours, imparting a dry mouthfeel like you’ve got a piece of paper stuck on your tongue. It can also evolve the malt flavour in ways the brewer didn’t intend – pale beer can mutate into honey or sherry qualities while dark beer can get these sherry notes plus dried fruit, which many actually enjoy, but it can also reduce the depth of malt flavour and leave it thin.
In aged beer oxidation can be a good thing, especially if the beer is stored correctly, as it can add complexity and depth. It can also be a bad thing in aged beer if it’s turned sour and lost all its flavour. Oxidation in aged beers is a complicated one and very much an individual taste depending on individual beers.
The oxidation process can also bring back foes from fermentation such as acetaldehyde (which can then turn into acetic acid), while diacetyl can speed up the staling process.
Any oxygen in beer after fermentation is bad news but heat and motion can speed up the oxidising process. Shipping beers internationally in non-chilled containers can result in the beers arriving at their destination oxidised (so if you’ve ever been disappointed by an exciting new import from afar then consider the distance it’s travelled and the cargo conditions). Likewise, kegged beer sitting in direct sunlight can suffer the effects of oxidisation (the light is ok here, the heat isn’t). Beer needs to be kept cool and still for it to have the best chance to stay fresh.
Beer oxidises. Thankfully there are ways of softening the blow of opening what you think should be a great beer only to find that it’s going bad. While it may develop interesting depth in some beers, it’s not necessarily a good thing, particularly in fresh beers. Do you like the flavour of oxidised beer?
I used this post to help me write this blog.