The primary cause of oxidized beer is the introduction of air to beer after the initial stages of fermentation. It is very easy to simply state that oxidation can be minimized by reducing air pick-up during the process, but like many things in life, this advice is much easier to offer than it is to practice. Before jumping into some of the things that can lead to oxygen pick-up in the process, I do want to touch on how different beers can be more or less sensitive to oxidation.
Many specialty beer styles use crystal malts for color and flavor. I find these malts interesting, because sometimes they can be used to terrific effect, yet other times they can muddy the flavor profile in a beer. Unless you are willing to rewrite the rule book and accept a paradigm shift, some styles seem impossible to consider without crystal malts, but when these grains are entirely removed from a recipe and replaced with a variety of higher kilned malts, such as Munich types, an “aha” moment often times follows. That is the interesting part I previously mentioned.
These malts can also be frustrating, especially when it comes to oxidation. Reading the literature leads many brewers to believe that crystal malts help ward off oxidation, but practical experience often times conflicts with this belief because beers containing more than about 5% crystal malts can really start to taste pretty unpleasant when aged. It seems to me that the crystal malts prevent beers from tasting like wet cardboard when oxidized, and instead take on an intense blackcurrant aroma, sometimes noted as Ribes or catty, coupled with more pronounced caramel flavors. It could be that you are fond of crystal malts and your malt choice is causing some issues.
Beers with lots of aroma hops can also present special problems since these pleasant aromatics have a limited life in beer. It can be very disheartening to witness the loss of hop aromas in really nice beers that do not seem to be old enough to be damaged by age. IPA brewers have come to realize that hoppy IPAs need to be consumed when fresh. Perhaps your limited shelf-life is a function of the hoppy beers you like to brew. Or maybe not?
Beer styles that are very delicate in nature, such as Pilsners, light lagers, and extra pale ales are also more likely to show the signs of oxidation. The subtle styles are like white shirts; they are great when clean, but can be really unpleasant when slightly stained. Oxidation can certainly put a stain on an otherwise excellent brew. If you don’t think your problem has to do with crystal malt, hop aroma or delicate profile, keep reading!
Oxygen in beer changes many of the compounds formed during fermentation into compounds that have different properties. The key thing to note here is that oxygen changes fermentation products, and explains why adding oxygen to wort does not ruin our beer. During fermentation, yeast produce carbon dioxide and create a positive flow of CO2 out of the fermenting liquid. This flow of carbon dioxide, even in open fermentation vessels, effectively blocks air from entering the beer; when fermenters are closed and equipped with airlocks there is effectively zero oxygen that comes into the system.
Whenever beer is racked from one vessel to another, the risk of oxidation is very real. Three things about racking that can result in oxygen pick-up are splashing, the environment of the vessel your beer is entering, and a leak in the racking hose. The simple act of splashing liquids exposes the liquid to the gas in the environment and increases the transfer of gas from the environment into the liquid. Although directing liquid flow down the side of a container is a great way to reduce splashing, it also increases liquid area and increases gas transfer . . . do not address splashing by directing beer flow to the side of the carboy.
The gas environment in the empty vessel is arguably as, or more critical, to oxygen pick-up during racking as splashing; if there is no oxygen in the empty vessel, oxygen pick-up is not possible. While completely removing oxygen from an empty vessel is difficult to do, it is pretty easy to remove almost all of the oxygen. The easiest way to remove the oxygen from a keg, is to fill the keg with water and push the water out with carbon dioxide. You can do the same thing with a carboy, minus the gas pressure; fill the carboy with water, insert a gas hose that has a slow flow of gas coming from the end, start a siphon, and displace the water with gas. You can accomplish the same thing by inserting a gas hose in an empty container and allowing a slow flow of gas into the bottom of the container. Since CO2 is heavier than air, a blanket of carbon dioxide forms in the bottom of the vessel and pushes air out of the top as more carbon dioxide is added. Regardless of the method, it is very important to change the environment of the receiving vessel.
The third thing that can cause problems when racking, a leak in the racking tube, is not always obvious. If the hose connected to the racking cane is not sealed, air can suck into the beer flowing from the line. The same problem can occur with pumps if the pump inlet hose is not properly connected, or if you are using a pump with a leaky mechanical seal (most homebrew pumps are magnetic drive and do not have a mechanical seal). If you’re using this answer like a checklist and have not come across a suggestion that seems likely to solve your problem, you may be picking up air when packaging.
The primary sources of oxygen pick-up during bottling are the same as with racking. Ideally beer is gently filled into a carbon dioxide environment, using a filling tube without air infiltration from leaks. When carbonated beer is bottled using a counter-pressure filler, the beer can be foamed or “fobbed” immediately before crowning; crowning on foam is a great way of pushing air out of the bottle headspace. If you are bottle conditioning, you probably will not have enough carbonation in the beer being bottled to generate foam (many commercial brewers that bottle condition package their beer with enough dissolved carbon dioxide to allow fobbing before crowning). Even if you are bottle conditioning, a counter-pressure filler can be used to purge air from empty bottles.
Oxidation problems are often best solved by working backwards through the process because excess air in the package is guaranteed to result in oxidized beer. If you are not sure if packaging is your problem, try doing a test where you bottle part of a batch and keg the other portion in a keg that has been filled with water and evacuated with carbon dioxide. Hopefully I have given you enough ideas to solve your shelf-life woes!