Flavour Changes Over Time

In my years as a BJCP judge, one of the things that was consistently noted, was that for the vast majority of beers, the fresher it was, the higher it scored. Back when Canada had a Brewer of the Year circuit that allowed brewers to enter every single competition, I knew more than one brewer who would spend the few weeks before competition brewing a dozen or more beers just to ensure that their beer was fresh. 

An argument can be made that it was very difficult for new brewers to break into the competition circuit because of the constant brewing required, not to mention the expense. It's entirely possible for one brewer to be just as good as another, but they don't have the time or money required to brew fresh for every competition, especially since there were 12 a year - but that's a moot point now. 

What is important, especially for brewers who aren't planning on consuming an entire keg within a month, is understanding that freshness matters because flavour changes. The largest culprit is, of course, oxygen, which is a fantastically reactive element and why almost every single food preservation method seeks to exclude it. It isn't the only culprit, though, so keep reading until the end if you've done every oxygen reduction method you can think of and still find your beer staling quicker than you'd like. 

Packaging

Repeated studies have shown that post fermentation is where most of the damage from oxygen can occur and for most home brewers, that means moving from the fermenter to the package. (I'm assuming no-one has a home centrifuge yet). At the commercial side, they'll be flushing lines with deaerated water and purging with CO2, which is perfectly within the capabilities of most home brewers as well. 

Where things get sticky is the actual choice of packaging. Let's go from worst to best:

  • PET bottles - these are a popular choice for shipping because they don't break, but for flavour stability, they're awful. The bottles themselves are permeable to oxygen (although they used to be even worse) and they come with caps without oxygen scavenging linings. 
  • Glass bottles - the most popular choice. They're cheap, plentiful and recyclable. They withstand pressure well and the bottles are less permeable to light. They're heavy to ship though, and the caps still permit oxygen to enter. You can mitigate that somewhat by switching to oxygen scavenging caps, but that is a temporary solution as SO2 will release oxygen over time. 
  • Cans - the best choice for small packaging. It's impermeable to light and gasses, it's light, recyclable and durable. The downside is that the equipment is expensive and it takes a bit of practice to reduce oxygen to sub-0.5 ppb levels that are needed for minimal change. Modern canning lines will fill the can with foam first and then fill the can from below, which appears to do the trick. 
  • Kegs - More difficult to fill than cans. Filling with foam and pushing it out won't work with kegs because they're too large and the foam will collapse. Filling with deaerated water and pushing out with CO2 is probably your best bet, but it does require a lot of wasted gas. Sankes are much better than cornies because soda kegs are designed for soda pressures. Most beer has less pressure than that and it causes seals to leak slowly. The also suffer the same issues as bottles with oxygen ingress and there are no oxygen scavenging keg seals. 
  • Casks - pretty damn good because of live yeast. The only issue is live yeast becomes dead yeast over time and then there are issues with those sorts of flavours. The other issue is that the cask must be filled almost to the brim because secondary fermentation will only use about 1/3rd of the oxygen in the headspace. If you're bottle conditioning, the same applies, so you have to balance the reduction of oxygen over the possibility of a slight gushing once opened. 

The Hot Side

While people were vigorously debating whether hot side aeration was a thing, LODO (low dissolved oxygen) folks were vigorously ridding themselves of every trace of oxygen they could find. There are a couple of places where this shows up.

  • Enzymatic oxidation - LOX enzymes in particular
  • Non enzymatic oxidation - this shows up with divalent cations (iron, copper especially) which catalyse oxygen reactions. 

And so they took it upon themselves to reduce oxygen at every single step of the process, including adding tannins via gallotannic acid or mash hopping in the hope that these would precipitate out metal ions. Unfortunately it doesn't appear those last two methods actually do much in the way of iron or copper adsorption, and it has been found that manganese, naturally present in barley is at least as responsible for non-enzymatic oxidative reactions as copper or iron

That said, there are no studies currently that suggest hot side aeration has any noticeable effect on beer in any way, and that's likely because the oxidation that is happening is on lipids which, if the boil goes according to plan, shouldn't make it into the beer in the first place. At the homebrew and small brewery level, however, it's undeniable that trub separation is less than ideal, and so LODO brewing might not be a bad idea, but it's more of a mitigation strategy at that point. You may do better to focus your efforts on the quality of your boil and trub separation, rather than on attempting to eliminate oxygen from your mash as this appears to be inevitable with today's technology. 

Heat

The last place that changes come from is from heat. Heat creates unstable aldehydes that degrade over time, though this doesn't appear to be an issue in malting, but rather boiling. You may have been told at some point to ensure your boil is "robust" but that's not an invitation to crank the heat as much as possible. The key is the temperature differential between the heating source (element, direct fire or jacket) and the liquid inside. Circulation also helps improve the chances that fewer aldehydes will form, at least until the boil starts in earnest as there is a much more even heat. Here, homebrewers have an advantage over commercial brewers as they have a smaller volume of liquid to heat and circulation is much easier and more effective. Most of us never get to the scorching stage, but nonetheless, the slower the rise, the better.

These aldehydes are responsible for a lot of the "weird" or unnamed flavours in beer where you know something is off, but you can't put your finger on it. It just doesn't taste the same as it should. These aldehydes will degrade. There is nothing you can do about it, even if you eliminate every scrap of oxygen. Your goal is simply to make sure there are fewer of them. 

In short, beer is an organic product and it will degrade over time. There is much you can do to ensure that happens at a slower pace though, even at the home brew level.