What happens when you age a beer ?

With Goose Island’s announcement that they will start flash pasteurizing their beloved and highly sought after Bourbon County barrel aged beers, I’ve seen many questions how this pasteurization will affect the aging potential of these potent beers.
To be able to gauge this impact, perhaps we need to understand what exactly happens when one decides to age a beer first. So here goes.

A few disclaimers. I am not a scientist. Sure I’m a beer lover, probably more educated than most, and have read up quite extensively on most aspects of beer. And yes, I have a relatively solid beer cellar and have aged several beers. Some for 10 years or more. So some of this is based on personal experience.
Second, there is unfortunately very little true scientific, read chemistry, research material out there. The best I have found is a study by Bart Vanderhaegen at the Centre for Malting and Brewing Science, Katholieke Universiteit Leuven. But even that one dates back to 2005. Another good read is “Cellarmanship” by Patrick O’Neill, but that focuses heavily on English cask ales. Yet it does provide some insight in the overall aging process.

I won’t delve into whether or not one should age a particular beer. You can download a handy-dandy little flowchart here (with credit to Andrew Schwab who originally developed it). And there’s many, many articles out there on how to properly store beer for aging. Google is your friend in that regard.

So what happens during the aging process ? We can broadly categorize the effects in two categories:

  • Biological reactions from yeast and bacteria
  • Chemical reactions

Yeast and bacteria

When a bottle contains live yeast and/or bacteria, biological processes will continue to happen. Yeast, be they Saccharomyces or Brettanomyces, will continue to convert any residual sugars. In general, yeast produces alcohol and CO2 as byproducts of this process, but these little critters are quite amazing and produce a host of other flavor and aromatic compounds such as esthers and phenols. Aging a bottle containing live yeast will ultimately result in a sweeter beer, sometimes of higher strength and with more carbonation as more of the remaining sugar is converted to alchohol and CO2. More often than not the aging process will provide more toffee-like aromas and flavors. But aging too long can lead to yeast autolysis, the point at which the yeast walls break down and yeast cells “explode”, which will result in meat-like, soy sauce or even burnt rubber flavors which are often undesirable.
Bacteria, most commonly Lactobacillus and Pediococcus, are even more tricky. They will produce chemical compounds such as lactic or acetic acids, rendering the beer more sour, and other funky flavor compounds commonly referred to as barnyard, horseblanket, etc. And yes, those can be a good thing, especially in Belgian Lambics, Flanders Red or Oud Bruin style beers.
It’s a complex and long process, often a result of an interplay of bacteria such as pediococcus, which produces diacetyl, and brettanomyces, which can clean up the diacetyl. Brewers do this intentionally. Consider the Belgian lambic brewers of the Pajottenland or Orval trappist. Adding live yeast, usually with a fresh dose of sugar, in the bottle is called “bottle conditioning” and the effects can be amazing. However, when bacteria get introduced unintentionally, we call the beer “infected” as these sour, funky or acetic flavors and aromas become undesirable in the beer. Since flash pasteurizing pretty much kills any remaining yeast or bacteria that may find their way into the bottle after fermentation, any aging effects of yeast or bacteria, whether intentional or unintentional, will be eliminated.

Chemical reactions

This is where it really becomes a terrain in which much more research is needed. We do know certain chemical compounds such as the alpha acids, delivered by hops and which give beer its charateristic bitterness, are broken down quite early in the aging process. Likewise the fruity esters our yeast so dilligently produced during fermentation, are the first ones to dissipate. On the other hand, several compounds created by the complex Maillard reactions that happen during the brewing process, seem to continue to develop and deliver burnt, caramel and even sherry-like flavors. Since darker beers have more of these compounds, the results of aging become more apparent in these beer styles. Checmically speaking, the Maillard reaction is still a bit of an enigma, but we do know it produces linear aldehydes, esters, dicarbonyl compounds, etc.. It appears that these continue to interact on a chemical level during the aging process, resulting in the desirable flavors and aromas mentioned above.
The, probably, least understood chemical reaction of these is oxidation. It is a complex chemical process that can create the dreaded trans-2-nonenal compound which is often perceived as a stale, cardboard or wet-paper like flavor. There is still a lot of unknowns with regards to which chemical precursors are responsible for this off-flavor (fatty acids, other lipids, lipoxygenases,…), but we do know that when beer is exposed to oxygen, this off-flavor will eventually develop. Depending on the beer and the oxygen levels, this process may take as little as a few weeks to as long as a few years. Only experimentation and regular tasting of your aged beer will tell you how fast the oxidation process is taking place.

So what about Goose Island beers ?

What will be the effect of the flash pasteurization of the Bourbon County beers ? It is for certain that the results of the biological aging process from yeast and bacteria will be removed. In the case of Bourbon County, who suffered from a bacterial infection in the 2015 edition of the beers, that may not be a huge impact. The beers, even though they do (or rather “did”) contain live yeast cells, are not intentionally bottle conditioned and hence the impact of continued yeast activity was always relatively small. The undesireable effects of any potential introduction of unwanted baceteria are definitely removed and hence we shouldn’t see those re-occur like they did last year.
The results of the chemical aging processes described above should not be impacted by the flash pasteurization. Even though complex and slow in a high alcohol environment, they should continue to occur as before. So if you are so inclined, continue to buy them, stash them and age them properly. Only time will tell what the eventual results will be.

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