By Greig McGill
“Love”, sang glam-rockers The Sweet, “is like oxygen. You get too much, you get too high. Not enough and you’re gonna die.”
I’m pretty sure that the group known for such gems as Ballroom Blitz, Fox on the Run, and Little Willy were also keen brewers and were applying their experience with oxygen to a crafty love simile. I see what they did there. Unfortunately for us, and consistent with a lot of over laboured lyrical imagery, it’s just not the full story.
Before yeast is pitched to begin fermentation, if you give them too much oxygen, they will indeed get too high, but oxygen in this stage is simply an energy source, used to power the Krebs cycle. This is the aerobic stage of fermentation where yeast uses the oxygen derived fuel to breed and produce CO2, and adenosine triphosphate (ATP) which is used to power the anaerobic stage of fermentation where we get the good stuff – alcohol! If your yeast are “too high”, in that they have too much oxygen, they will reproduce more than usual, meaning each cell needs to work less to ferment the wort. This can mean reduced fermentation character, and degraded and lagging fermentations from later generations of this yeast, as it becomes selected to favour aerobic rather than anaerobic fermentation.
Conversely (and obviously, given the above) “not enough, and [yeast’s] gonna die”. Not enough oxygen means very little fuel for the Krebs cycle, which in turn means just the ATP already extant in the cells. That’s going to be one stressed out fermentation, taking a long time, probably not attenuating to nominal stated levels, and almost certainly producing a bunch of esters, fusels, and other by products you probably don’t want in your beer. You probably don’t want to re- use yeast from such a fermentation either.
While this is something a brewer needs to be aware of, and be sure to optimise in order to produce excellent beer, oxygen introduced before fermentation does not contribute to oxidation in the finished beer. The yeast will maintain their aerobic cycle until all available oxygen is used up. This is very likely true also for oxygen introduced during fermentation, but I am not aware of any research in this area so the jury is still out. There is plenty of anecdotal evidence that it is beneficial, and brewers such as Russian River and Stone (you’ve heard of them, right?) do this with several of their bigger beers to promote healthy fermentation even into double digit ABV numbers. The caveat is that these breweries know their systems and their yeasts intimately, have precise control over the amount of oxygen introduced, and labs to back up their sensory evaluations with good data. The takeaway? By all means, if you’re brewing a big beer, play around with oxygen during fermentation without any real worries about oxidation of the final product. Just don’t assume it will be beneficial. Experiment, document your findings, and find your own process that works for you.
One final note on oxygenation of wort that can have an impact on oxidation in the final beer; hot side aeration – the myth, the legend, the cause of religious wars amongst brewers. Whether you believe HSA has an effect on your finished beer or not (and the jury is so out on this that it’s barely visible with a telescope), the fact is that you will chemically bind oxygen to various compounds present in the wort if you aerate that wort in any way while it is at a higher temperature than 26.67 degrees Celsius. These bound compounds break down over time, releasing that oxygen back into the beer and becoming an oxygen source that will allow the beer to oxidise. I’d suggest minimising HSA where practical, but certainly don’t lose any sleep over it if you get a little. Unless you’re packaging for maximum lifetime and product stability, my advice is to be aware, but not to be worried.
So what exactly is oxidation? We’ve made it this far just saying “oxidation is bad, mmmkay?” I guess I should probably clarify what it is! Firstly, oxidation is simply a loss of electrons from a molecule during a reaction. It need not involve oxygen at all, but was named oxidation because oxygen is extremely reactive and is kinda ubiquitous here on Earth (and in most of the universe, to be fair), so was the first observed cause of such a reaction. In beer, there are many compounds that will react with oxygen, losing electrons, and thus transforming into other compounds. The most common is trans-2-nonenal. It contributes what many describe as a papery, cardboard-like, or sherried note to beer. Oxidation tends to dull hop flavour and aroma, and changes malt character in several ways – and not all are unpleasant. A good aged barley wine just wouldn’t be the same without some of the malt complexity oxidation introduces. In general though, if you want your beer to remain as close to your vision for as long as possible, you need to minimise the amount of oxygen available in your packaged beer.
Like any chemical reaction, heat will speed up oxidation, so keeping your beer cold is always a good idea. It might seem like stating the obvious, but I’ve seen many a home brewer who kept the fruits of their labour in a case in the garage. I’ve also seen many a commercial brewer send their beer to bars that store it at ambient temperature!
You will always have some oxygen in your final packaged beer. The goal is to reduce it to very low levels. Without an expensive dissolved oxygen (DO) meter, you are unlikely to know for certain if you have succeeded in keeping oxygen to the sub-100 parts per billion (or ideally, much lower) level professional brewers strive for. Instead, just consider all the ways your beer can come into contact with air.
Transfers are the most common. If you transfer your beer to another vessel from the fermenter, you should take all possible steps to minimise oxygen uptake. Professional brewers will pack hoses and pumps with de-aerated water, purge all vessels (often several times) with CO2, and ensure all fittings are 100% airtight. This can be a large pain for home brewers, but if you plan to keep your beer as fresh as possible for as long as possible, you might want to consider some version of this regimen at home. Any improvement counts for some reduction in DO, so something is better than nothing!
Bottling and kegging both count as packaging, so the advice above follows, but in addition think about the surface area of your beer. Kegs expose less beer (as a percentage of volume) to air, and also are airtight. Bottles do allow some air ingress, and this will cause oxidation over time. Consider kegging your beer and filling bottles on demand using something like the Beer Gun when you need bottled beer.
Dry hopping is another common way of introducing oxygen to a beer. Some brewers take the trade-off in some aroma loss in order to dry hop during active fermentation, and thus have the yeast “scrub” the oxygen away. Others use various methods of CO2 purging their hops in sealed containers or expensive dry hop systems that recirculate the wort through a sealed and purged chamber containing their hops. Whichever method you use for dry hopping, consider all the ways in which oxygen can be introduced and do your best to minimise them.
Finally, while oxidation begins immediately, its impact on beer is relative to the amount of oxygen introduced, and the time until the beer is consumed. Even if you take no precautions against oxidation, but you drink your beer within a few weeks of packaging it, you probably won’t notice too much degradation. Brew more beer and share it with more people so it goes faster. Everyone wins!