Oxidation can happen to any beer that comes into contact with oxygen after fermentation. With the huge popularity of hop-forward beers in recent years, this has come much more into focus. Oxidation can also be caused by light (UV rays), but in this article we focus on oxygen as the main source of the problem.
What is oxidation?
At the start of fermentation you want oxygen in the wort to get the yeast off to a good start. Once the yeast has got going and used up the oxygen in the wort, we don't want oxygen in the beer, as we can end up with an oxidised beer. Oxidation is a reaction between the oxygen in the air and the fully fermented beer. The colour turns browner and the flavours are considerably diminished, especially in hop-forward beer. Instead, flavours like wet cardboard/paper and harsh hops come through. In dark beer styles, flavours more reminiscent of raisins and sherry can appear; to some extent these are desirable, especially in strong examples, and they mainly result from long storage.
How does beer get oxidised?
Beer mainly gets oxidised through splashing and vigorous stirring after fermentation. There is no need for paranoia here, but it is always wise to take precautions. Think, for example, about how you move the finished beer from the fermenter to the keg or bottles. Do you pour it, or do you transfer it gently with a tube at the bottom of the bottle/keg? Do you use a siphon and splash around in the fermenter before transferring? Do you stir vigorously to mix priming sugar solution into the beer for natural carbonation? Is there a point in the process that can draw in air during transfer? All of this, and more, can play a part. If you struggle with oxidised beer, this process is usually what you should take a closer look at.
How can I avoid oxidation?
Heavily hopped beers such as NEIPA and similar are particularly prone to oxidation. These also benefit greatly from being kegged rather than bottled, as it is very difficult to bottle-condition these styles without exposing them to a lot of oxygen. If you still want to try bottle-conditioning hop-forward beers, they should be drunk within a fairly short time. Keep the bottles cold, and fill them as full as you can without too much air in the neck of the bottle.
If you go for kegging, which is the best solution, it is well worth purging the keg thoroughly with CO2 before transferring the beer from the fermenter. You do this by filling the empty keg with CO2 and releasing the pressure two or three times with the pressure relief valve. The oxygen then leaves the keg, and the CO2 settles like a blanket at the bottom.
Also be aware of the risk of oxygen during cold crashing (chilling the beer after fermentation). When the temperature drops, the air in the fermenter contracts. If a regular airlock is fitted, it will draw in air and, in the worst case, oxidise the beer. There are several solutions to this, for example the practical CO2 Harvester Kit. We would also point out that oxygen does not dissolve easily in liquid, so the greatest risk of oxidation arises when oxygen can be «forced» into the beer through splashing in the fermenter or during transfer from fermenter to bottle or keg.
Closed transfer
Now we are on to those who are really serious about reducing oxygen in the keg: closed transfer. This can be done in several ways, depending on the fermentation equipment used.
If you use a pressure fermenter, for example a unitank from Brewtools or SS Brewtech, or a PET pressure fermenter such as the Apollo Snub Nose from Keg King, this is simple. Just attach a transfer line to the outlet of the fermenter and the other end to the Cornelius keg. Connect a spunding valve to the keg's CO2 post, set it to a slightly lower pressure than the fermenter, and open the tap. This makes the transfer completely closed, and the beer won't be exposed to air until it is poured into your glass.
Closed transfer with a regular fermenting bucket
If you use a standard non-pressurised fermenter, you will need a little more equipment to achieve this. Again, there are several ways to do it, but let's take a standard plastic fermenting bucket with a tap as an example. There are several solutions, but the simplest may be to fit a weldless bulkhead with a ball lock CO2 post in the lid of the fermenting bucket. Alternatively, you can put together a solution that connects through the rubber bung for the airlock. There are many options, but the most important thing is that it is airtight.
Purge the Cornelius keg thoroughly a few times (as described above), and release the pressure to evacuate the oxygen. Attach a tube to the tap, and the other end to the liquid out post of the keg (with a black ball lock). On the keg's gas post, connect another ball lock with a tube running to the bulkhead in the fermenter lid. Then just open the tap and let the beer transfer by gravity. Since we have connected the beer to the keg's liquid out post, it will flow through the dip tube right to the bottom of the keg, which is free of oxygen, with as little splashing as possible. And no pressure will build up, since the keg is connected by tube to the top of the fermenter. Only CO2 will move from the keg over to the fermenter as the keg fills with beer.
This image simply illustrates how it is done. A cheap solution for closed transfer that works!
We want to stress once again that there is no need to be completely paranoid about this, but it pays to think a little about how you handle the beer after fermentation. It is well worth using a couple of tubes and a little extra time on the transfer rather than ruining a beer with half a kilo of precious hops.
