Achieving the proper level of dissolved oxygen in wort is one of the most important factors in a successful fermentation. Most homebrewers understand this in a general way, but whereas many of us are willing to go to some trouble to control fermentation temperatures or use proper pitching rates, few of us seem willing to make the same effort to ensure that our yeast have enough oxygen to perform at their very best. This is to some extent understandable. After all, traditional aeration methods such as shaking the fermentation vessel or spraying wort can be quite effective and usually result in decent beer. But there are risks associated with these methods, especially in bigger beers.
– stuck fermentations
– under-attenuated beers
– long fermentation times
– yeast stress (which can affect the viability of yeast re-pitched into a new beer)
– and off flavours (a common outcome in homebrewed beers)
So how can we minimise these risks? The answer, short and simple, is to use pure oxygen, the most efficient and reliable method of oxygenation open to the adventurous home brewer.
The article below approaches this topic from two angles. The first section explains how to determine the ideal level of dissolved oxygen to help minimise the risks listed above. The second section explains how to put together an oxygenation system to dissolve pure oxygen into your wort in an accurate and predictable manner.
Section one – determining your oxygen requirements
To determine the ideal level of dissolved oxygen in wort you need to work backwards as follows:
- Start by determining the strength of your wort
- Use this figure to calculate how much yeast you need to pitch
- Use the yeast pitch rate to calculate how much oxygen is required to ensure a healthy fermentation
In practice, the higher the yeast pitch rate, the more oxygen you will need to dissolve into solution. (There are exceptions to this rule – for example, Belgian Trappist brewers sometimes deliberately underpitch to encourage increased ester formation for a more complex flavour profile – but we can ignore these cases for present purposes).
With this rule of thumb in place, let’s look at a concrete example to help get our bearings. Suppose we want to brew a medium strength ale with a starting gravity of, say, 1.048. According to Chris White, the owner of White Labs, one of the world’s leading yeast supply companies, we would need to pitch roughly 180 billion cells for a standard 20 litre batch (White, 2010, p. 122). The screen shot below illustrates how you might calculate this figure yourself using the Mr Malty Yeast Pitch Calculator (this calculator was created by Jamil Zainasheff, the co-author, alongside Chris White, of the book Yeast, from which I am taking most of the raw data for this article).
Ok, so now that we know that a 20 litre batch of medium strength ale requires roughly 180 billion cells, how do we use this information to work out the ideal level of dissolved oxygen? Luckily for us, we don’t have to! White Labs have already done all the work for us, and according to their research, the minimum recommended level of dissolved oxygen for a medium strength ale is 8-10 parts per million (White, 2010, p. 13). To put this in perspective, it is interesting to consider the following table from the Wyeast Laboratories website, which records the effectiveness of various methods of aerating or oxygenating wort:
|Siphon Spray||4 ppm||0 sec.|
|Splashing & Shaking||8 ppm||40 sec.|
|Aquarium Pump w/ stone||8 ppm||5 min|
|Pure Oxygen w/ stone||0-26ppm||60 sec (12ppm)|
You can see here that the most that can be dissolved into solution by using aeration (as opposed to oxygenation) is 8ppm. And this will only be achieved if you splash and shake a vessel very vigorously and using plenty of head space, or spend at least 5 minutes pumping air using an aquarium pump and stone.
But even if you take full advantage of these aeration methods, you will still only achieve a level of dissolved oxygen of 8ppm, the minimum required for a medium strength ale. To be on the safe side, we really should aim to dissolve at least 10ppm. In fact, certain yeast strains (for example, some lager strains) might require levels levels as high as 12-14ppm. And stronger worts (say above 1.077) might require even higher levels.
At this point, it might be helpful to lay out some simple guidelines:
- Medium strength worts (e.g. 1.035 – 1.060): 8-12ppm
- Lager yeast strains: 10-14ppm
- Strong worts (1.060 and above): 12ppm –
As we discussed earlier, the ideal level of dissolved oxygen in each case is determined by the yeast pitch rate, which is a function, in turn, of the strength of the wort, or in the case of lager yeast strains, the need to pitch roughly double the cell count to compensate for the colder fermentation temperatures. In all of these cases the basic rule of thumb applies: the more yeast you pitch, the more oxygen you will need to ensure a healthy fermentation.
Warning: just as there are risks associated with under-oxygenating wort, there are also risks in over-oxygenating wort. Above 14ppm oxygen can start to stress yeast or even become toxic, so be careful not to push it too far!
Section two – how to piece together and use an oxygenation system
A basic oxygenation system can be created using the following components:
- Can be sourced from Wholesale Welding Supplies
- €19.50 group buy price
- Pressurised to 110 bar
- Can be sourced from Wholesale Welding Supplies
- €23.00 group buy price
- Will only fit oxy turbo bottles
- The outlet barb is 4mm in diameter
3. Aeration Stone
- Widely available (e.g. from the Hop & Grape)
- Approximate cost is €15 but if you look you should be able to get a better price
- 0.5 micron works best but a 2 micron stone will work fine too
- Easy to use if attached to an aeration wand or keg dip tube that can extend to the bottom of the vessel (e.g. Williams Brewing system)
- The barbs on air stones are generally 6mm
- 5mm internal diameter tubing is the best compromise. It can be tightened on to the regulator barb with a jubilee clip and softened in hot water to fit over the barb on the air stone
- I got mine in B&Q. It doesn’t need to be rated for high pressures
- I can’t remember how much I paid but it wasn’t more than €3
- You need a flow meter that can read flow rates between 0.5 – 2 litres per minute (I’ll explain why in a moment)
- The flow meter goes inline between the regulator and the aeration stone
- I paid just under €12 for mine (shipping was free from China)
- For the cheapest option, use the following search:
- Go to ebay.com (not ebay.ie)
- Search for “oxygen flow meter”
- Filter the results by selecting “price + shipping: lowest first”
I’m rounding everything up for caution’s sake:
- Oxy turbo bottle – €22.00
- Regulator – €25.00
- Air Stone – €15.00
- Tubing – €3.00
- Flow Meter – €15.00
Total approximate cost (not including P&P): €80.00
This will cover you for roughly 110 brews (possibly more – see below).
How to Oxygenate Wort using Pure Oxygen
According to Chris White, (2010, p. 79), the ideal injection rate for pure oxygen in a 1.077 wort is 1 litre per minute. This is the most efficient way to achieve the required 10-14ppm. For a weaker wort, say 1.048, and using the same flow rate of 1 litre per minute, you would only need about 45 seconds to achieve the same oxygen levels. If you decide to inject the oxygen at 0.5 litres per minute, you would need to double the injection time (i.e. 2 minutes for a 1.077 wort and roughly 90 seconds for a 1.048 wort). You get the idea…
These calculations assume that you have a way of controlling the flow rate. For this it is advisable to use a flow meter. If you do not, you will have no easy way to tell exactly how much oxygen is making its way through the regulator, which operates via a valve with no built in gauge.
Once you have cooled your wort to pitching temperature there are two main methods of oxygenation:
1. The simple injection method
- Boil your air stone for a few minutes and sanitise the attached wand (if using)
- Work out exactly how long you need to inject the oxygen and have a timer on stand-by
- Open the regulator to release the oxygen (do this just before submerging the air stone to ensure it doesn’t flood with wort)
- Force the air stone to the bottom of your fermentation vessel
- Keep a close eye on a timer to ensure you don’t over-oxygenate the wort
- If lots of bubbles break on the wort surface turn down the flow rate. These bubbles should burst inside the liquid.
- Close the valve on the regulator
- Rinse and boil the air stone to keep it clean and sanitary
2. The combined shake and inject method
- If your are stingy you might want to aerate the wort prior to topping up from you precious oxygen bottle to save oxygen
- Shake the fermentation vessel vigorously to achieve roughly 7-8ppm
- Complete the first 3 steps in the simple injection method
- Top up the wort with a quick burst of oxygen (i.e. reduce the injection time by about 3 quarters)
- Follow the final three steps of the simple injection method
Please be very careful when using pure oxygen. It is highly flammable and if ignited will burn furiously. It should go without saying, but please don’t use it anywhere near a naked flame (or any other kind of flame for that matter)
This guide grew from a thread on the NHC forum and is really a group effort. Special thanks to Hop Bomb, Ciderhead and Will_D for helping to move everything forward. We originally thought oxygen would be too expensive and too difficult to source to make it worthwhile, but Ciderhead discovered that Wholesale Welding Ltd sell reasonably priced oxygen bottles and Will_D explained how these smaller bottles (which most of us had written off as not containing enough oxygen) actually had enough to oxygenate roughly 110 brews. Hop Bomb went on to organise a very successful group buy.
If you have any questions about any of the information contained in this article, feel free to join the discussion on the original thread.
White, C. & Zainasheff, J. (2010) Yeast: the practical guide to beer fermentation. Brewers Publications, Boulder.