Article - Wort Aeration

By Sam Slaney

Wort aeration/oxygenation
An important part of a healthy fermentation is the introduction of oxygen to the wort as soon as the yeast is pitched. This goes against the general rule in brewing of avoiding oxygen intake in the brewing, fermenting and conditioning processes, and also in the packaged beer (leading to staling of the beer).

Why introduce oxygen?
There are many advantages to aerating/oxygenating the wort:

• Higher attenuation
• Healthier fermentation cycle
• Less undesirable fermentation by-products (eg. diacetyl and esters)
• Reduction of lag times

These are brought about by an increased and healthier yeast population. 

How does oxygen improve yeast health and population?
Yeast undergoes a short aerobic (requiring oxygen) stage before commencing the anaerobic (no oxygen) stage in which sugars are converted to ethanol and carbon dioxide.

In the aerobic stage, oxygen dissolved in the wort is consumed by the yeast to aid yeast growth and the formation of ergosterol used in the fermentation cycle.  If there is not enough oxygen dissolved in the wort, yeast growth and energy reserves will suffer, resulting in a weaker fermentation.

What beers benefit most?
It is especially important in lagers to reduce ester formation and improve fermentation cycles resulting in a cleaner tasting, drier beer.  It is also important for high gravity beers, such as bocks, barley wines and strong Belgian-style ales, to achieve fermentation with complete attenuation.

It is not as important for normal gravity ales where some esters are appropriate, and also ester driven beers such as German weizens.

Recommended oxygen levels for most worts range between 8-15ppm dissolved oxygen (DO).  For ester driven brews, such as weizen, DO levels of <5ppm are appropriate.  However, without a DO meter, these numbers are useless.

The boiling process drives all the dissolved oxygen from wort, so it is necessary to aerate/oxygenate.

How to aerate/oxygenate wort
Aeration can be achieved in a few ways. With all methods it is important that aeration/oxygenation is performed before the onset of fermentation (< 6 hours).  It is also important that aeration/oxygenation is performed on chilled wort (below 25oC) to prevent oxidation. Oxygen is less soluble in wort as the gravity increases, so for stronger beers double the time you stir or pump into the wort.

Splashing, stirring and shaking
A common way that many brewers use is simply by splashing chilled wort from the boiler into the fermenter and/or stirring the wort in a way to introduce air. This is a simple method and doesn’t require any additional equipment, except a sanitised spoon and some upper body strength. This works well and is used by many prize-winning brewers, however the level of DO is usually well below 5ppm using these techniques.

Aerating using an aquarium pump
Pumping filtered air into the wort is a good way to aerate the wort, and a simple aquarium pump can be used, in conjunction with a 2mm sintered stone (a very high porosity outlet). To avoid pumping airborne bacteria and mould in the air into the wort, a sterile syringe filter is used in-line. This simple set-up allows easy aeration of the wort, and usually takes between 10 and 15 minutes of pumping to achieve the desired level of oxygen dissolved in the wort. One problem is this extended period of pumping causes a great deal of foaming, which commonly spills out of the fermenter. Reducing the pumping rate or an additive such as No-Foam stops this from occurring. Using air a maximum of 8ppm oxygen can be achieved. To gain higher levels of DO, pure oxygen needs to be used.

A simple aeration kit using an aquarium pump, syringe filter and a 2µm diffusion stone.

Oxygenating using pure oxygen
The best way to get oxygen into your wort is by using pure oxygen. This speeds up the process and makes oxygen levels more predictable by the use of a flow meter on the regulator. Due to the higher pressure, a smaller aeration stone can be used (0.5mm). Oxygenation times of about 1-2 minutes give the desired levels of oxygen in the wort. Obtaining an oxygen cylinder from a gas dealer can be relatively expensive though.

An oxygenation kit using an oxygen bottle, regulator, syringe filter and a 0.5µ diffusion stone.

Conclusion
Aeration or oxygenating your wort to increase DO levels is an easy way to improve the quality of your beer. All of the equipment needed for aeration and oxygenation is supplied by Grain and Grape. We also have a DO meter for testing our own beer, so if you come and hassle us with a fresh sample of aerated wort and one of your finest brews as payment we’ll test it for you.