Although Fermentation is an anaerobic process, (i.e. takes place in the absence of oxygen or air) most Brewers know it is necessary to introduce air into the wort at the start of a fermentation for it to proceed satisfactorily, This is because the Yeast requires oxygen to produce unsaturated fatty acids and sterols for cell wall formation which it can synthesise from brewers wort in the presence of oxygen. If sufficient oxygen is not present the yeast cells will he unable to reproduce (bud) and there will be insufficient yeast cell numbers to complete the fermentation, resulting in high final gravities, and possibly unwanted incomplete fermentation flavours such as VDK (which has a flavour like butter scotch) and aldehydes (which have a flavour of bruised apple skins).

To avoid this problem it is necessary to introduce oxygen (air) into the wort before adding the yeast or during the early stages of the fermentation (during the first 24 hours). The level of oxygen required depends on the wort gravity (the higher the original gravity the greater the demand far oxygen) and is dependent on the yeast strain. Oxidisation levels between 6 and 16 % mg/l are usually required up to highest gravity worts. Using air (which contains 1/5 oxygen) the normal maximum saturation level is 8 mg/l; where higher dissolved oxygen levels are required pure oxygen is used. Precise control of oxygen is required since excessive oxygen can produce excessive yeast growth and may even poison yeast. The level of wort oxidation influences ester production and hence the flavour of the finished beer.

Oxygen reacts with wort and beer components to give oxidised and stale flavours and so, apart from its role in stimulating yeast growth, oxygen should be kept out of wort production and the post fermentation stages of the beer. It is therefore important to confine the addition to the active stage of yeast growth.

Historically there has been a debate as to whether to aerate wort when it is hot or cold. The overwhelming majority of Commercial Brewers all aerate the cold wort, based on sound quality considerations. The relative merits are summarised below:

Hot Wort Aeration:
Advantages:	Disadvantages
Hot wort is more sterile, hence less prone to air borne infection	Oxygen is far less soluble in hot wort and hence less is available to the yeast
Where a heat exchanger is used, this acts as a good system to ensure all the oxygen is fully dissolved.	Oxygen reacts very fast with hot wort (half life circa 8 minutes at 70oC) hence less is available for yeast.
	Hot wort oxidation can lead to unpleasant stale and oxidised flavours in the beer.
	Hot wort oxidation leads to colour formation due to melanoid reactions.

 

Cold Wort Aeration:
Advantages	Disadvantages
Oxygen is more soluble in cold wort and hence more is available to the yeast.(8 ppm Oxygen from air at 20oC)	Cold wort is more prone to infection and hence better micro control (air filtration) is required.
Oxygen reacts slowly in cold wort (half life circa 200 hours at 20oC).	Must provide adequate mixing to ensure the oxygen is dissolved.
Negligible oxidation in the presence of active yeast.
Negligible colour pick up.

There are a variety of ways of introducing air/oxygen in the cold wort, and in practical home brewing all these systems can be used. Clean air (preferably passed through an air filter) can be bubbled into the cold wort before or immediately following pitching using a small pump such as a fish tank pump. For high gravity beers (Original Gravities of 1060 and above) several aerations are recommended within the first 24 hours after pitching to give vigorous yeast growth (it is unlikely that any yeast can be poisoned by air alone, so there is little risk from over aerating).

If no pump is available use a clean spoon to agitate the cold wort and continue the mixing when the yeast is pitched. It is useful to give the brew a good stir or shake during the first 24 hours to dissolve further air in order to keep the fermentation going. It is also equally effective to aerate the yeast before pitching and this can be done by shaking, stirring or even by air pump. If the fermentation starts to slacken within the first 24 to 36 hours then further aeration may help to stimulate the yeast. After 36 hours aeration is generally of no value as the yeast has passed through its growth phase and lost its requirement for oxygen.

Although aeration is a very important part of the brewing process it must be remembered that there are a number of other causes for defective fermentation, such as dead yeast, insufficient nutrients, incorrect pH residual cleaning fluids etc., which can cause the fermentation to fail and no amount of air will overcome!