Hot Side Aeration

David Wilton gave a presentation on Hot Side Aeration a little while back, here’s a copy of the information he presented.

This topic has also been discussed (with Davids input also) on Jim’s Beer Kit:


It is useful to note the distinction between hot side and cold side aeration. Hot side aeration is the term referring to oxygenation includes events from mashing to wort chilling. When the wort is cool oxygen is added to the wort before the yeast is pitched. At this point oxygen acts as a valuable yeast nutrient. Once the fermentation has started oxygen returns to being negative, at this point referred to as cold side aeration (Fix, 1999). Fix (1999) belives that HSA is the primary cause of staling in beer due to this careless aeration of the wort whilst it is still hot.

Temperature & Time are Key

As John Palmer so wisely puts it “Aeration is Good, Oxygenation is Bad” (Palmer, 2006)! The distinction between aeration and oxygenation is related to both the point at which it is and the temperature. The temperature which aeration is distinct from oxygenation is 27C (80F) (Palmer, 2006). HSA is fundamentally different from CSA because of the speed of redox reactions at elevated temperatures. For example introducing 1ml/L of oxygen at 70C will start reactions that consume the free oxygen in seconds (Fix, 1999). Doing the same at 20C will have virtually no affect, as Dr Bamforth puts it “if oxygen were really that reactive we would have all gone stale by now!” Free oxygen at 20C will remain inert (un-reactive) for days and in practical brewing situations will be removed by yeasts / absorbed by melanoidins. However, having said that the most crucial stage in cold side aeration is at bottling. As little as 1 mg/l of air in a 300 ml bottle is sufficient to neutralise all the melanoidins ability to absorb and fix oxygen in a light larger (Alexander, 2009). Boiling wort is too hot to absorb oxygen.

Chemical Pathways


Trans-2-Noneal is one of the must discussed and studied of staling compounds. It is a class of long chain unsaturated aldehydes with very powerful paper and cardboard like flavours. Short chain aldehydes are precursors to T-2-N, and they along with any T-2-N present in the beer are called the T-2-N potential. The short chin precursors are somewhat neutral in flavour, but are readily transformed into longer chins by aldol condensation.

It was once thought that just about all oxidative defects (including the presence of T-2-N) were attributed to headspace in the beer. However, it was shown that the T-2-N potential was set by the wort production. The importance of Hot Side Aeration on this potential was first identified by Dr George Fix in Zymurgy, 1992.

More recently studies have shown that eliminating hot side aeration is not the entire answer because there are other pathways leading to T-2-N. It was still found to be present in European beers produced with very modern facilities which have been optimised to both hot and cold side aeration. As Dr. Charles Bamforth (1999) puts it what does the brewer always do when things aren’t going right? Blame the Malster! It has been shown that enzymes present in the barley (lipase and lipoxygenase) are key to the enzymatic pathway leading to T-2-N. Both pathways require oxygen, however, the oxygen required for the enzymatic pathway is so small that it is virtually impossible to avoid in the brew house.

Lipase and lipoxygenase are thermally unstable and thus 96% of these enzymes are lost in kilning. Unfortunately, because of the very low flavour threshold of T-2-N not much is needed before flavours appear. 

Myth or Reality?

There is no lack of scientific evidence that oxygenation of hot wort can have deleterious consequences for the stability of bottled beer (Alexander, 2009). However, there are no shortage of people who do not believe it is an issue.


Hot-side aeration can be demonstrated in medium and large commercial breweries because the brewing equipment is so big that splashing is a really dramatic event. Think of liquid flowing through a six-inch pipe at 400 gallons per minute and cascading 12 feet through the air before hitting the bottom of a tank. This is not roughly stirring a five-gallon mash with a wooden spoon (BYO)

However, it is argued that scale works in the other direction. The problem for home brewers is that the degree of oxidation is related to surface to volume ratios and so our small brews are more susceptible (Alexander, 2009).


Wheeler points out that traditional highly regarded breweries such as gales, Harveys, Hook Norton Jennings etc. took no precautions whatsoever to eliminate air from hot wort throughout the brewing process (Garrod, 2008). Whilst Wheeler does not believe in HAS because of these practices, he does point out that many proponents of HSA now point towards the early mash as being the greatest source.

Preventing in the home brew environment


Achieving reasonable wort clarity in lautering is important (Fix, 1999).

The grain in the mash should be underlet or infused with hot water from the bottom up. By infusing in this manner, stirring of the grains to insure uniform mixing of the grain and hot water is not necessary. By not stirring the water into the mash, hot oxygen reactions can be reduced. (Millspaw, 1992)

The melanoidins formed at 77°C/170°F are more stable than those formed at the lower temperatures of conventional mashing. By adding these specialty malts only in the mash out, the brewer can make his mash more efficient by optimizing saccarification, maximizing the formation of melanoidins. This will lead to smoother and rounder flavours from the specialty grains, as well as more stable and clearer beers (Millspaw, 1992).

Transferring to the boiler

In practice if the power is switched on immediately the heating element is covered any uptake of oxygen is immediately driven off by the action of the boil (Alexander, 2009).

Post Boil

Older books used to advocate pouring the boiling hot wort from one bucket to another to add oxygen and cool it. Unfortunately, the wort is still hot enough to oxidise and not aerate. Pouring down the side of the bucket to minimise splashing doesn’t help either since this increases the surface area of the wort exposed to the air (Palmer, 2006). Ensure the wort is below 27C before aerating air (Palmer, 2006)


In fresh beers both T-2-N and its precursors are bound up with natural sulphur compounds from yeast metabolism. However, after a lag, which is reduced if thermal or mechanical abuse occurs, the effects of T-2-N become discernible.


Additives could be one solution, however, the effective additives tend to have unacceptable side effects and the neutral ones rarely seem to work (Fix, 1999). The most likely addative to be of any use in the home brew environment is Potassium Metabisulphate (Campden Tablets) added to the mash. These haven’t gained favour in the commercial world because above certain (?) levels they must be stated on packaging and we “can let the wineos have that” (Bamforth, 1999). It may therefore be an option to us, the bisulphates will bind to the staling aldeydes and masks their presence. However, these bonds are rather short lived in beer, and when they are broken, the staling aldehydes fully reveal their presence (Fix, 1999)

The effects of oxygen pick-up after fermentation are more apparent and severe than the effects of hot-side aeration. If you are thinking about changing your brewing procedure to avoid oxidation, you should begin addressing oxygen pick-up from the end of the process and work your way forward toward mashing (BYO)


Alexander, J (2002). Hot Side Aeration (HSA). Brewers Contact. 9, 2-5
Also published at


Bamforth, C. (1999). Hot Side Aeration. The Brewing Network

Fix, G. (1999). Principles of Brewing Science, Second Edition.  Brewers Publication

Garrod, P. (2008). Hot Side Aeration. Brewers Contact. 8, 4-5
Also published at

Millspaw (1992). Hot Side Aeration and Beer Stability. Zymurgy. 15.
Also published at


Aldehydes – A chemical Precursor to Alcohol. In some cases alcohol can be oxidised into aldehydes creating off flavours

Trans-2-Noneal (T-2-N) – A class of long chain unsaturated aldehydes with very powerful paper and cardboard like flavours.

Enzymes – protein based catalysts that effect specific biochemical reactions

Redox – Describes all chemical reactions in which atoms have their oxidation state

Polyphenols – A polymer of phenols that contribute to haze and staling reactions

Melanoidins – Strong flavour compounds produced by browning (Millard) reactions.