Hot Side Aeration is one of those topics that divides homebrewers. Some believe others don't.
Personally I avoid aeration at any stage the wort is above 40C.
The following is from
Brewing Techniques and is pretty old but gives a reasonable explanation.
BIAB brewers may argue over the final paragraph though.
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HOT-SIDE AERATION
Q: Oxygen is obviously an oxidant, and we all know of its detrimental effects in beer (postfermentation). I am also aware of yeast metabolism and its need for adequate oxygen during fermentation. I am under the understanding that aeration during sparging and transfer from mash to kettle is where the danger exists. Apparently, introducing oxygen at this stage can create oxidation precursors leading to early oxidation of the final product, sort of like a "beer free radical."
My question is, What exactly is hot-side aeration? At what stage of brewing do I need to be concerned with it? How do I avoid it? And what effect does it have on the final product?
DM: First, to correct one statement: Yeast needs oxygen for growth, not fermentation. This is important because I don't want anybody to think they should aerate their beer during fermentation. That would ruin the flavor. You need to saturate the wort with air and pitch your yeast as soon as possible after you have chilled it. But once fermentation begins, no more air should be introduced.
The phrase "hot-side aeration" refers to the hot side of the process, which is wort production. Your understanding is basically correct. Wort contains melanoidins and tannins that are readily oxidized at high temperatures. If air is introduced during wort production--in other words, on the hot side --these substances will be oxidized and later, in the finished beer, they can turn around and give up their oxygen to alcohols that were created during fermentation. An oxidized alcohol is an aldehyde, and aldehydes are the bad guys that are responsible for all the stale, old-beer flavors we have all encountered in far too many imported brews.
That answers your first question: Hot-side aeration is any incorporation of air into hot wort, and you need to be concerned with it at all stages of wort production.
How to avoid it: Don't let your hot wort splash during transfers or fan out as it flows down the side of a vessel. Use hoses or tubing to fill kettles gently from the bottom. Be as gentle as possible when recirculating your wort to clarify it before running it into the kettle. Cool your hot wort in the kettle or else siphon or run it out through a hose or pipe to the heat exchanger. Basically, look at your whole brewing operation critically and decide what changes need to be made to minimize hot-side aeration.
What effect does it have on the final product? The first thing I usually notice when a beer oxidizes is that the hop aroma is gone. Then comes a stage where the flavor seems not quite right, but without any distinct off-flavor. Finally come the stale, cardboard, sherry-like or toffee-like flavors.
The insidious thing about hot-side aeration is that it can cause staling even in a beer that has been carefully handled during fermentation and subsequent stages. A freshly bottled beer may have a dissolved oxygen content of almost nothing and yet oxidize in a matter of weeks if the hot wort was mishandled.
Now for the good news: Brewpubs and home brewers don't have nearly as much to be concerned about as shipping breweries because they keep their beer close to home rather than sending them out into the cruel world where they can suffer all kinds of insults. Storage temperature has a lot to do with how fast beer oxidizes. So does agitation. There really is something to the old saying that beer doesn't travel well. Getting knocked around in the back of a truck does take a toll on packaged beer, and temperature is an even bigger factor. But even at low temperatures, kept absolutely still, beer will oxidize eventually. That is why the best plan is to store it cold and drink it quick.
Another thing you may want to keep in mind is that filtered beer is much more susceptible to oxidation from any source than bottle- or cask-conditioned beers, because live yeast acts as an oxygen scavenger. I don't know whether you filter your beer or not, but if you do, you have an extra reason to be careful with hot-side aeration.
One other factor is a concern, especially if you find that some hot-side aeration is unavoidable in your brewing process. The factor is wort clarity. Be sure to recirculate your wort in the lauter tun to get it as clear as possible before running it into the kettle. Cloudy first runnings have a large proportion of long-chain fatty compounds (lipids) that are precursors of many staling substances. If you can minimize the precursors, you can greatly slow the appearance of oxidized flavors in your finished beer."
Cheers
Nige
