Should I oxygenate my wort?

This is a big question. We will try to cover it briefly here. There is a huge body of literature written on this subject both for homebrewing and professional brewing. Why add oxygen at all? Yeast has two metabolic pathways. The one it prefers is to eat sugar and O2 to make CO2 and water. This is a very high energy pathway and it is much preferred by the yeast. It will continue on this path until all O2 or sugar is depleted. Since there is so much energy available in this pathway the yeast will use this excess energy to reproduce and also to replenish it's glycogen reserves. The second pathway, called fermentation, the yeast takes sugar and makes CO2 and alcohol. This is our favorite pathway. :) We add O2 in order to get new healthy cells and also to have the yeast with as much glycogen as possible for the main ferment. The higher the original gravity is, the more glycogen (therefore the more O2) will be needed to keep the yeast healthy until the end of fermentation. Under-oxygenating will cause all sorts of off flavors and is much more of a problem that over-oxygenation. When yeast has too low of glycogen reserves it will be "stressed" during ferment and not complete it's pathway to alcohol leaving intermediate chemicals behind. This can include esters, fusel alcohols and diacetyl. How can I add O2? Splash racking or shaking the carboy is the first method homebrewers try. It only adds a small fraction of the O2 required by the yeast but it is much better than doing nothing at all. The other disadvantage is you are shaking outside air in and it could be contaminated with organisms. The most likely is wild yeast. Injecting air through a sanitary filter is another method. This is the method used by Anheuser Busch. It works well for yeast strains with low O2 requirements and also for low gravity worts. The advantage is that it is hard to over-oxygenate. The disadvantage is it is impossible to get enough O2 in for a barley wine or a double IPA. And it is only close to good enough for a regular IPA. The rule I was taught was: Air is Ok to 1.044 but not past. Injecting O2 directly. This allows you to set almost any O2 level you desire. However it is very important not to over-oxygenate as this is quite easy to do with pure O2. You only want enough O2 to quadruple your yeast, any more will start to strip flavor components from your wort and leave the finished beer tasting watery. With a flow meter it is very easy to control the amount of O2 you inject into your wort. In the brewery I work in I have a constant wort flow rate, wort temperature, oxygen flow rate and length of hose that allows me very repeatable results when I oxygenate during cooling.

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