Yeast needs several things to work well and do its job. First, it needs food. Yeast feeds off of the fermentable sugars in the wort and in the process converts it to CO2 and alcohol. Next, it needs to be able to work within the proper temperatures for its particular style. Many ale yeast strains work best around 68 degrees Fahrenheit. Finally, yeast needs OXYGEN which brings us to WHY we need to aerate our wort.
It is often written that the best things we can do to improve the quality of our home brew is to do full-boils of our wort*, control fermentation temperatures and take special care with sanitation. All very useful and practical suggestions but I submit that aerating your wort ranks just as high in importance.
*Note: While full-boils of your wort can significantly increase the quality of your beer they also greatly deplete the amount of oxygen naturally contained in the water thus further increasing the need to aerate.
Let's take a look at what happens with under-oxygenated wort:
- Long Lag Times. This will be one of the first problems you may notice if your wort has not been properly aerated. Lag time is that crucial period when yeast cells replicate and fermentation begins. We want shorter lag times to lessen the chance of contamination and reduce the chance of off flavors ending up in our finished beer.
- Incomplete Fermentation. Here again, it comes down to the yeast. To achieve the desired final gravity of your particular beer style you want your yeast to ferment completely to achieve the desired Final Gravity and ABV percentage. If your yeast does not have enough oxygen it will eventually cease functioning thus resulting in incomplete fermentation. Assuming proper pitch ratios, fermentation temperatures and life-sustaining oxygen, the yeast will continue to be healthy and thrive despite the rising alcohol production which can kill the fermentation cycle in poor environments.
- Off flavors or "fruity" tastes. When it comes down to it we are really just trying to make the best tasting beer we can. As we previously alluded to, long lag times can lead to off flavors ending up in our finished product. Another by-product of poor yeast cell production (from poor aeration for instance) is excessive ester production. An over abundance of esters can lead to an unwanted fruity character to your beer.
|William's Oxygen Aeration System|
- Splashing. This is a simple method of allowing the wort to "splash" into the primary fermentation container either along the side, through a strainer or off the bottom of the vessel itself. This will allow a little additional air to make its way into the wort before the yeast is pitched.
- Agitating or Shaking. This method simply involves vigorously stirring or whisking the wort after it is in the fermentation container for several minutes or longer. Alternatively, you can cap the fermentation bucket or carboy with a sanitized lid and shake for 1-2 minutes or as long as you can stand it. This isn't as practical since a typical 5 gallon batch of wort is quite heavy! Note - method 1 and 2 can be done in combination with one another to improve the result.
- Injection. There are 2 basic ways of injecting oxygen into your wort. The first involves purchasing a simple aquarium pump along with a filter and diffusion stone. These are readily available from pet stores or home brew stores. Simply sanitize the hose and stone that will come in contact with your wort, submerge and power on for 25-30 minutes. The second, and most preferable, method is to inject pure oxygen into your wort. All of the methods above introduce "air" into your wort. Since air only contains just over 20% oxygen you need to inject way less pure oxygen into your wort for it work. Much like the aquarium set-up, you can purchase a relatively inexpensive oxygen valve, hose and diffusion stone to get the job done. To this, you will add a pure oxygen tank (for around $10 from Lowe's or Home Depot) and you are set (I like this one from Williams Brewing). The good news is that instead of injecting air into the wort for 25-30 minutes you only need 45-60 seconds of pure oxygen to give the yeast its much needed boost. A word of caution though - More isn't always better when it comes to injecting oxygen into your wort. Too much oxygen can actually stress the yeast and in extreme cases kill it so stick with the prescribed times.
Well, that's it. I hope you have luck aerating your wort and I'm sure your home brew will be all that much better.