Oxygenation

Human beings require air to breathe and sustain life. The air we breathe is composed of approximately 78% nitrogen, 21% oxygen, 0.93% argon, 0.039% carbon dioxide, and 0.031% other gasses. Although oxygen is not the most abundant component of the air we breathe, it is the only one that humans utilize. Once oxygen is absorbed into our blood, it is pumped throughout our body and provides our cells with oxygen in order to produce energy. Unlike human beings, the brewer’s yeast Saccharomyces cerevisiae can live and reproduce both aerobically (in the presence of oxygen) or anaerobically (in the absence of oxygen), making it a facultative anaerobe. Even though small amounts of oxygen can be found in wort, most of it is forced out of solution while boiling. Because of this, we homebrewers are responsible for providing yeast with the necessary oxygen for a healthy fermentation. During the earliest part of fermentation yeast are absorbing oxygen in order to make unsaturated fatty acids and sterols which are important components of their cell membranes. Changes to cell membrane structure help yeast bring in nutrients necessary for growth, and make them more tolerant to ethanol produced later in the fermentation process. During this initial preparation stage of fermentation, yeast utilize all of the oxygen found in wort and when they are ready to begin reproducing, they do so anaerobically (without oxygen). Regardless of the oxygen content of the wort at this stage of fermentation, the high sugar content forces them to break down sugars in a way that produces carbon dioxide and ethanol; a process called the Crabtree effect.

With the exception of zinc, oxygen is the only major component necessary for fermentation that is either not found in wort or cannot be produced by the yeast themselves. Unlike zinc, which is often supplemented by the addition of yeast nutrient, oxygen is added through a variety of methods by the homebrewer. During the first ten hours of fermentation the majority of the oxygen in wort is absorbed by yeast to strengthen their cell membranes and make them more tolerable to the harsh conditions of fermentation (low pH and high ethanol levels). Oxygen is one of the most important factors for a healthy fermentation because it provides the necessary components for yeast to produce sterols and unsaturated fatty acids, both of which are required to make up the cell membrane of new cells. Yeast reproduces by budding a new “daughter” cell out from their own cell, a process that requires a great deal of energy and cellular material. Unsaturated fatty acids are one of the materials that make up the cell membrane and thus are required in large amounts during budding of a new cell. When yeast cells reproduce they also require elements naturally found in wort like amino acids and free amino nitrogen. These wort elements are needed inside the cell, so yeast need to pass them through their cell membrane as quickly as possible. Oxygen plays an important role in this by helping the yeast make compounds called sterols that allow their cell membrane to bring inside nutrients more easily. Think of this process like one of those high-intensity cooking shows that when the timer starts the participants run around the kitchen like crazy gathering all their ingredients at their station. During the early stages of yeast cell reproduction the yeast are in a state of “gathering” all that they need to reproduce quickly.

Although there are many factors that can influence the amount of oxygen required such as yeast strain, fermentation temperature, and original wort gravity, a general recommendation for the average beer is 8-10 parts per million (ppm) of dissolved oxygen at the beginning of fermentation in order for proper yeast cell growth. While too little oxygen can impair yeast reproduction, over-
oxygenation prior to fermentation is not a huge concern since all of the unused oxygen is usually released out of solution following primary fermentation. One important thing to note on oxygen levels, however, is that it is much harder to dissolve oxygen into your wort when the gravity and temperature are higher. High gravity beers (greater than 1.065) will required more dissolved oxygen in order to keep the yeast happy in this harsh environment. If the yeast are deprived of oxygen during the critical initial stages of fermentation, they will not be able to effectively produce cell membranes and grow new cells. As a result, low levels of dissolved oxygen in homebrewed wort can lead to problems like poor attenuation, stalled fermentations and can produce off flavors. In addition, when yeast health is compromised in this way it is unlikely they will be of adequate health for re-pitching into subsequent batches.

Methods of Aeration

There are many methods for adding oxygen to your homebrew wort and each has their own advantages and disadvantages. Cost is one of the factors that influences which method a homebrewer chooses to use for oxygenation. When choosing a method for aeration based on cost consider this; the price difference between basic aeration (shaking) and more complex methods (using pure oxygen) is less than the price of two 5-gallon (19-L) batches of homebrew. This may seem like a lot but the benefits to the quality of your beer will be tremendous. Yet in the end, it is always better to use the simplest method of aeration over nothing. Here are a few of the most common methods used to add oxygen to homebrew wort.

Siphon Spray During Transfer

One of the easiest and most cost efficient ways to add oxygen to your homebrew wort is to spray or splash your cooled wort as you transfer it to your fermenter. This can be done in any number of ways including pinching the end of the transfer hose with a sanitized binder clip, or siphoning into your fermenter through a funnel with a mesh screen. The goal is to produce a foamy layer of bubbles on top of your wort using the most sanitary method possible. Using this method should produce approximately 4ppm of dissolved oxygen (see table to the right), which is about half as much as
the recommended minimum for a healthy fermentation.

Shaking the Fermenter

Another simple and low cost method that can add up to 8ppm of dissolved oxygen to your wort is shaking your fermenter (see table, right). Once all of the cooled wort is transferred, simply shake or swirl your fermenter. If you see foam on top of the wort then it is working. Inevitably, you may be doing this without thinking about the benefits of adding oxygen to your wort. When I first began homebrewing I always wanted to make sure the yeast were adequately mixed with the wort so I would shake and swirl the carboy. Incidentally, this vigorous movement also added oxygen. One thing to consider when choosing this method of aeration is that the amount of dissolved oxygen is variable and depends on how well and how long you shake your fermenter.

Aquarium Pump with Aeration Stone

It only took a few batches of homebrew for me to realize that haphazardly shaking my glass carboy filled with wort was not the safest method of aeration, so I decided to take the plunge and buy an aquarium pump with 0.5-micron stainless steel aeration stone (see picture on facing page). This type of pump pulls in air, pushes it through a sterile filter, and then releases it as tiny bubbles through the aeration stone to easily dissolve in the wort. At the time I made the switch to the pump, I felt like it dramatically improved the quality of my homebrew. In retrospect, it was probably because this method of aeration was better at consistently adding around 8ppm of dissolved oxygen to my wort compared to shaking. Major disadvantages to this method are that since the pump is using air (21% oxygen) as the source of oxygen, it can take a lot longer to reach the desired dissolved oxygen level and has a lower saturation point than pure oxygen. The other thing to be aware of when using a pump is that you must use a sterile filter so that no mold spores or contaminating bacteria get into the wort. Also remember not to get the filter wet otherwise it will not work properly. Once, when I was first starting out with my aquarium pump, I accidently got my filter wet and didn’t have a replacement. As a straight-up MacGyver move (I’m showing my age here with my TV references), I stuffed a three piece airlock with cotton balls and ran my aquarium pump air through that prior to going into my fermenter. It worked like a charm! While the pump and stone technique has significant advantages over spraying or shaking, I’ve learned that for slightly more money you can use a far superior method of aeration with a pure oxygen system.

Pure Oxygen with Aeration Stone

There are a few methods for using pure oxygen when aerating your wort and generally these methods will provide the optimal level of dissolved oxygen, around 12ppm (see table above). In addition, using pure oxygen is sterile so bacteria and wort spoilage organisms cannot grow in it. As with all things homebrew, you can go the simple route or you can go with a more complicated setup. I find that the easier the equipment is to use, the more likely I am to use it.

Probably the most common method for aerating wort with pure oxygen involves using a small tank of pure oxygen, a regulator valve to release the oxygen, a metal wand and an aeration stone. With the exception of the oxygen tank, all of these items can be purchased at your local homebrew shop or online homebrew retailer. Purchasing the pure oxygen can be a bit tricky since companies that sell industrial gasses are not allowed to sell it without a prescription. A friend of mine once told me that hardware stores carry hand torch oxygen tanks that work perfectly (see picture on page 92). In order to use this set up, simply put the sanitized wand and attached aeration stone into the wort and turn on the gas for 30-60 seconds. In my experience, a slow flow of oxygen is better than a faster flow. When finished, rinse the wand and stone in sanitizer or boil the stone to clean out the pores. Handle the aeration stone carefully though, because oils can clog the pores, so try not to touch it with bare hands.

Recently, I purchased an inline oxygenation set up to use with my counterflow plate wort chiller. It has bells and whistles; it even has a really cool sight glass so I can see the wort flowing through as it makes its way to my fermenter. The other thing it has on it right now is a lot of dust. Since buying it I have made three batches of beer and I never seem to have the time to put it all together during the end of my brew day. Instead, I use a wand and aeration stone. Sometimes the simplest method is the best! If you do choose to use an inline method of aeration, simply aerate the wort as it leaves the wort chiller on its way to the fermenter.

With all methods of wort aeration, a significant amount of foam will be produced, especially when using an aquarium pump or pure oxygen. Foaming is worse with air because nitrogen has a very low solubility, and since air is mainly nitrogen there is lots of foaming with air. One way to combat the excessive foaming is to add a food-grade antifoam product. These are typically silicone-based, but I have also seen a few that use vegetable oil. Antifoam products work by breaking the surface tension of the wort, which prevents the foam from forming.

When to Add Oxygen

Aeration of homebrew wort can be done following cooling while transferring to a fermenter (splashing/siphon spray), immediately after transferring, or after pitching yeast (shaking, using air or using pure oxygen). I prefer to oxygenate my wort immediately before pitching my yeast, but I know others like to do it after they pitch. Whichever way you choose, make sure to keep it consistent in order to help diagnose a problem if one arises.

I have heard of some homebrewers adding pure oxygen to their yeast starter, but I would advise against this because yeast strains have varying oxygen requirements and without this knowledge and precise control of oxygen level with a regulator you can easily under or over-oxygenate your starter. Some reports also suggest that aerating yeast may require up to 30% more yeast to be pitched to achieve similar fermentation times as wort aeration. In addition, excessively high levels of oxygen can be toxic to yeast. Yeast growth will also benefit from a continuous source of oxygen, which is not practical with a pure oxygen set up. Instead, low levels of oxygen can be incorporated into the starter by either shaking it frequently or using a stir plate.

When Oxygen is Bad in your Homebrew

Although oxygen is a necessary ingredient required for optimum fermentation, it is not always a homebrewer’s ally in the quest for making an amazing homebrew. Ideally, the only time you want to have oxygen in your beer is during the initial stages of fermentation. Unfortunately, it is not reasonable to keep every molecule of oxygen out of your beer, so it is best to minimize exposure during specific steps of the brewing process.

Hot Side Aeration

Make sure that when you are adding oxygen to your wort you do so when it is cool (less than 80 °F, 27 °C), otherwise it can cause stale flavors by oxidizing melanoidins in the beer. During hot side aeration the dissolved oxygen also has the potential to temporarily bind with wort factors that can break apart over time when the yeast is no longer active to break it down. This will oxidize the beer leaving cardboard-like flavors and aromas present in your final product. If you bottle condition your beer, the yeast that produces the carbonation can reduce the oxygen to a degree,
but I don’t recommend relying on this as a method of reducing oxygen in your beer.

Late Fermentation/Following Fermentation

As mentioned earlier in this story, during the bulk of fermentation, the yeast no longer have a requirement for oxygen so adding it considerably after pitching yeast has the potential to produce aldehydes, diacetyl, and sulfur-containing compounds that will give your beer off flavors. In addition, oxygen post-fermentation can cause chill haze by reacting with polyphenols and tannins, and lead to oxidation and staling flavors and aromas once packaged. Oxygen post-fermentation is such a huge concern for commercial breweries that some of them go to great lengths to minimize it.

Alternative to Adding Oxygen

Although oxygen is a necessity for early stages of yeast fermentation, it can also have negative effects in beer. One of the main issues with oxygen in beer occurs when beer is aged for long periods of time (months). During the initial stages of fermentation, when yeast are absorbing oxygen, they produce byproducts that are pre-cursors to staling compounds. As the beer ages these pre-cursors are converted into the staling agents and can produce oxidized off flavors. While doing some research on the long-term effects of oxygen in beer I came across a series of interesting articles that described the potential of various oils to bypass the need for oxygen, when they were added to the wort or yeast starters pre-fermentation. The theory is that by adding the oil, which is comprised of unsaturated fatty acids, a brewer can bypass the yeast’s production of unsaturated fatty acids from dissolved oxygen. Although this sounds like an April Fool’s Day prank, it actually makes sense.
Before I exchanged my oxygen tank for a jug of olive oil, I was curious about a few things such as the oil’s effect on head retention, lag-time, and production of off flavors. New Belgium Brewing Company and their Assistant Brewmaster, Grady Hull challenged my skepticism when I found a description of some of their work on this subject. What they did was replace oxygenation with olive oil in their yeast five hours prior to pitching it into a batch of beer.

They chose olive oil because it can easily be found in most grocery stores and contains 55–83% oleic acid, an unsaturated fatty acid that is naturally produced by brewers yeast. What Mr. Hull and the team at New Belgium found when they compared these beers to regular oxygenated batches was fascinating. They detected slightly less head retention, but no change in pH or attenuation when compared to a batch that was oxygenated. What they did find was that there was a slight increase in ester production as measured by a gas chromatograph (a fancy chemistry machine) and by the tasting panel, but the tasters agreed that it was still within the style guidelines for that particular beer. They noticed that the fermentation took longer with the addition of olive oil as compared to oxygen. They also noted that as the concentration of olive oil was increased, the beer was more similar to the oxygenated version. The main goal of the work was to assess whether olive oil was a good alternative to oxygenation for beers that would be stored long-term. They concluded that olive oil “aerated” beer had improved warm storage characteristics (i.e. less oxidation) and increased flavor stability with little to no effect on flavor quality. Although this work has not prompted me (or New Belgium) to switch to olive oil aeration, it addressed my concerns and provided some interesting insight into an alternative to wort oxygenation and how to reduce oxidation potential when storing beers for extended periods of time.

Concluding Thoughts on Oxygenation

I encourage even the most novice of homebrewers to incorporate some aspect of wort aeration into your brew day. The benefits will outweigh the negatives as long as you remember to be as sanitary as possible and only add oxygen early in the fermentation process. Yeast requires oxygen for the initial steps of fermentation, and it is our responsibility as the brewer to decide which method of aeration is right. Although splashing and shaking can create a reasonable amount of dissolved oxygen in wort, my recommendation is to use some method of pure oxygen with an aeration stone, as this is the quickest method to produce an optimal and consistent level of dissolved oxygen in your wort prior to the start of fermentation. Whether you decide to add oxygen to your wort by shaking your fermenter, use an elaborate inline oxygenation system or even add a drop or two of olive oil, efforts to oxygenate your wort will be one of the most important things you can do to enhance yeast health, ensure full attenuation and improve the overall quality of your homebrew. The methods outlined here are not exhaustive, but instead should provide you with a general understanding of the importance of aerating your homebrew as well as some commonly used methods to achieve adequate dissolved oxygen for a healthy fermentation. If you are hesitant to go all out in your homebrewery witha pure oxygen system, try some of the more basic methods first. Your yeast and your palate will thank you.

references:
Takacs, P. and Hackbarth, J.J. “Oxygen-enhanced fermentation.” (2007). MBAA Technical Quarterly. 44; 104–107.
Smart, K. “Brewing yeast fermentation performance.” (2000).
John Palmer’s How to Brew, Chapter 6
Hull, G. “Olive oil addition to yeast as an alternative to wort aeration.” (2008). MBAA Technical Quarterly. 45 (1); 17-23.
www.oliveoilsource.com

Written by Christopher Wood