Nick Carr on September 16, 2015 2 Comments Photo Credit: Flickr Much is made of oxygen as a super villain in the brewing world. Oxygen stales beer creating “cardboard like” flavors and it decreases a beer’s shelf-life. The act of boiling wort forces much of the oxygen in the wort out of solution, which we would think is good. And it would be great, but for one small problem. Yeast needs oxygen to reproduce. Yeast uses oxygen to produce unsaturated fatty acids (UFA) and sterols, which make up the yeast cell walls. The formation of both UFAs and sterols is complex and they go through many steps to get to the needed end result. Oxygen is an important factor in several of these steps. With weak cell walls yeast have reduced membrane integrity resulting in; poor regulation of permeability, the inability to withstand higher levels of alcohol, and decreased cell reproductive health. So oxygen, at least at this stage, is needed. It creates healthy yeast, and healthy yeast means better attenuation, more complete fermentations, lower fermentation times, less chance of a stuck fermentation, and fewer off flavors. When looking at wort aeration and oxygenation there are some things to consider. The gravity of the beer will affect the amount of oxygen required. Is there enough oxygen taken in during aeration of a yeast starter? How much oxygen is enough? Aeration over multiple yeast generations. When do you want to aerate your wort? Is there a substantial difference between methods of aeration? Are there alternatives to aeration? Let’s look at these one at a time. Beer Gravity and Required Oxygen The higher a beer’s gravity the higher the pitch rate and the higher the pitch rate the more oxygen will be needed. Oxygen solubility drops as the gravity of the wort increases. So, if you are brewing a beer 16 °plato or higher (>1.065 SG) you not only have to increase the oxygen in it –because your pitching more yeast, but you also have to deal with the reality of it taking longer to put that oxygen in. So, whatever your preferred method of aeration (different methods are discussed below) you will have to do it longer for both these reasons. This will take some experimentation to find the length of time needed to get proper amounts of solubilized oxygen with a given aeration method, but a good place to start would be about twice as long as you aerate for an average batch of beer. Also, if you are going to be doing lots of high gravity beer it may be worth investing in a few special pieces of brewing equipment designed to help with oxygenation. In Yeast: The Practical Guide to Beer Fermentation, White and Zainasheff suggest any beer higher than 22 °Plato (1.092 SG) be oxygenated with pure O2 because air just doesn’t have enough oxygen to offset the sugars already dissolved in the beer. Though adding oxygen after start of fermentation is usually frowned upon because of the possibility of off flavors, there is something to be said for hitting a high gravity fermentation with a second “minimal” dose of oxygen. You would want to do this before peak fermentation activity is complete and, as stated by White and Zainasheff, after the first cycle of yeast budding has taken place; about 12 hours into fermentation. Oxygen and the Yeast Starter For average gravity beer there is some evidence that the simple process of preparing a yeast starter gives the yeast enough oxygen. The process of creating a yeast starter includes agitating the mixture several times which introduces oxygen. A yeast starter kit can help you accomplish this. There is also data from an experiment you can read about here that suggests aeration via aquarium pump or agitation, plus leaving the starter unsealed to the air — in this case covered with tin foil — created the highest yeast cell counts. As with all experiments, we have to remember that the biggest factor speaking to the quality and usefulness of an experiment is repeatable results. When to Aerate/Oxygenate We have to define a couple terms here. First, “hot side aeration,” which is the addition of oxygen back into the wort after the boil but before the wort has cooled. Second, “cold side aeration,” which is the opposite, or adding oxygen back into the wort after the boil and after the wort has cooled. Most brewers consider any hot side aeration a bad thing. Hot side aeration can cause oxidation and lead to a beer going stale a lot sooner than anticipated. That’s why most consider hot wort a “no splash zone.” How much this hot side aeration actually affects the beer is up for debate, some brewers even consider it non-consequential. Either way it seems a good idea to limit excessive splashing of hot wort. Why take the chance? By the same token don’t automatically think your beer is going to be a disaster if you happen to agitate before it’s cooled. So, we want to quickly cool the wort down to near pitching temperatures — in the neighborhood of below 80°F — before we purposely start aerating. The Importance of Aerating Multi-Generation Yeast Homebrewers usually use new yeast for every batch of beer, but some do harvest their yeast for reuse. If you are going to harvest yeast to pitch into another beer it is even more important that the yeast have the oxygen to reproduce, build healthy cell walls, and store nutrients. You could say reused yeast is reliant on two different aerations, that of the beer it has come from and that of the beer it is going into next. How Much Oxygen It is near impossible to not introduce some amount of oxygen during transfers. But this is nowhere near the volume needed for healthy yeast growth. As mentioned earlier, most yeast strains, at average pitching rates, into average gravity worts require close to the same range of oxygen. This range is 8 to 10 parts per million (ppm). To get your wort into this saturation range you have a number of options. Methods of Aeration/Oxygenation There are several aeration/oxygenation methods available to the homebrewer. They are all relatively simple, but differ in their effectiveness and equipment needed. They include splashing, stirring, shaking, using an aquarium pump, or using pure oxygen. The Difference Between Aeration & Oxygenation: The difference between aeration and oxygenation is a matter of quality. Aeration is the process of adding oxygen via the air, while oxygenation is the process of adding pure oxygen to the wort. With aeration it is impossible to get more than 8 ppm into the wort because dry air is made up of only 21% oxygen. Oxygenation on the other hand can give you substantially higher numbers. Wyeast labs published a chart that gives some idea of the effectiveness of the methods we will consider here. A second chart published in Yeast: The Practical Guide to Beer Fermentation from White Labs is much more conservative with the numbers. As an example, the White Labs chart puts the “shaking” number at around 2.5 ppm after 5 minutes, while the Wyeast chart has it at 8 ppm after only 40 seconds. So use these as references only. Spraying: This is easily done during the transfer to the fermenter. You can secure your hose at the top of the fermenter so that the wort falls, splashing against the bottom. Easy, quick and simple. You can also create a spray at the end of the siphon hose. This can be accomplished by using a siphon spray wort aerator attachment, capping it lightly, pinching it off, or creating your own attachment by adding a small piece of capped tubbing with very small holes perforating it. Note: This method is the least effective, but it will add more oxygen to the wort than doing nothing at all. And it can be combined with the next method to increase O2 uptake further. Agitation (Stirring/Shaking/Splashing): This is where you can take out some of your frustrations of a long brew day. Don’t be gentle. Sanitize a wooden spoon or spatula and stir like crazy. Splash around. I’ve even gone as far as sanitizing my blender, than pouring several quantities of the wort through it. You can also pour the wort back and forth between sanitized buckets or carboys. With smaller batches, say 5 gallons and below, shaking the fermenter is also a good option. Be careful though, especially if using glass carboys! Be sure any openings are plugged well. You can lift and shake, or if using a plastic carboy, you can lay it on its side and gently roll it around. I often combine spraying and some sort of agitation, which if the Wyeast chart is to be believed, gets you up to the lower end of the needed O2 range. Tip: When agitating a sealed fermenter the amount of headspace will determine (to a point) how much oxygen gets into the wort. The more headspace the more O2 available to be absorbed. Purging the headspace with pure oxygen will further increase the O2 available, though if you can do this you can likely just use the oxygenation method. Wort Aeration System: If the thought of all this work has you groaning, or you consistently make larger batches, the next method is for you. You can buy a wort aeration system, all setup and ready to go. This system takes advantage of an aquarium pump to push air through a filter and then a diffusion stone, infusing sanitary air into the cooled wort. The porous stone creates thousands of very small bubbles. These small bubbles have a larger surface-to-volume ratio than bigger bubbles, allowing more oxygen to be absorbed on the way to the surface. If you don’t want to buy any brewing supplies, you can also make a pretty simple aeration system by buying an aquarium pump and diffusion stone. It just depends on how much you want to DIY. It’s also worth noting that you should always sanitize any part of your equipment that will come in contact will your cooled wort, including your wort aeration system. This goes for whatever system you use, whether you buy one or build it, sanitation is incredibly important. Note: This method is easier than agitation, but still can only put the wort in the lower 8 ppm range. Oxygenation: In this method a pressurized gas cylinder replaces the aquarium pump and you feed your wort pure oxygen. Much higher dissolved oxygen levels can be achieved. This becomes especially important when brewing high gravity beer. Most small oxygen cylinders don’t have any sort of flow rate gauge attached, so it is hard to determine a useful oxygen flow and amount of time. Generally about 60 to 120 seconds will get you in or above the 8 to 10 ppm range. Gauging flow can be done, to some extent, by watching the bubbles. As flow increases more and more of the small bubbles will combine making bigger bubbles and more of these bubbles make it to the surface. Now, as mentioned earlier smaller bubbles have a larger surface-to-volume ratio then do larger bubbles, so it stands to reason to have smaller bubbles. Also, the more agitated the surface, the more oxygen is escaping and going unused. Find a flow that gives you the most amount of small bubbles with the least amount of churning at the surface. Also, over oxygenation becomes a concern with this method. Most yeast strains can handle high amounts of oxygen, but at some point yeast don’t make any more sterols. At this point, if there is oxygen left in the environment the yeast use it to create more flavor compounds, which can equate to flavor problems. The excellent book Fermentation, Cellaring, and Packaging Operations edited by Karl Ockert gives the detrimental amount at more than 40 ppm. This is simple enough to avoid: don’t oxygenate your average gravity beer for more than about 3 minutes. For higher gravity beer you’ll have to experiment to find the happy place. Note: You lose head forming compounds through excessive foaming. So, oxygenating or pump aerating too long and too vigorously will likely result in a finished beer with very little head or head retention. Is Aeration Necessary? Some research has looked at the possibility of getting the building blocks for UFA’s from a different source and forgoing aeration entirely. This was the idea presented in an experiment done by Grady Hall at New Belgium Brewing Company as part of his postgrad work. The experiment is quite interesting and Mr. Hall’s full PDF paper can be found here, and it’s definitely worth a read. In short the experiment tested whether olive oil, a rich source of oleic acid (a precursor of UFAs), could be added in place of aeration. The fermentations of four different batches of New Belgium’s Fat Tire Ale were setup; each with no aeration, and varying amounts of olive oil added to the yeast starter 5 hours before pitching. All four experimental batches fermented slower than the aerated controls. The batch dosed with the most olive oil, still minuscule at 1 milligram per 25 billion yeast cells, had a fermentation time of 94 hours as compared to the 83 hour fermentation of the control. All fermentations reached terminal gravity which seemed to speak to good levels of UFAs. The experimental batches had more esters and less fusel alcohols than the controls, but were close enough that a tasting panel said they were within the range of the beer’s usual profile, and the panel actually found the higher esters more to their liking. A follow up tasting panel found that after the beer had been aged warm for three weeks oxidation was less noticeable in the experimental beer. New Belgium found the results were similar enough to the Fat Tire standard that they packaged and sold the beer. Maybe you drank one. How does this affect the average homebrewer? Well, first we have to understand that our homebrew is (hopefully) treated better than most commercial beer. We don’t have to worry about it being trucked across the country or stored under less than ideal conditions. So, how stable you want your beer really depends on your batch size and how long it takes you to drink it all up. But if you do want to try this approach there are a couple things to think about. First, the oleic acid in olive oil only contributes to the building of UFAs, not sterols. So, as Mr. Hall suggests at the end of the paper experimentation would have to be done with adding some form of sterol or aeration, plus the olive oil. The one place this might be pretty pertinent for the homebrewer is when brewing high gravity beers. The possibility of using olive oil in conjunction with aeration could eliminate the need for oxygenation, or at the very least cut down on oxygenation times. Second, is the amount of olive oil. The amount used in the experiments is very small, bringing it down to a homebrewing level makes in infinitesimal. To do the dilutions you’d need micropipettes and a lab grade scale. But, looking around on different forums, some brewers have gotten decent results by sticking the tip of a toothpick or other small instrument into the olive oil, then stirring it into the yeast starter. Obviously some experimentation is in order here. But, this might not be a bad way to give your yeast a little extra umph; especially for bigger beers and those looking for alternatives to having to oxygenate. Quick Tips Overview: Clean and sanitize everything that will touch your wort. Aerate/Oxygenate wort below 80°F. Smaller bubbles are better than big bubbles. Higher gravity beers solubilize oxygen at a slower rate than average gravity beers. Keep foaming to a minimum. Optimum wort O2 levels is 8 to 10 ppm. You can only get 8 ppm through aeration to get higher rates you need to oxygenate. If harvesting or using harvested yeast pay even closer attention to aeration processes. You probably realize by now that it will take some experimentation to find the exact right process depending on your aeration method, yeast strain, and wort gravity. It isn’t hard to brew a single 5 gallon batch of beer then split it into five smaller fermenters so that you can try different things; whether it be different amounts of O2, different aeration methods, olive oil, or some combination. Keep meticulous records so that when you find something that works you have plenty of information available to make great beer repeatable. Cheers!