by Don Put (Brewing Techniques)
Definition of terms: Beginning brewers sometimes get a bit confused by this subject because they’ve been told not to introduce air into the finished beer, but that they need to aerate the cooled wort. Some of the confusion no doubt results from using the terms “wort” and “beer” interchangeably, when in fact they are two distinct terms for different ends of the beer-making process. Aeration is very important in the wort stage but can ruin the finished beer.
Wort refers to the prefermentation state; beer refers to the postfermentation malt beverage. Wort can be labeled either sweet (before boiling and hop additions) or hopped (after the boil and hop additions).
For the purpose of this article, aeration refers to the diffusion of air (the atmosphere contains approximately 21% oxygen by volume) into the chilled wort, while oxygenation refers to the diffusion of pure oxygen into the chilled wort. Either method of introducing oxygen will produce the same end: a healthy, active yeast population that will reward the brewer with a clean-tasting finished beer.
The differences between aeration and oxygenation have more to do with the brewer’s preference (a preference based on the style being brewed and the available or favored equipment) than what might be the “right” or “wrong” way. Some brewers choose to use direct injection of pure oxygen into the chilled wort (either by bubbling it through a diffuser in the fermentor or by placing the diffuser into the cooled wort stream as it enters the fermentor); others prefer to use oil-free sterile compressed air. These two methods have virtually replaced the more traditional aeration techniques such as using open Baudelot wort coolers (where wort is aerated as it flows down the outside of the cooler in the presence of air). On the homebrew scale, it is preferable to use a process that will minimize the chilled wort’s contact with unfiltered, contaminated air.
Why aerate your wort? Most brewers learn early on that yeast need a certain amount of oxygen to accomplish their task quickly and thoroughly; knowing when and why to aerate, however, has confused many a beginning brewer. Let’s take a look at why oxygen is needed and when to add it.
Why Aeration Is Important
The postboil hopped wort is essentially devoid of oxygen, so something must be done first-thing to remedy this. Yeast need oxygen in the early stages of the fermentation process (the aerobic stage) to promote cell health and teproduction. For most yeast strains, a dissolved oxygen level of 8 ppm is optimal, and this degree of aeration is usually obtainable. Fortunately, 8 ppm also coincides with the upper limit for wort satutation, so there is little danger of overdoing it (more on this later).
This level of dissolved oxygen is important not only for fermentation, but also in the preparation of yeast starters. Although you probably aren’t going to start culturing your own yeast this week, suffice it to say that yeast need oxygen during their early phase of activity, regardless of specific application.
Knowing when to aerate: The cooler the wort, the better the oxygen uptake. If the postboil wort is aerated while it is still hot, the undesirable effects of hot-side aeration will no doubt appear in the finished beer, causing it to stale quickly.
Although it is sometimes hard to determine exactly when hopped wort is cool enough to aerate, it is safe to assume that introducing air or oxygen after the wort has cooled to below 70 °F (21 °C) will give very satisfactory results.
To better understand the importance of wort aeration, let’s look at what happens to yeast in the early stages of fermentation.
Aerobic stage: Lag phase. Once the yeast have been pitched into a new environment — your freshly brewed and cooled wort — they start to acclimatize themselves by adjusting to the new environment’s temperatute, specific gravity, pH, wort composition, and level of dissolved oxygen. This is usually called the lag phase. The yeast begin to use their glycogen reserves (energy stores similar to our fat cells) to provide energy so they can synthesize enzymes and a permeable cell membrane. Oxygen is a necessary component of these processes. The cell membrane controls the passage of nutrients from the wort into the cell and assists in cell wall construction.
Sterols are important during the development of the cell membrane and other cell components. This cell development will play a vital role in the yeast’s ability to metabolize wort sugars into alcohol and carbon dioxide without contributing off-flavors or off-odors. The creation of sterols requires molecular oxygen as well as a variety of fatty acids, triglycerides, and lipids.
A strong cell membrane enhances the yeast’s alcohol tolerance, which is especially important when the specific gravity of the wort is over 1.060 or so. Higher gravity worts increase the osmotic pressure on the cell wall and membrane, which is potentially hazardous to the health and viability of the yeast. If the yeast have not been raised in a similar gravity wort, they have a tough time getting themselves into an equilibrium with the surrounding solution once they are transferred. The gravity inside the cell is different from that outside, and the cell wall and membrane have to allow a slow diffusion to reach a stable condition. If the cell wall and membrane are weakened due to a lack of oxygen during the critical development stage, the yeast will rupture and die. It is also harder to get the desired level of dissolved oxygen in higher gravity worts because the higher the specific gravity, the lower the wort’s oxygen saturation limit. That is why those “big” beers require more diligent aeration practices and the pitching of large, healthy yeast starters.
Some off-flavors and off-odors — especially banana, solvent, or fruity odors and tastes — are caused by esters, and esters can result from oxygen deprivation during this early fermentation phase. Esters are desirable in some styles (Trappist-style ales, for example), but are considered a serious flaw in many others. Ester levels in the styles that require them can be more easily controlled by increasing the fermentation temperature or by selecting a yeast strain known for higher ester production. One method of increasing ester levels that is not appropriate is depriving yeast of oxygen in the initial aerobic stage of fermentation.
Logarithmic phase. The next phase is called the logarithmic, or log phase. This phase of explosive growth results from the ability of healthy yeast to “bud” — a form of asexual reproduction. Budding produces a birth-scarred mother cell and a scar-free daughter cell. This reproductive process continues through successive generations, resulting in an exponential increase in the yeast population.
Healthy yeast cells can survive many birth scars, so “mother” cells can produce more than one daughter cell, and these daughters in turn produce their own daughter cells. Yeast that have weak cell walls from lack of oxygen during their developmental stages cannot survive the necessary repetitive budding that produces a yeast population that is able to ferment the wort cleanly and quickly. Yeast can also reproduce during the fermentation or anaerobic stage, but they reproduce much more intensively in the presence of oxygen because of the added energy stores available for their metabolic needs.
Anaerobic stage: When the available oxygen in the wort is used up, yeast start producing carbon dioxide and alcohol. This phase is called the anaerobic stage. At this point, it is important to minimize or, ideally, eliminate the introduction of oxygen.
Introducing oxygen after the yeast enter the anaerobic phase can result in increased levels of diacetyl — with its characteristic buttery flavors — and may even cause the Pasteur effect, in which the yeast return to the aerobic phase, causing a cessation of alcohol production and an increase in undesirable off flavors. (Note: Late aeration can actually result in lower ester levels in the finished beer. The negative effects of late aeration, however, clearly outweigh any potential advantage here.)
Aeration techniques and devices are as varied as the brewers who develop and use them. Whether you use the simple “shake-the-fermentor-until-you-drop” method, the kitchen faucet-type aerator, the air pump and aquarium stone setup, or the more advanced method of injecting sterile air or pure oxygen into the wort stream, you are aiming for the same result: to get as much oxygen into the cooled wort as possible.
In the past, brewers have been concerned about overoxygenating the wort; that is, getting the dissolved oxygen levels up to where they are injurious to yeast (somewhere up in the high teens mg/L). Recent tests by George Fix have shown “that there is no way such levels can be reached with beer wort no matter how much oxygen is injected.” As to the discrepancies between Fix’s new data and the more traditionally held beliefs about oversaturation, Fix states:
It is a common error to report the highest dissolved oxygen meter reading as the actual value achieved. When I inject oxygen, I will get readings in the 20s (mg/L); however, if I wait a few minutes for the system to reach equilibrium, the meter will drop to below 10 mg/L. It takes some effort just to get 8 mg/L dissolved, because dissolved oxygen levels steadily decrease as soon as the oxygen supply is turned off. It has been my experience that it is only the fully dissolved oxygen that is relevant to yeast, and the maximum saturation value is well below harmful levels.
Shaking and splashing: Shaking the filled carboy is probably the aeration method most often used by beginning brewers. It is cheap, reliable, and with low- to normal-gravity worts works out very well. Actually, this method can be combined with the splash-as-you-rack/pour-from-the-kettle technique for even greater aeration of the cooled wort. The idea here is to create disturbance of the air/wort interface so that the oxygen-bearing air can diffuse into the wort.
The obvious drawback to these methods is that the cooled wort is exposed to air that contains probable contaminants; after all, the air that ends up in the carboy is unfiltered. This will only cause a serious problem when the air is especially dirty or when the brewer pitches too little yeast. Underpitching gives the contaminating organisms a chance to reproduce in numbers large enough to affect the flavor of the finished beer. Also, unless the carboy/fermentor is opened up occasionally during the shaking and splashing process to let fresh air in, only a finite amount of oxygen is available for diffusion into the wort.
An improvement over this method, at least as far as getting more dissolved oxygen into solution, is a two-bucket transfer technique that has been suggested by Jim Busch. Busch describes the process as follows:
Sanitize two plastic buckets. Place the chilled wort in one bucket. Next, vigorously transfer the wort from one bucket to the other from a height of at least one foot. This will allow you to obtain the highest dissolved oxygen levels short of using the aquarium pump/oxygen rank and aquarium stone methods. The wort should be transferred at least four times.
This method is cheap and reliable, but like any of the aeration techniques involving atmospheric oxygen it suffers from the same risk of exposing the wort to possible contamination. In all such cases, however, the risk can be minimized by pitching sufficient amounts of healthy yeast.
Aquarium Pump & Oxygen Stones: The next step up is the aquarium pump/Oxygen Stone with filter combinations. This setup suggested by Dave Miller. Miller describes the use of an aquarium pump as follows:
Aerate the pitched wort until foam reaches the top of the fermentor. Then, let the foam subside for a few hours and repeat. A few hours later, repeat again. The idea is to bubble air through the wort for at least 15 min during the first 6–8 h after cooling and pitching.
An aquarium pump system is a fine, inexpensive way to aerate wort while keeping it from being exposed to airborne contaminants. You can also replace the air pump with a bottled, regulated oxygen supply.
Another Option in the Use of Aquarium Stones
Roy Paris suggested soaking the stone for a few hours in vodka rather than washing the stone in a chlorine solution. Although you may get a little alcohol in the wort, you will avoid any chance of introducing chlorine and resulting chlorophenols.
Paris also believes that although activated carbon filters remove odors, they are not efficient filters of particulate matter. He uses a second stone in a jar half filled with 3% hydrogen peroxide (the common drug store variety). Hydrogen peroxide is an oxidizing agent that will remove odors, and its mild antiseptic properties will destroy airborne microflora. In addition, any particulate matter is washed out in the peroxide solution. Any peroxide washed into the wort is quickly broken down into water and oxygen, which at this point in the process is desirable. The accompanying graphic shows what the setup looks like.
A quick word about filters: A filter is a cheap way to clean the air of possible contaminating organisms. Even though pure oxygen from a cylinder is sterile, I use a filter between the regulator and the injector in my system to make certain that any possible delivery-line contaminants are not passed into the cooled wort.
Pure oxygen injection: Many advanced home brewers and professional brewers use direct injection of pure oxygen into the chilled wort to achieve an adequate dissolved oxygen level quickly. Oxygen cylinders are available from welding supply companies in a variety of sizes and prices; you also have to buy a regulator or a regulated flowmeter. (See box for cautions about using cylinders of pure oxygen.) Once you buy a cylinder, you pay only the cost of the oxygen for subsequent fills.
A lot of people get confused about the different grades of oxygen available. Most processors of bottled gas offer three grades of oxygen: industrial, medical, and UHP (ultrahigh purity). Although the grades differ in purity, the industrial grade, which is sold as welding grade, is 99.8% pure. This is the grade usually supplied to the food industry. Note also that the contaminants are only other gases that occur naturally in our atmosphere; the purity has nothing to do with bacteriological contamination. Most suppliers do offer industrial-grade oxygen in bottles that have never been used in welding applications to ensure that the bottles have not been fouled by the unintentional introduction of other welding gases, such as acetylene.
Safety Tips for Using Bottled Oxygen
Pure oxygen is a hazardous gas. For safety’s sake, please note the following important safety considerations when using bottled oxygen.
From Oxy-Fuel Welding, Cutting and Heating Guide (Victor Equipment Company, St. Louis, Missouri, 1988).
The above examples are just a sampling of a few methods for dealing with wort aeration. Remember, aeration techniques and devices are as varied as the brewers who develop and use them. Ultimately, the most important thing is not how you do it, but whether you do it.
Once you realize the important role oxygen plays in the fermentation process, it’s relatively easy to avoid the pitfalls of wort oxygen deprivation — high terminal gravities, stuck fermentations, and high levels of flavor-ruining compounds, to name a few. Whatever your budget, you can develop an aeration/oxygenation technique that will produce a healthy, clean fermentation and a fine finished beer.
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