New brewers are cautioned that oxygen is bad for their beer, and commercial breweries go to great lengths to reduce the oxygen introduced during packaging to the parts per billion (ppb) level. However, there is one time during the brewing process when oxygen is of great benefit. Yeast requires abundant amounts of oxygen for healthy reproduction. The yeast cells reproduce by budding, that is, new cells are formed and then separate from the existing cells. The chemical building blocks for the new cell walls are sterols and unsaturated fatty acids, and a major element in these compounds is oxygen.
If starved for oxygen during this critical time, the yeast will stop reproducing. Additionally, the walls of the new cells will be fragile and can easily rupture, preventing the cell from doing its fundamental work of metabolizing the sugars in the wort to produce ethanol and carbon dioxide. Without a sufficient population of healthy yeast, fermentation may stall prematurely or be incomplete, the beer may be too sweet and underattenuated and other undesirable compounds and flavors may result.
It's in the air
Where does the yeast find a source of this essential element? Fortunately the earth's atmosphere contains approximately 21% oxygen, which dissolves readily in water. However, the solubility of gases in water (and wort or beer) is greatly dependent upon the temperature. As the temperature decreases, the solubility increases. For example, at sea level and freezing (32 °F or 0 °C) pure water can hold up to 14.6 milligrams per liter (mg/L, equivalent to parts per million, or ppm) of dissolved oxygen. At a typical lager fermentation temperature of 50 °F (10 °C), the saturation level decreases to 11.3 mg/L, while at an ale fermentation temperature of 68 °F (20 °C) it is 9.2 mg/L. At boiling (212 °F or 100 °C at sea level), it is essentially zero.
As the wort is boiled, virtually all of the dissolved oxygen is driven off, resulting in extremely low levels even after it is chilled. Therefore it is necessary to supplement the wort with additional oxygen so that the yeast can reproduce properly. This is accomplished by aerating or oxygenating the wort. Those homebrewers who boil only a partial volume of concentrated wort and top off the fermenter with cold water enjoy some advantage in terms of aeration. The cold top-off water as it comes from the tap is likely to contain 3-4 mg/L of dissolved oxygen and vigorous splashing of the water can increase this.
The sugars and other solids dissolved in the wort also affect the oxygen solubility, which decreases as the specific gravity increases. This can be significant at higher wort gravities. For example, the saturation level decreases more than 3 mg/L when the specific gravity increases from 1.040 to 1.100. This is despite the fact that higher gravity worts place more stress on the yeast and require more oxygen.
Is enough enough?
How much oxygen is required for healthy yeast reproduction? There is some disagreement about this among brewing scientists, but the minimum level is considered to be 5 mg/L, and the optimum demand for oxygen by some yeast strains at higher specific gravities increases to as much as 12–13 mg/L, which can be beyond the saturation level.
Excessive oxygen can be toxic to the yeast. While this is true, it is unlikely to be a problem from a practical standpoint. Oxygen toxicity occurs at levels above that of saturation in the wort. As oxygen is added during aeration, any excess beyond the saturation level is quickly bubbled off into the air where it can do no harm.
The evidence that the yeast quickly consumes the dissolved oxygen is clear. Within only a few hours after pitching the yeast, the level of dissolved oxygen in the wort drops to nearly zero. To some extent it is possible to "pre-load" the yeast with sterols and unsaturated fatty acids, reducing the dissolved oxygen requirements. Some dry yeast manufacturers claim to enhance their yeast with these components, but there has been little research into its effectiveness. Contin-uously aerating the yeast during propagation and prior to pitching also reduces the oxygen demand; however, this is seldom practical for homebrewers who are making starters from liquid yeast.
Almost everyone agrees that wort aeration is desirable, but it is more difficult to quantify the amount. The instrument for measuring oxygen levels in liquids is a dissolved oxygen meter. Unfortunately DO meters are expensive ($500 or more), need frequent calibration and the sugars in the wort tend to foul the probe. This shortens the life of the probe and requires rather frequent replacement (adding another $150 or more to the cost). This places them beyond the means of all but the most dedicated homebrewer, and only a few larger craft breweries use such a tool.
However, the vast majority of commercial breweries do add pure oxygen or compressed air to the wort immediately after it is chilled. The most common method is to introduce the gas via an aeration stone placed in the line between the wort chiller and the fermenter. Major American lager brewers, for example, strive for a dissolved oxygen level of 9-10 mg/L in their wort.
Letting the air in
Several different aeration practices are used by homebrewers. Some merely let the chilled wort fall from a distance into the fermenter, acquiring additional oxygen from the air as it splashes. Another technique is to vigorously stir or shake the wort in the fermenter. A sanitized spoon or paddle can be used, or a paint stirrer can be purchased at a paint store or home center and attached to an electric drill. This will provide an increased level of dissolved oxygen above that of the tap water, but it is far from optimal, especially for lagers and higher gravity beers. Additionally, caution is required when using fragile carboys and other glass fermentation vessels.
Somewhat more effective is pouring the wort repeatedly between sanitized buckets from a height of several feet (1 m) or higher. It's possible to achieve dissolved oxygen levels of 6–7 mg/L via this method, but there is the risk of spilling and the fact that it should be done repeatedly (at least 5–6 times) and can be time-consuming and impractical for batches larger than 5 gallons (19 L).
An alternative is the Wortwizard (http://www.wortwizard.com), which utilizes the chilling water flow and the Venturi principle to act as both a wort pump and aerator. This device is placed in the cold water line and slightly restricts the flow. A small hole pulls in the ambient air, creating a partial vacuum that is used to draw the wort from the kettle to the fermenter and also to provide aeration. The manufacturer has not provided any data on the resulting level of dissolved oxygen.
Getting directly to the point
It is also possible for homebrewers to inject air or oxygen directly into the wort, much like commercial breweries. The goal is to increase the contact area between the gas and liquid so as to decrease the time necessary to achieve the proper level of dissolved oxygen.
Merely inserting a hose into the wort would be highly inefficient. A far better device for diffusing the gas quickly and efficiently into the wort is a porous aeration stone, which produces extremely tiny bubbles that dissolve in the wort rather than being lost into the air. Stones suitable for wort aeration have a pore diameter of 0.5 micron or less.
Inexpensive ceramic aeration stones intended for keeping live fish can be found at aquarium shops, but these should be considered disposable. They tend to crumble in the relatively acidic environment of the wort and are very difficult to clean afterward. Permanent aeration stones made from sintered stainless steel are more expensive (typically $10–25) and can be purchased from homebrew shops and suppliers. With proper cleaning and sanitation (boiling in water for several minutes is usually sufficient), a stainless stone should last a very long time. If your brewing water is hard and contains a lot of dissolved minerals, it may be necessary periodically to soak the stone in a solution of commercial lime remover.
An aquarium pump can be used as a source of air for the stone. There are several factors to consider if you use this method. The first is that the volume of air delivered by the pump is relatively small. Coupled with the fact that air is only approximately 21 percent oxygen, this means that a considerable length of time may be required to adequately aerate the wort. One hour or more per 5 gallons (19 L) of wort is typical.
Furthermore, the ambient air is hardly sterile. Airborne wild yeast, bacteria and other microorganisms can contaminate the wort — especially during warm weather and in humid climates. Therefore, it is highly recommended to filter the output of the pump. A sterile HEPA filter with an effective size of 1.0 micron, intended to remove impurities from the air for allergy sufferers and those with breathing disorders, is available at pharmacies and from some homebrew suppliers. The HEPA filter should be replaced regularly.
A rudimentary disposable filter can be made from sterile cotton balls soaked in isopropyl alcohol, enclosed in a larger diameter piece of tubing and inserted into the air line.
The pure stuff
More effective and much quicker than an aquarium pump is to use pure oxygen from a compressed gas cylinder and regulator. Hardware stores and home centers sell brazing torch kits that can be adapted for homebrew wort aeration. These are also available at some homebrew shops and suppliers. They include a small oxygen regulator that attaches to a disposable cylinder. When this is connected to an aeration stone, it is possible to adequately aerate 5 gallons (19 L) of wort in 45–60 seconds. One disposable cylinder will oxygenate approximately 40–50 gallons (150–190 L) of wort before needing replacement at a cost of $8–12.
Larger regulators and refillable cylinders are available from welding gas suppliers. If you brew larger batch sizes or relatively often, it may be less expensive in the long run to buy or lease your regulator and gas from a welding shop. An explanation is in order about the various grades of oxygen. Industrial grade is the most commonly available type and can be found at any welding gas supplier. Aviation grade has more stringent requirements for the water content and medical grade requires a prescription and more detailed recordkeeping on the part of the manufacturer and dealer.
For brewers, these distinctions are largely unimportant, for it is not cost effective to manufacture the grades differently. (All are separated from highly compressed liquid air at extremely low temperatures.) More-over, pure oxygen is a very inhospitable environment for microorganisms, so bacterial contamination is not really a problem. Because of this, filtration is not necessary as it is with air.
As might be expected, aeration of the wort tends to produce foaming, which of course is also an indication of its relative success. This is not considered undesirable, but sometimes the foam can overflow the fermenter. A commercial food grade foam control agent, available at homebrew shops, can greatly ease any problem.
A small amount of foam reducer — anywhere from a few drops to a teaspoon per batch — is effective at reducing the foam, yet it does not later diminish heading of the finished beer because it breaks down during fermentation. Many commercial breweries use foam control agent to effectively increase their fermenter capacity.
Mo' better air?
You may wonder if additional aeration is beneficial after the yeast is pitched. There is some indication that the yeast will continue to reproduce and consume oxygen for as long as 24 hours after fermentation begins. The risk to aerating this late is that not all of the oxygen will be used, and it will end up contributing to oxidation and the papery and sherry-like flavors that are the signs of staling. Some commercial breweries give their wort additional oxygen or air after a period of 12–24 hours, especially for high gravity beers. It may be worthwhile for homebrewers to do the same in cases where there is a long "lag time," that is, a period with little or no apparent fermentation activity. However, once there are signs of vigorous fermentation, it is wise not to introduce additional oxygen into your wort.
Whatever method you use to provide oxygen to your wort, your yeast is likely to be grateful for this essential element and will reward you with flavorful beer that is properly fermented and fully attenuated. Some of the best advice I can give for successful homebrewing can be summed up in this simple mantra: pitch a large population of healthy yeast and aerate the chilled wort well.
Bill Pierce is BYO's "Advanced Brewing" columnist.