Dissolved Oxygen: How Much Is In Your Wort?
By Dennis Davison (Brewing Techniques)
Most brewers recognize the importance of infusing wort with good quantities of dissolved oxygen, but how much dissolved oxygen does your method actually achieve? A prominent home brewer reviews the various aeration/oxygenation methods, presents testing methods that can be used to measure dissolved oxygen, and shares the results of his comparison of the efficiency of various methods.
As the years pass and home brewing grows in sophistication, home brewers seek out more and more information to help them make that perfect beer. The rewards of information are clear: If we can understand the chemistry involved and use it to our advantage, we might just make a Pilsner Urquell clone in our home brewery that is indistinguishable from the original.
Dissolved oxygen (sometimes abbreviated as DO) is one of the areas that, despite its importance, has generated very little data for use by home or small-scale commercial brewers. You can’t find volumes of information written on the subject, and a book dedicated to dissolved oxygen certainly won’t make it to the New York Times best seller list. This article is designed to fill the gap in the particular area of dissolved oxygen.
The Importance of Dissolved Oxygen
Why is dissolved oxygen in wort so important? The answer is found in the life cycle of the yeast. Yeast begin their fermentation activity with an aerobic stage, during which the yeast go through a respiration process. During respiration, yeast absorb the available oxygen and store it for future use. Yeast will grow and multiply very readily during this respiration phase.
The growth is what’s important to home and microbrewers. A healthy yeast crop produces a healthy, fast fermentation. The quicker the fermentation begins, the less the chance that other bacteria will affect the beer. Slow, sluggish fermentations give these bacteria opportunity to grow and produce off-flavors. Some yeast strains — forms of Pediococcus and Lactobacillus — require little or no oxygen to reproduce. Saccharomyces strains, which are the workhorses of beer fermentation, require oxygen to reproduce. In all but the rarest circumstances, dissolving sufficient amounts of oxygen into wort is of paramount importance.
To date, all articles on this subject have dealt with techniques for dissolving oxygen into hopped wort. This article addresses the question of how much oxygen can be dissolved using the various techniques available to home and commercial brewers.
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Methodology
The test data that follow are from 10 beers of various gravities, ranging from 11 °P (1.044 O.G.) to 24.8 °P (1.105 O.G.). Each beer was produced using all-grain methods and with full 7-gal boils lasting 90 minutes. The worts were chilled to 66 °F (19 °C) using an immersion chiller. The resulting wort was verified as having 0 ppm dissolved oxygen.
The yeasts used in these tests were Yeast Lab American Ale, Wyeast #3068 Weihenstephan and #1762 Belgian Abbey II, Brewtek British Draft Ale , and Schmidt #118 Lager, which gave a good cross section of the yeasts used by home brewers. Each 6-gal wort was split into two separate carboys, and each carboy was pitched with the same variety and quantity of yeast. Each yeast strain was pitched into two separate gravity batches to note any effects of gravity, yeast, and dissolved oxygen. All worts were rechecked for dissolved oxygen levels before testing oxygenation methods. Several types of oxygenators and several stones were used on each split batch. All tests were performed at 66 °F (19 °C).
I found no major differences in obtainable dissolved oxygen levels based on wort gravity. Further research remains to determine optimal levels of dissolved oxygen within beer. DeClerck states that levels of 6.4–7.8 mg/L (ppm) of oxygen are adequate for a well-aerated wort. From the initial data, I have found that this value range is reasonable and obtainable for most brewers. Yeast strains may play an important role in what the desired level of oxygen should be, and future experimentation should deal with this question as well. Wort composition (with trub or with trub removed, with spent hops or without) may also play a role in how much oxygen is dissolved. The worts used in this study contained minimal amounts of trub and spent hops.
This article provides a range of dissolved oxygen readings obtained using some of the most commonly used methods.
Aeration Methods
The methods used to dissolve oxygen into wort span a range that I call low-tech, mid-tech, and high-tech.
Low-tech methods: One typical low-tech method for diffusing oxygen into wort is the simple splashing technique every home brewer started out using. Just siphon the wort into your fermentor and splash it around. Variations on this theme have been devised, including tubes with holes in the sides that suck air into the wort as it passes and tubes with obstructions at the ends to force the wort to spray in various directions, giving it a larger surface area and thus allowing it to absorb more air as it splashes. Low-tech methods also include the simple bunging and shaking of a carboy. These methods require little more than imagination or a modicum of spare equipment and cost the brewer next to nothing.
The data show that levels as high as 7 ppm can be obtained using these methods, but to obtain such high levels requires the use of several of these methods in combination. The only disadvantage to these low-tech methods is that you run a chance of contaminating your beer with whatever airborne bacteria may be present in your brewery.
Mid-tech methods: Mid-tech methods have sprung up out of home brewers’ resourcefulness. One of these mid-tech methods is the use of aquarium pumps and diffusing stones. A variation on this approach includes the use of a scuba tank as a source for the air to be pushed through the stone. Both of these methods use normal atmosphere, so you are back at the mercy of airborne bacteria unless you use an in-line biological filter to capture contaminants before they make their way into your wort.
Mid-tech methods also can result in levels of 7 ppm, and they require minimal investment or resourcefulness to obtain the necessary items.
With the wide variety of diffusing stones available to home brewers today, I was unable to test everything on the market, but I selected a cross section of what’s available. Figure 1 shows the collection of diffusers I used for the present study. The accompanying box provides more background on diffusers and their use in brewing applications.
High-tech methods: The most efficient way to dissolve oxygen into wort is to use pure oxygen and a diffusing stone. Currently, two manufacturers supply such systems for home brewers. This type of system might be more costly than an aquarium pump, but the results you can obtain are far superior. With aquarium pumps, the air you inject is only 21% oxygen; with a pure oxygen setup, you inject 99% pure oxygen, or 4.7 times as much oxygen by volume.
Methods for Measuring Dissolved Oxygen
Several methods are available for measuring dissolved oxygen in wort. The simplest method for home brewers is a colorimetric test sold in aquarium stores or obtained from companies that use the test to measure dissolved oxygen levels in boilers. Colorimetric test kits are available in several varieties that involve the use of drops of different solutions in a premeasured vial or an ampoule kit. One of two chemicals is used in this kit — either Rhodazine D, which creates shades of pink, or Indigo Carmine, which creates shades of blue.
I have experimented with both and have found the ampoule-type kits with Indigo Carmine to work the best for reading dissolved oxygen levels in wort. The blue shades are not natural in beer and can be easily distinguished; the pink tones seem to blend more readily into a beer’s color. The vial tests consist of a small premeasured vial in which you place the liquid for testing. You then add a certain quantity of either Indigo Carmine or Rhodazine D to the vial. This method requires you to add perfect drops, and occasionally you get a drop with a large air bubble, leaving you wondering whether you added the proper amount of agent. Ampoules are much easier to handle. These vacuum tubes contain the agent in the tube. All you have to do is snap off the tip of the tube in the solution that’s going to be checked. The vacuum within the vial draws liquid inside, and the liquid changes color in response to the dissolved oxygen content; the higher the concentration, the deeper the color. The vial or ampoule is then compared to a color standard. These standards are similar to a paint card with varying shades.
| Table I. Comparison of Various Racking and Splashing Methods of Wort Aeration | ||||||||||
|
| Gravities | |||||||||
| Aeration Method | 1.044 | 1.045 | 1.05 | 1.051 | 1.054 | 1.065 | 1.069 | 1.071 | 1.074 | 1.105 |
| Splashing, with no agitation |
| 0.7 |
| 2.5 | 2.0 |
| 0.9 | 1.4 |
| 0.8 |
|
|
|
|
| 2.5 |
|
|
|
|
|
|
| Splashing, with additional agitation |
| 1.5 | 3.4 |
| 2.0 |
| 2.2 | 2.5 |
| 1.8 |
|
|
|
| 3.4 |
|
|
|
|
|
|
|
| Shaking for 1 minute | 6.4 |
|
| 4.5 | 5.4 |
|
|
|
| 3.5 |
| Shaking for 2 minutes | 7.0 |
|
| 6.0 | 7.2 |
|
|
|
| 6.2 |
| 12-second burst of pure O2 at 10 psi | 3.5 |
|
| 8.5 | 5.8 |
|
|
|
|
|
| 24-second burst of pure O2 at 30 psi | 5.5 |
|
| 14.0 | 13.2 |
|
|
|
|
|
| 45-second burst of pure O2 at 30 psi |
|
|
|
| 18.9 |
|
|
|
|
|
| Aerating tube with holes, additional splashing | 1.4 |
|
|
|
| 1.6 |
|
|
|
|
|
|
|
|
|
|
| 1.6 |
|
|
|
|
| YCKC diffuser with aquarium pump for 25 minutes |
| 3.9 |
|
|
| 2.6 |
|
|
|
|
| YCKC diffuser with pure O2 at 10 psi for 20 seconds |
|
|
|
|
| 3.4 |
|
|
|
|
| LB #2 stone with aquarium pump for 20 minutes |
| 1.5 |
|
|
| 4.7 | 2.8 |
|
| 6.9 |
|
|
| 2.9 |
|
|
| 7.0 | 3.4 |
|
| 7.3 |
| YCKC diffuser with pure O2 at 10 psi for 45 seconds |
|
|
|
|
| 2.6 |
|
|
|
|
| LB #1 stone at 10 psi for 20 seconds |
|
|
|
|
| 5.7 |
|
| 5.3 |
|
|
|
|
|
|
|
| 9.0 |
|
| 9.7 |
|
| LB #1 stone at 10 psi for 30 seconds |
|
|
|
|
| 13.7 |
|
|
|
|
|
|
|
|
|
|
| 16.9 |
| 9.8 |
| 8.7 |
| Level of DO after 1 hour with yeast present |
|
|
|
|
| 13.2 |
|
|
|
|
| Gulfstream with pure O2 at 10 psi for 30 seconds | ||||||||||