Dissolved Oxygen: How Much Is In Your Wort?

10/30/2015

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.

 

---

 

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.

 

To browse our extensive selection of liquid and dry yeast, starters, and nutrients, click here!

 

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			11.6					9.5	9.2
			10.9
			17.4					16.4	18.1
Racking tube with holes, no additional splashing	0.9
10-second burst of pure O2 at 20 psi	7.8						10.5	17.5
Wort aerator, no splashing									0.8
Wort aerator, with splashing									1.5
Scuba tank air, for 20 seconds		2.0					2.4
		4.4
Scuba tank air, for 60 seconds		4.1					3.1
		5.1
		5.1					4.3
		5.9
		6.4					5.8
		7.1

 

The Skinny on Stones

Although diffusing stones offer a great potential for infusing oxygen into wort, they generally require sufficient pressure to make them work properly. Not all of the stones can be used with an aquarium pump. Less-expensive aquarium pumps produce ~2 psi pressure, and these pumps can’t push enough air through stones that are smaller than 4 microns. The Gulfstream and Liquid Bread Stone #1 both require ~3 psi to work. A stone’s pressure and micron rating, however, is not generally the deciding factor in working at lower pressures. What is important is the “breakaway point,” which is defined by the fineness of bubble formed and passed through solution. Some smaller-micron stones can be used with aquarium pumps, but none were used this way during the series of experiments outlined in this article. Microbreweries and brewpubs generally use a direct in-line injection system. I know of one microbrewery that uses the Gulfstream stone fitted into a T fitting that comes off the heat exchanger so that the cool wort is injected with oxygen as it is pumped into the fermentor. Stones that are more porous are less efficient. They allow large bubbles to pass through the wort and escape to the surface, carrying off a good portion of the oxygen with them. The stone from American Science and Surplus is just too porous. As a good rule of thumb, if it looks like 70-grit sandpaper, stay away from it. Ideally, the surface should look like fine emery cloth, or just-polished metal.

 

I used a kit produced by CHEMetrics for comparisons with the dissolved oxygen meter. You will need to obtain a selection of colored gelatins, available from a camera store or theatrical supply company. These transparent gelatins should have light-golden to brown hues. The proper gelatin to use depends on the color of your beer — which is based on the volumetric size required for the test kits. In most cases, a 10 °SRM beer color will only look like 4 or 5 °SRM because of the reduced volume (generally less than 1 oz). If you wish to try this measurement experiment, brew a light-colored beer.

The colorimetric tests were not designed for use with wort, and the readings can only be approximations. The best way to measure dissolved oxygen levels is to use a meter.

Although by far the easiest method, using a dissolved oxygen meter is also the most costly. The market offers table-top and hand-held electronic meters that start at about $ 600. For a microbrewery or brewpub, that kind of investment for your lab is well worth the cost. The meter that I obtained was the Hanna HI 9142.

The meter’s probe has a membrane that covers a polaro-graphic sensor and has a built-in thermistor for temperature compensation. A thin, permeable membrane isolates the sensor elements from the testing solution, but allows oxygen to enter. When voltage is applied across the sensor, oxygen that has passed through the membrane reacts, causing the current to flow and thus producing a digital reading.

According to the instruction manual that comes with the meter, a flow of liquid is required to replenish the liquid that contacts the membrane. This membrane in idle solutions will allow all of the oxygen surrounding it to be absorbed through itself. Once absorbed, the area around the membrane will have a reduced volume of oxygen, giving lower readings on the meter.

The meter was used exclusively for the test data supplied, while the colormetric tests were used only to check their reliability against the meter. I drew 500-mL samples into a graduate, immersed the probe, and then raised and lowered the probe to ensure an adequate, even sampling (stationary probes will display a lower ppm count).

I checked this sampling method to see if additional oxygen was being introduced. First, I moved the probe up and down in the graduate to obtain a reading. Once a stable reading was recorded, I left the probe idle for 3 minutes, after which time the readings dropped by about 1 ppm. By moving the probe up and down in the graduate again, the readings stabilized at the first figure after 30 seconds and remained stable for up to 3 minutes with the same slow movement. I found artificially elevated readings only when the probe was splashed in the graduate, which caused air bubbles to rise through the solution. Slow, gradual movements are the key to accurate readings.

My first measurement experiments used the colorimetric test. After I purchased the meter, I was able to verify that the colorimetric test data were accurate to within 1 ppm of the readings obtained using the digital meter. The colorimetric test requires an eye for color, but satisfactory results can be achieved.

 

Safety Issues

Several safety issues arise when dealing with the substances used in this article. Pure oxygen: The use of pure oxygen is a major concern. Oxygen is a catalyst — it will not burn on its own, but provides open flame the nourishment it needs to fuel the fire. Always use pure oxygen in a well-ventilated room with all flames extinguished (cigars, cigarettes, and so forth). It is best used outdoors, where furnaces and hot water heaters will not be affected by its presence. Electrolytes in meters: If you use a dissolved oxygen meter, the electrolyte in the probe should not be placed into contact with the main volume of the beer. This electrolyte can be dangerous at best. All beer samples used to measure dissolved oxygen should be discarded after use. Gas cylinders: Always use extreme care when handling or transporting gas cylinders. Any charged gas cylinder is a potentially dangerous projectile. Never leave a gas cylinder standing on end unattended. If the cylinder falls and breaks off at the neck, you’ll have a hole to patch if you’re lucky; if you’re unlucky, you’ll be dead. Always store cylinders strapped against a wall or lying on their side, well propped so as not to roll.

 

It is important to note that dissolved oxygen meters, like pH meters, need to be calibrated. Calibration varies depending on the manufacturer. Calibration in essence requires reading a solution known to be at 0 ppm (sodium sulfite). Once you calibrate to 0 using one of the two potentiometer pods, hold the dried-off probe in the air while holding down the Cal button. Adjust the other potentiometer pod to a factor determined by your elevation. I calibrated the meter fresh out of the box and verified the calibration before each use.

 

Test Results

 

The rack-and-splash method: All tests were performed on 3 gal of wort in a 5-gal carboy. The first dissolved oxygen reading was taken after each was racked into carboys using the old standby method of racking and splashing. This was the most unreliable of all the tests because it depended on the amount of splashing achieved.

In these tests, I obtained readings of 0.7–3.4 ppm (Table I). The 0.7 ppm reading came from just racking and letting the wort splash against itself in the bottom of the carboy, whereas the 3.4 ppm reading was from constant vigorous shaking of the racking tube. As Table I shows, the use of aerating tubes with holes or other spraying devices seemed to make no difference. The results fall within the same range as the results obtained without these devices.

The shake, rattle, and roll method: The next most common method of aeration is the standard shake, rattle, or roll. Though this method does not yield consistent results, it does increase dissolved oxygen levels significantly. The idea is to have as much headspace as possible in the carboy. If you filled a carboy to the brim, the small volume of air left in the headspace will yield little if any dissolved oxygen in the wort. A larger head-space, however, provides enough air to get reasonable oxygen dissolution in the wort. Since these tests were performed on 3 gal of wort in a 5-gal carboy, the use of a 6.5-gal carboy with 5 gal of wort would require at least double the shaking time shown in the data. Four-minute shaking times should give an adequate level (around 7 ppm).

 

Table II: Dissolved oxgyen Levels Using Scuba Air*
Time (seconds)	DO level (ppm)
20	0.2
80	0.9
140	2.1
200	3.6
*Scuba tank with 5 gal of wort at 10 psi.

 

The method I used was to bung the carboy with a solid stopper and rock the carboy on edge for periods of 1–2 minutes. Results varied, but they did show a pattern: 1 minute of shaking produced oxygen levels of 3.4–5.0 ppm; 2 minutes of shaking yielded 5.2–7.2 ppm (Table II). This method also showed great variance, however. These data suggest that splashing the wort vigorously while racking and following up by shaking the carboy for 2 minutes will provide satisfactory levels of dissolved oxygen for the yeast.

Aerating with diffusion stones: As mentioned earlier, not all stones will work with a standard aquarium pump. If you plan on purchasing a pump, get the biggest pump you can find. Based on the initial data I collected, results are not promising when using standard pumps. If you use this method of aeration, you might just want to rethink your techniques.

The stones tested were the Gulfstream stone, Liquid Bread #1 and #2 stones, the generic stone from American Science and Surplus, the two aquarium stones, and the Yeast Culture Kit Company diffuser (Figure 1). After running air through the stones at 2 psi under 3 gal of wort in a 5-gal carboy for 20 minutes, I recorded levels of 1.0–2.6 ppm. The diffusing stones gave the worst performance. The best performer was the plastic diffuser from the Yeast Culture Kit Company.

The aquarium pump method using standard pumps just doesn’t cut it. At the levels obtained in these experiments, you’d need to aerate for 1–2 hours for just 3 gal, or 1 hour 40 minutes to 3 hours 20 minutes for 5 gal, to obtain the required levels. This is unacceptable to me and to most brewers. Because you will need to turn the pump off when foaming begins to fill the carboy, your times can be increased by at least another hour while you wait for the foam to subside. High-pressure pumps can reduce the time required. I have noticed that as air pressure increases, the amount of oxygen dissolved also increases (see Table III).

If you plan on using an aquarium pump or compressed air, you will need to prefilter it to remove airborne bacteria that may decide to ferment your wort. From the standpoint of contamination risk, the aeration stones are just about as good as shaking the carboy.

Using a scuba tank for a compressed air source, though, makes the stones much more effective. Scuba tank air is prefiltered compressed air and contains 21% oxygen, just like our atmosphere. Prefiltering removes oil from the air, but not airborne contaminants. Most of these contaminants are not dangerous to humans, but they are dangerous to beer. A biological filter should be placed in-line to remove anything that can contaminate your beer.

Using various diffusing stones and a scuba tank as an air source — run at 10 psi for a duration of 3 minutes in a 5-gal wort — I achieved dissolved oxygen levels of 3.2–4.0 ppm. These levels are much better than when the stones were used with a standard aquarium pump.

 

Table III: Pure Oxygen with Various Stones, Pressures, and Durations
	Time (seconds)
Pressure (psi)	10	20	30	40	60	120
10	3.5*	5.5	8.4	12.2	17.3	16.8
10	3.3	4.5
10		3.4
20	7.8
30	5.8	12.3
*Data is in parts per million.

 

Scuba tanks do get expensive, however, and you probably don’t have one sitting in the closet. And scuba tanks do not solve the problem common to all stones — foam formation in the carboy’s headspace. If you don’t provide an adequate headspace in the fermentor, you’ll have to do the 3 minutes of aeration over short intervals to allow the head to subside.

Oxygenation: Oxygenation, as opposed to aeration, seems to be the most efficient method. Microbreweries and brewpubs use this method, so why not home brewers?

Two companies that sell almost identical systems for home brewers are Liquid Bread, with its Oxynator and Gulfstream, with its Xygen. These are complete systems; each comes with a stainless diffusing stone, a 99.9% pure disposable oxygen tank, and a regulator. I have since moved up to a 6-lb hospital oxygen tank, but I performed the first tests in this study using the Oxynator.

According to the directions, you are supposed to give two 20–30 second bursts on a low setting to dissolve enough oxygen into the wort. My first problem with this was that since the regulator has no gauge, what is considered low? Figure 2 shows a little calibration strip that I created to gain consistent results. With a calibration strip like this, you can see approximately how many pounds per square inch you are injecting. Mark your knob at the hose connector when the regulator is turned off. As you turn the knob, remember which full turn you are in. The first full turn will not release any oxygen. When you turn the knob another two-thirds of a turn, oxygen will come out at 5 psi. With the valve fully opened, oxygen comes out at 30 psi.

 

Aeration Equipment Manufacturers
Brewers Resource 409 Calle San Pablo #104 Camarillo, CA 93012 Order line: 800/827-3983 Information: 805/445-4100 Fax 805/445-4150 Gulfstream Brewing Products 6331A Woodville Highway Tallahassee, FL 32311 Tel. 904/421-6902	Liquid Bread Inc. 1007 La Quinta Drive Orlando, FL 32809 Tel. 407/888-3533 Fax 407/888-3531 URL:https://www.pmark.com/Ibread The Yeast Culture Kit Co. 1308 W. Madison Ann Arbor, MI 48103 Tel. and fax 1-800-742-2110 e-mail: yckco@aol.com URL: https://oeonline.com/~pbabcock/yckco.html

 

Initial tests varied. Pressure probably fluctuated between 20 and 30 psi before the addition of a second gauged regulator and the development of the calibration strip, so they will not be used as controls. Nevertheless, with two 30-second bursts in 5 gal of wort I recorded readings of over 11 ppm. Then I set the pressure at 10 psi. With one controlled burst for 30 seconds in a 5-gal batch, I obtained readings of 3.1–4.0 ppm. After a second 30-second burst, I recorded a grand total of 6.0–8.0 ppm (Table III).

 

Other Findings

 

Using pure oxygen, I obtained readings well over 19.0 ppm. The most noticeable difference in fermentation was that worts with levels greater than 10 ppm of dissolved oxygen would start fermenting later than worts with levels around 7 ppm. This can be easily explained: The wort with higher levels of dissolved oxygen has a longer aerobic stage while it absorbs the greater quantity of oxygen. Once these more oxygen-rich worts started to ferment, however, they would ferment much more vigorously than the worts with lower oxygen levels.

Higher dissolved oxygen levels will also reduce the flocculation capacity of your yeast; the yeast in these experiments stayed in suspension longer when the worts started with higher dissolved oxygen levels. Though this has not been scientifically analyzed, I might assume that the increased yeast growth during the aerobic stage and limited amounts of calcium available per yeast cell are factors in this phenomenon.

Attenuation and ester production in worts of the same gravity but differing levels of dissolved oxygen were equivalent. Comparing beer made from worts with dissolved oxygen levels in the 6.0–8.0 ppm range and those made from worts with levels greater than 12.0 ppm, the flavor characteristics were indistinguishable. The reduced flocculation of yeast at 65 °F (18 °C) did give the beers made from the higher dissolved oxygen levels a yeasty flavor, but cold conditioning and extended aging eventually flocculated the yeast.

Fermentation starting times of four hours for most ales at an average homebrew pitching quantity of a 1-L starter were quite common when using a pure oxygen method. Once you have tried pure oxygen, you’ll never want to go back to those antique techniques of aeration. If you use a hospital oxygen tank and tank and regulator that show flow rates in liters per minute, a 1-L flow is approximately equivalent to 15 psi.

Several other devices have recently hit the market: a no-foaming stone from Brewers Resource and a 0.5-micron stone from Gulfstream. Both of these items will require further investigation and study.

 

Final Observations

 

Dissolved oxygen meters are useful not only for measuring the oxygen content of wort but also for measuring the effluent oxygen levels of your brewery. As government begins to concern itself with biological oxygen demand (BOD) in the effluent in many areas of the country, these meters will enable you to check your effluent to see if it does have a high demand for oxygen. We need to save the planet for future generations, and if we can do our part from the start we won’t suffer higher taxation on beer-related products to bring the environment back to normal.

Home brewers, in general, underpitch yeast. Most starters from liquid yeasts on their own cannot provide a large healthy crop of yeast for robust fermentation of a 5-gal batch. Higher levels of dissolved oxygen in starters and in wort, however, will help increase the production of yeast and get your fermentation going more quickly. If you look at the volume of yeast pitched in microbreweries, home brewers would need to pitch close to 200 mL of slurry — not starter, but pure yeast — to reach the same saturation point in a 5-gal batch.

Related Products

All contents copyright 2024 by MoreFlavor Inc. All rights reserved. No part of this document or the related files may be reproduced or transmitted in any form, by any means (electronic, photocopying, recording, or otherwise) without the prior written permission of the publisher.