Aerating With Oxygen

Before jumping into the mechanics of oxygenation, I want to touch on oxygenation versus aeration. Yeast require oxygen to grow since oxygen is a component of healthy cellular membranes. When brewing fermentations are lacking in oxygen, fermentation rate, yeast health, and beer flavor are all affected. The simplest and cheapest way of adding oxygen to wort is through aeration, since air is comprised of 21% oxygen. The primary challenge with this method is that the solubility of oxygen from air is about 8 ppm (8 mg/liter) in 12 °Plato wort and begins to drop as wort gravity increases. This is not a major problem up to about 18 °Plato if you have a good aeration method and plenty of healthy yeast. For these reasons, many brewers prefer using oxygen instead of air as the source of oxygen when brewing higher gravity brews.

One major difference between aeration and oxygenation is that the latter requires more control because the aeration method cannot result in too much oxygen in wort, but using pure oxygen can. Practical experience from brewers who routinely oxygenate wort demonstrates a range of real issues with excess oxygenation. Fermentations often begin vigorously with lots of yeast activity and growth, but end up stalling before fermentation ends. And yeast harvested from these batches has lower viability and vitality compared to yeast from batches with less oxygen going into fermentation. Beer aroma is also affected by wort oxygen levels — with increased sulfur production and reduced ester production being two flavor notes associated with increased oxygen. To complicate the discussion, all of these factors are yeast strain-dependent. The bottom line is that yeast need oxygen during the early stages of fermentation and more problems result from insufficient oxygen than too much.

I have used the sort of set up you describe to oxygenate yeast during propagation and think I can give you some helpful pointers about this method. I totally agree with the idea that oxygen should be delivered at a low flow rate. This really does two things for you. The first is that the low flow rate, especially when put through a small diffusion stone, can result in nearly 100% transfer of the oxygen into solution. I will get back to the significance of this in a moment. The other practical result of oxygenating at a very low flow rate is that you can more easily time and control the oxygenation process, where small variation in oxygenation time have little effect on the process. I don’t have any data comparing the performance of 0.5 micron stones to 2 micron stones, but believe based on the availability of sintered stones intended to facilitate gas diffusion that pores in this size range work well for the application. The system I built for small scale yeast propagation (30 gallon/114 L batch sizes) used a 2 micron aeration stone.

So let’s begin with the type of regulator. The regulator I purchased for my project was a medical-grade regulator made by Victor, with a regulated flow range from 16 mL/min. to 500 mL/min. The advantage to producing a very slow and steady gas flow through porous stones is that the small bubbles release from the surface of the stone and flow as small bubbles into the liquid mass. If the flow rate is too great, the bubbles have a tendency to coalesce. This phenomenon occurs when two bubbles bump into one another and form a large bubble. This process can rapidly repeat, especially if there is a flooded effect on the stone surface. On a macroscopic level, coalescence leads to large bubbles that float through the liquid and escape at the surface. This process can be seen when boiling water in a pot. Very small steam bubbles adhering to the bottom of the pot gather with other small bubbles to form larger bubbles and the steam bubble rises through the pot and creates turbulence as the bubbles rise and burst at the surface. So what is the big deal with coalescence?

The purpose of wort oxygenation is to dissolve oxygen into wort. If small oxygen bubbles coalesce and rise to the surface of your fermenter, this means that the gas transfer process is inefficient. Although the inefficiency is not going to break the bank, it does make process control difficult because you don’t know how much gas dissolves into the wort unless you have a dissolved oxygen meter laying around. This brings up a fundamental question; how much oxygen is needed? I will skip the subject of determining what works best for a given beer type, yeast strain, or fermentation method and use an easy to manipulate target of 10 ppm (10 mg/l) oxygen. This is right in the middle of the range typically used in breweries.

To make this easy I will use a nominal batch size of 20 liters (about 5 gallons) to determine the amount of oxygen that is desired in the wort following oxygenation; and that amount is 20 liters x 10 mg/liter or 200 mg (0.2 grams). The molecular weight of oxygen is 32 grams per mole, so 0.2 grams is equal to 0.00625 moles. One mole of an “ideal gas” occupies 22.4 liters (at standard temperature and pressure), and this tells us that 0.00625 moles is equivalent to about 0.14 liters. So the target total volume of oxygen dissolved in this 20-liter wort volume is 0.14 liters or 140 milliliters (or 7 mL of oxygen per liter of wort). To scale up based on volume just multiply 7 mL/L by wort volume, or to scale up by desired oxygen content, scale up from 10 ppm.

OK, so here is the major assumption in this discussion: All of the oxygen that flows through your gas regulator and into the wort actually dissolves into the wort. This is the really nice thing about oxygenating with oxygen as opposed to aerating with air. Air contains about 79% nitrogen and you will always have undissolved nitrogen bubbles escaping wort, making it difficult to determine what is happening with the oxygen. When using oxygen you don’t want to see bubbles making it to the top of the fermenter. This is hard to do with wort, but you can tune your system with water. Remember, coalescence is not the idea and the desired result is a slow flow of small bubbles leaving your stone that disappear before rising to the surface. The truth of the matter is that there will be some loss in this process, but not much if the bubbles are very small.

So let’s return to the control of this process. The target is 140 mL of oxygen in your 20 liter batch, and you have a regulator with flow controller that is able to be dialed back to 16 mL/min. If you set this at 20 mL/min. and run for 7 minutes you will have the dose required. Add in some inefficiency and your target oxygenation time is somewhere in the 8-10 minute range. Like everything in brewing, you need to dial this in based on what actually works for your process. I hope this information is useful!

Response by Ashton Lewis.