Oxygen Quality

There is one very major distinction separating bottled carbon dioxide from bottled gases like oxygen and air, and that is the primary use of the gas. While bottled carbon dioxide has a wide number of industrial-type uses, bottled oxygen and air are sold specifically for use in respiration systems in addition to use in industrial applications. The cleaning requirements, or lack thereof, of these industrial users is far different from those of the buyers of breathing gases. This is why the valves on oxygen and compressed air bottles intended for use in respiration systems are physically different than valves found on cylinders used, for example, in welding. Suffice to say, it is easy to obtain bottles of oxygen and compressed air to use for homebrewing without having to worry about the gas or cylinder.

But this does bring up a good brewing question. What are the practical differences between compressed air and oxygen in the brewery? The one difference that really matters to me is how the source of oxygen affects oxygen solubility. When air is used as the oxygen source in brewer’s wort, the solubility of oxygen is about 8 mg/liter or 8 ppm (based on 12 °Plato wort). Changing the oxygen source to pure oxygen increases the oxygen solubility by approximately fourfold. This dramatic difference can negatively affect yeast membranes.

Aside from yeast health, wort oxygen influences the production of nearly all yeast-related aroma compounds present in beer. On paper, one can develop a convincing biochemical argument demonstrating that too much oxygen results in beer that is “too clean” and lacking in aroma, especially those associated with esters. And based on practical experience some brewers find that too much oxygen results in fermentations that stall after a very vigorous start that also tend to throw off lots of acetaldehyde and sulfur. The bottom line is that wort oxygen level plays a very key part in beer flavor development, and is one of those factors that varies with yeast strain and wort properties.

Brewers who successfully use pure oxygen control its delivery into the wort stream during cooling using a variety of methods. The most common method is metering oxygen based on wort flow rate in order to deliver a measured amount to the batch. As long as the oxygen is introduced as very small bubbles, it is safe to assume that nearly all of it is absorbed into the wort. This assumption makes it easy to meter oxygen with a flow meter into a known volume of wort to yield a concentration without verification with an expensive, and arguably unnecessary, oxygen meter.

Pure oxygen can really help out when brewing higher gravity beers because the solubility of oxygen from air diminishes as wort gravity increases (for more information on this topic, read up on Henry’s Law and how gas solubility in liquid is influenced by the concentration of other gases). Aside from these types of beers, however, I prefer using compressed air for wort aeration because wort simply cannot be over-oxygenated using this method, making aeration one less detail that can go wrong during the brew day.

At Springfield Brewing Company we use dry, oil-free compressed air from an air compressor to supply our wort aeration panel. This panel has a pressure regulating valve, gas rotameter for flow measurement and control, and a sterile filter. I believe the latter feature is pretty important to the practical brewer because compressed air can contaminate wort. And the suitability of a gas for breathing purposes has nothing to do with this particular concern. Since pure oxygen is toxic to most forms of life, I don’t get too concerned about oxygen as a potential source of contaminants. So if you decide to use compressed air, consider an in-line sterile filter.

 

Response by Ashton Lewis.