This post was last updated on April 20, 2020.

As with any other beer ingredient, carbon dioxide (CO2) quality is essential to finished beer quality, contributing to sensory outcomes, beer foam, mouthfeel, and shelf stability. The quality of CO2 is generally managed by the supplier. Brewers have a role in ensuring their CO2 supply is free from contaminants, and appropriately handled in the brewery.

Gases like nitrogen, oxygen, and argon are typically sourced from “air,” but commercial CO2 is generally collected as a by-product from various chemical industries, such as ethanol production, fuel combustion, natural gas production, and chemical synthesis. Over time, a supply shortage in one industry may result in new feed gas sources for carbon dioxide; contaminants and quality can vary based on those sources. Because freight costs can be high, CO2 supplies are customarily relatively local to their customers, and diverse sources are constantly being evaluated for sustainable supply reasons. CO2 manufacturers should routinely test feed gas sources for compositional changes, as well as monitor the quality throughout the production process. Storage and trans-fill depots should also have purity monitoring systems to screen incoming lots for quality.

Standards of purity are controlled by various regulatory bodies such as the Compressed Gas Association (CGA), International Society of Beverage Technologists (ISBT), European Industrial Gases Association (EIGA), and the FDA in the U.S. For instance, the EIGA states that Food Grade CO2 should be at least 99.9% pure and that “each facility producing carbon dioxide for the food and beverage industry should have a documented system for quality management” and “a formal assessment of food safety risk, including the raw gas process and feedstock, using the HACCP methodology is a legal requirement and shall be implemented at all plants producing carbon dioxide for use in foods.”

Brewers may first become aware of a CO2 supply or quality shift through sensory panel results, or in extreme cases, based on customer feedback. In the U.S., beverage grade CO2 will be at least 99.90% pure; many other molecules can comprise the other 0.10%, including water, oxygen, and hydrocarbons such as benzene, acetaldehyde, and other sensory active molecules. To put that into perspective, 0.10% equates to 1,000 parts per million, which is 1-4 orders of magnitude higher concentration than many flavor-active hop components. So understanding your CO2 supply, source, and purity is important for ensuring the quality of your beer brands.

Managing CO2 Quality

Working with Suppliers:

Communication with your CO2 supplier is key to forecasting CO2 supply risk, as well as quality.

• For carbonated beverages, specify and use either ISBT Purity Grade (“beverage grade”) CO2 or Food-Grade CO2.
• Suppliers should provide a certificate of analysis (COA) upon delivery of CO2 to document that the actual lot meets the required purity specifications.
• Periodically audit the quality management practices of your CO2 supplier and request that they verify their quality through an independent ISO-certified lab.
• Ask your supplier what proportion of their CO2 supply derives from ethanol production, petrochemical production, or other sources (this may allow you to assess supply risk).
• Ask your supplier what steps they are taking to ensure that beverage grade CO2 quality and quantity requirements will be met in the event of supplier feedstock source changes.

Additional Resources:

• Guidelines for CO2 Purity; Table of Potential Impurities Based on Industrial Source
• Why the Grade of CO2 Gas You are Using is Important
• Elements of a Certificate of Analysis
• FDA Code Related to CO2 Purity and Use as a Food Substance

Managing Deliveries:

Brewers receiving high pressure cylinders can request an actual COA (rather than a typical COA) to document purity of the CO2 being delivered. Brewers receiving bulk CO2 to refill large receivers may face additional and different issues, and can engage their suppliers in dialogue to understand supplier efforts to ensure beverage grade purity is retained at the time of delivery:

• Inquire about routine delivery truck tank maintenance.
• Inquire about supplier steps to guard against cross contamination of beverage grade CO2 across different customer types.
• Bulk CO2 tanks should not be allowed to run out of CO2 and bleed dry at any time, otherwise oils and headspace gas may contaminate CO2 lines.

Preventative Maintenance:

• Liquid CO2 transfer lines and compressor oils should be compatible with liquid CO2 and food grade; hoses from feed storage tanks must be carefully chosen to avoid leaching of plasticizers which could come in contact with beer.
• Users of mini or large bulk storage tanks should periodically test their liquid CO2 for buildup of non-volatile residues: Maintaining CO Quality in Bulk CO Storage Vessels.
• Consider in-line filtration of your CO2 to scrub unwanted chemicals, aromas, and moisture: Brewery Case Study: CO2 Gas Purity and Filtration.
• CO2 gas supply lines in breweries and cellars should never have condensation or standing liquid in them.

Additional Resources:

• Safe Carbonated Drinks: Cheers to Improved Quality, Production of CO2
• Carbonation Demystified – Brewers Association 2011 PowerHour (CO2 Supply Chain Purity content begins at the 40:25 mark)
• Duffell, C. Beverage Appearance and Flavor Protection from Carbon Dioxide Quality Excursions. Technical Quarterly Master Brewers’ Association of the Americas. Volume 42, number 1, 2005.

Sensory and Quality Testing:

• Sensory testing (easy, inexpensive): slowly bubble CO2 through sanitary brewing liquor for a short time, then assess against untreated liquor for attribute and/or difference testing via sensory panel. Never inhale CO2 directly.
  • This CO2 sensory method is shared thanks to, and with special permission from, Bill Simpson.
• GC-MS CO2 Analysis: Contaminants found in CO2 that can impact quality are typically found in the parts per million (ppm) or parts per billion (ppb) range and require highly sensitive instruments for detection. Third party labs can perform gas trace contaminant analysis, in the absence of a GC-MS in a brewery lab.
• Duffell, C, Scrafton, R and O’Leary, S. A novel analytical method for determining the quality of retail beverage gases. Technical Quarterly Master Brewers Association of the Americas. Volume 43, number 3, 2006.