Scavenging Oxygen

Oxygen is a problem for beer at all stages of the process following the early stages of fermentation because many of the flavor-active compounds created by yeast are changed, i.e., chemically oxidized, when exposed to oxygen. Unfortunately for us brewers, these oxidation reactions are very fast and many of the products of oxidation are easily detected. It’s also a bit of a bummer that yeast typically do not mop up oxygen faster than it can react with beer flavor compounds.

When I was a young brewing student in the early 1990s, the most common method used to measure air in packaged beer was with a Zahm and Nagel headspace gizmo. These setups have a piercing device that allows for the measurement of pressure and temperature in bottles and cans; pressure and temperature are then looked up in a Zahm and Nagel chart to determine how much carbon dioxide is dissolved in the beer. The Zahm and Nagel device can be equipped with a separate apparatus that allows gas to literally be shaken out of the beer sample where it flows into a special glass burette filled with sodium hydroxide. Carbon dioxide is consumed by the sodium hydroxide but nitrogen and oxygen (not much present in beer because of its reactivity) remain in the burette and are measured as headspace air.

In the early ‘90s, it was not uncommon for packaged beer to contain 0.5 mL, and often more, of headspace air. This equates to about 400 parts per billion (ppb) of dissolved oxygen (DO). Today, brewers become concerned when their package DO is above ~30–50 ppb of DO. Packaging technology has made enormous strides over the last 30 years, as have methods used to measure DO.

I tell this story because homebrewers have become hyper-concerned about oxygen because commercial breweries are hyper-concerned with DO. Our concerns as homebrewers are like those of commercial brewers, except for one main difference; homebrewers don’t have to worry about our beers sitting in some unknown place at some unknown temperature for extended time periods waiting to be picked from the shelf. I think it is very important for homebrewers to keep that fact in mind when worrying (or not worrying) about certain brewing details. 

Empirically, it does seem that bottle-conditioned beers taste oxidized less often than other beers. This observation is well-documented in the homebrewing and commercial literature and is usually attributed to yeast being able to scavenge oxygen. I have never been sold on this explanation because oxidative reactions are faster than oxygen uptake by yeast. Indeed, research over the last 20 years or so provide clearer explanations of these empirical observations. A study by D. Saison, et al., in 2010 titled “Decrease of Aged Beer Aroma by the Reducing Activity of Brewing Yeast” where aged beer containing aged beer aromas was almost entirely stripped of these staling aldehydes during refermentation (Journal of Agricultural and Food Chemistry 2010 58 (5), 3107-3115). A more recent study in 2023 by De Clippeleer, et al., related to non-alcoholic and low-alcohol beer (NABLAB) production, showed that yeast selected for NABLABs biochemically reduce wort aldehydes associated with worty and stale aromas, thereby greatly reducing these off-aromas (“An In-Depth Comparative Study between Commercial Alternative Brewing Yeasts in Low-Alcohol and Alcohol-Free Beer Production,” ASBC Meeting Abstracts, 2023).

I recently tasted three NA (non-alcoholic) experimental beers brewed at the Fermentis Academy in Lille, France. The control beer was fermented with SafBrew LA-01, a maltose-negative yeast (it doesn’t ferment maltose sugar). The first experimental beer was first kettle-soured with Lactobacillus plantarum before wort boiling and fermentation using the maltose-negative strain. The second experimental beer was produced by adding fruit aromas to the kettle-soured NA. The control beer had a perceptible wortiness, however, both beers that were kettle-soured lacked worty aromas associated with wort aldehydes, demonstrating that lactic acid bacteria also reduce aldehydes.

OK, time to wrap these tidbits of information into something useful. For starters, I am not suggesting that dissolved oxygen is not a problem in beer. Focusing on methods to minimize oxygen pick-up after the start of fermentation are, without argument, important to brewing. Many homebrewers these days have taken an anti-rack position because of concerns about oxidation. I use simple methods, including a carboy fermenter and a keg for my secondary. It’s very easy to fill a keg with sanitizer, blow it out using CO₂, recover the sanitizer for later use in the soon-to-be cleaned carboy, and rack beer from the carboy into the secondary without worrying about oxygen pick-up. I usually cold crash this beer after whatever time at fermentation temperature is required to finish the beer, store for a few days, and rack into another keg for serving. Up until now, minimizing oxygen pick-up has been relatively easy.

For those of us who bottle, there are a few options for filling. My preferred method is using a counter-pressure filler, even when my plan is to bottle-condition, because these fillers allow for beer containing CO₂ to be filled with minimal foaming, intentionally foamed or fobbed after filling to push gas out of the headspace, and then capped with minimal oxygen pick-up. Commercially available bottle-conditioned beers are usually filled with about 2.2 volumes of CO₂ so that fobbing before capping is possible. High package airs occur when this step of the filling process is not properly performed. This is where homebrewers tend to deviate from commercial norms.

Most homebrewers use something like a BeerGun® or a flexible hose that can be pinched to stop flow to fill bottles with still beer from a carboy, bottling bucket, or Corny keg. Still beer contains too little CO₂ to fob and always leaves headspace gas, which is 20% oxygen, in the top of the bottle. While it’s tempting to leave minimal headspace in the bottle, that trick often results in bottle breakage. We’re now back to the beginning of this story where bottled craft beer in the early 90s often had very high package airs. I drank my fair share of great microbrewed beer back in that time and enjoyed more fresh beer than oxidized beer. The game changed as more beers started to show up on warm shelves, in places further and further from the brewery, and sat for longer and longer time periods. You can control this at home.

The other thing homebrewers without sophisticated bottle fillers can do is dose fresh yeast with priming sugar before packaging. Fresh yeast will not only carbonate your beer faster than whatever happens to be hanging around after fermentation and aging, but it will be in a better metabolic state to reduce staling aldehydes that develop as oxygen reacts with alcohols. Most bottle-conditioned beers contain about 500,000 yeast cells per mL, roughly 10–20 times less than wort pitch rates, and brewers wanting to dose fresh yeast need not go overboard. This last bit is the key that recent research explains; active yeast converts staling aldehydes back to the compounds, primarily alcohols, they were prior to oxidation like an oxidation eraser. Thanks for the great question that led me to some interesting references!

Response by Ashton Lewis.