Cold-Side Considerations: It’s lurking out there . . . oxidation

This whole column can be summed up in the following statement: Oxygen is vital to the yeast but detrimental to the beer. Oxygen is utilized by the yeast to synthesize key nutrients that it needs to physically grow and reproduce. These nutrients can be essential lipids and sterols. Yeast ferment, which means they do not respire oxygen like we do. But yeast will utilize any oxygen in the wort to biochemically synthesize these nutrients, even if the oxygen is chemically bonded to other wort components — such that after fermentation the oxygen content of the beer is effectively zero. Or is it? There is a lot of debate on the detriments of hot-side aeration (i.e., oxidation of hot wort prior to fermentation) with some studies and many opinions that indicate that low dissolved oxygen brewing (popularly known as LODO brewing) can improve your beer’s malt aroma and flavor as well as flavor stability. By how much is difficult to quantify.

But our focus today is on total package oxygen (TPO), oxygen that is introduced to the beer after fermentation, most often during packaging. As Dr. Charles Bamforth states in his career retrospective, “Investigations over many years reinforced that flavor stability is a problem that should be addressed commercially ‘in reverse order,’ with the focus on beer in the trade first and then tracking back. Thus, I am at pains to emphasize absolutely that the two most important considerations should be the minimization of oxygen in the final package and the maintenance of beer at the lowest possible temperature (short of freezing) throughout storage and distribution. Only once this is assured is it worth paying attention to upstream.” ¹

Beer Oxidation 101

So, what are the mechanisms of oxidation and staling? Oxidation is a chemical reaction resulting in the loss of electrons to a substance. Actually, you can’t talk about oxidation without also acknowledging the other side of the coin, and that is reduction. In oxidation-reduction reactions (or redox reactions), one substance loses electrons (is oxidized) and the other substance gains the electrons (is reduced). Historically the terms were defined because oxygen was the primary reagent, and iron (for example) would be oxidized from Fe to Fe⁺², and the oxygen would be reduced. However, actual oxygen is not required — it can be any substance that gains electrons — and this is often the case in brewing. The important thing to understand is that “oxidation-caused staling” doesn’t always mean oxygen. More on the chemistry of beer staling later.

Flavor Stability 1: Temperature

To put it simply: Beer is best consumed fresh. There are a couple of styles where the oxidation products tend to be more pleasant than the fermentation products, but to me those clearly represent large opportunities for improvement in the fermentation of the style. In general, the first signs of oxidation and poor flavor stability are the loss of fresh hop and malt aromas. From there, the effects vary. Different beer styles stale differently. Different storage temperatures stale differently. For example, in Pilsner beers the effects that follow aging at 77 °F (25 °C) tend to be predominately a caramel character, while at 86–99 °F (30–37 °C), the flavor change is more of the cardboard (E)-2-nonenal.² In hoppy ales, low temperature storage (37 °F/3°C for 2 weeks) resulted in a hop character transition from tropical fruit, dank, and citrus character to more of a tea/herbal and spicy aroma.³ High temperature storage (86 °F/30 °C for 2 weeks) of the same beer demonstrated a general loss of hoppy aroma and an increase of malt-derived aroma. These are just a couple of examples of the effect of storage temperature. Temperature is the one factor that everyone agrees is significant in the staling process. In fact, several studies demonstrated the applicability of the Arrhenius equation, which suggests that the reaction rate for most chemical reactions increases by a factor of 2–3x for every 18 °F/10 °C increase in temperature. Beer that is stored at 32–39 °F (0–4 °C) was flavor stable for several months, compared to beer stored at room temperature, which exhibited detectable stale flavors after only a few weeks.

Flavor Stability 2: Total Package Oxygen (TPO)

It is generally agreed that after temperature, the next biggest effect on flavor stability is the oxygen that is introduced on the cold side, typically during packaging. The goal for commercially packaged beer is to be less than 50 parts per billion (ppb) TPO, as this is the generally accepted benchmark for maximizing shelf life.⁴ The question is, how can we minimize oxygen in the headspace when we bottle, can, or keg our beer? It’s not easy. Standing water will have an equilibrium concentration of 8–10 ppm at room temperature near sea level. That concentration decreases with warmer temperatures and higher elevations. Boiling the water for several minutes can drive it down to about 1 ppm. Keep in mind that 1 ppm is 1,000 ppb or 20x more than our goal of 50 ppb.

The next step is to understand the limitations of the materials we are dealing with. Plastics are generally permeable to gases, the amount may be small, but it is there, and 50 ppb is a low hurdle to overcome. This is the primary reason that plastic bottles for beer remain commercially absent. In addition, oxygen will absorb onto plastic surfaces and be released after filling due to the gas equilibrium change. One estimate for a 500 mL plastic bottle is that 352 ppb of oxygen will be available. The rate of desorption is temperature dependent, but 90+% will take place within one week and maybe 40–65% within one day.⁵ There are additional polymeric coatings that can be applied to the container to reduce diffusion and/or absorption, but these coatings interfere with recycling of the plastic.

The better option is glass bottles obviously. The only means of oxygen ingress or CO₂ loss in a glass bottle is through the bottle cap liner. Historically these PVC-based bottle cap liners have been a significant problem for oxygen. One study from 1990 indicates that diffusion through the bottle cap could allow as much as 4 ppm of oxygen per day!⁶ Fortunately, there are oxygen scavenging bottle cap liners now, as well as better materials, that may address this problem. (Hopefully they are being used!)

The best practices for filling containers is an inert-gas purge and capping, or sealing, on foam . . . let’s start with the purge. The idea behind the purge is to eliminate or replace the air in a bottle, can, or keg with an inert gas such as nitrogen or carbon dioxide, such that when the container is filled, there is no oxygen in the container that can mix with the beer as it is filled. In a perfect world, the gas flow would be very laminar, there would be no turbulence, and the air (and oxygen) would be displaced from the container without any mixing of the gases. In the real world, there is some turbulence, and some mixing, but the available oxygen is greatly reduced. This is why Corny kegs are recommended to be purged with CO₂ prior to filling.

Purging of bottles can be done as well, such as with a counter-pressure bottle filler or the Blichmann BeerGun™, but generally if you have a kegging system you aren’t bottling often — unless it’s for competition, in which case I highly recommend that you do purge your bottles. Lastly, there is the well-known recommendation to “cap on foam.” What this means is that as the bottle fills, you allow just enough turbulence in the beer to generate enough foam to fill the bottle neck, and indeed, emerge from the bottle, as you withdraw the fill tube. This foam is predominately filled with CO₂, and by capping on foam you eliminate any oxygen in the package. However, there is a bit more to this than meets the eye. The emergence and wiping away of foam from the top of the bottle before capping is important! Why? Because small bubbles want to become large bubbles due to partial pressure equilibria and physics. The small bubbles at the top of the neck quickly absorb oxygen from the air to try to come to equilibrium with the atmosphere. In other words, you want to cap on small bubbles of foam, after eliminating the big bubbles of foam that emerge from the just-filled bottle. You will have a much better chance of entirely eliminating oxygen from the bottle if you do.

Beer Oxidation 201

It is important to understand that oxygen gas, O₂, is not especially reactive when dissolved into wort; it doesn’t immediately react to form staling flavor compounds. Instead, the oxygen must be converted (i.e., react with other substances) into reactive oxygen species or ROS, which are much more reactive and able to initiate staling reactions. Ground-state molecular oxygen (O₂) can react with transition metal ions — primarily iron, copper, and manganese — to form superoxide anion (O₂^–) and from there can react with water to produce further reactive species such as perhydroxyl radical (OOH•), peroxide anion (O₂^-2), and finally to hydrogen peroxide (H₂O₂). These species can react further with the metal ions according to mechanisms such as the Fenton and Haber-Weiss equations (not shown) to produce the hydroxyl radical (OH•, not hydroxide, OH^–) and superoxide anions (O₂^–). The reactivity strength of these ROS increases as follows: Superoxide ion, perhydroxyl radical, hydroxyl radical.^{2, 7} (By the way, “radical” means that the atomic structure contains an unpaired valence electron, which is a bit of a loose cannon, as it were.)

These ROS tend to react with ethanol, which is the most abundant organic compound in beer, to produce the 1-hydroxyethyl radical and acetaldehyde. Most of the acetaldehyde in beer is produced by the yeast as they process glucose into pyruvate and pyruvate into acetaldehyde and finally to ethanol, for energy. Ideally, these aldehydes should all be reduced by the yeast by the end of fermentation, but oxidation can bring some back. However, ethanol is the least reactive of the alcohols in beer — reactivity increases here with molecular weight. This means that the fusel or higher alcohols may be almost as likely to become oxidized, even though the concentration is orders of magnitude less. Thus, acetaldehyde is not the only manifestation of oxidation and staling. Aldehydes are very significant organic compounds. They typically have strong aromas, such as vanillin, which is a phenolic aldehyde and the primary component of natural vanilla extract. There are three main types of aldehydes that are associated with stale beer, and these are: Fatty acid oxidation aldehydes (hexanal — green vegetative, and the classic wet cardboard aroma of (E)-2-nonenal), Strecker degradation aldehydes (formed from amino acids, such as methional — having a cooked potato flavor), and Maillard reaction aldehydes (such as furfural — having a caramel and bitter almond flavor).² These are the types of aromas and flavors most often associated with stale beer. However, these sorts of flavors are often the last to appear, when the beer has turned the corner from poor to bad. The first signs of oxidation or flavor degradation is often the loss of fresh malt and hop aroma. So far, scientists have not determined whether this loss is due to breakdown of aroma compounds or if the mechanism is more of a masking issue by formation of aldehydes, or if it’s a mix. It’s probably a mix, which would explain the mixed results. For example, recent work by Barnette and Shellhammer indicates that hop aroma loss is not due to breakdown of hop monoterpenes.³

The Bottom Line

There are a lot of studies that describe oxidation of aldehydes and the creation of compounds during malting, milling, and mashing that are associated with stale flavor, but the majority of these products are eliminated during the boil or reduced by the yeast during fermentation. This shifts the focus to the cold side and our efforts to minimize TPO by purging the bottles, cans, or kegs with inert gas and capping on foam. But keep in mind that the best thing you can do to preserve your beer’s flavor is to store it cold. Oxidation and staling are chemical reactions that are controlled by temperature.

References:

¹ Bamforth, C., The Horace Brown Medal. Forever in focus: researches in malting and brewing sciences, J. Inst. Brew., 126, 2020.
² Vanderhaegen, B., Neven, H., Verachtert, H., Derdelinckx, G., The Chemistry of Beer Aging – A Review, J. FoodChem, 95 (2006).
³ Barnette, B., Shellhammer, T., Evaluating the Impact of Dissolved Oxygen and Aging on Dry-Hopped Aroma Stability in Beer, JASBC, Vol 77 (3) 2019.
⁴ Skinner, A., Managing Dissolved Oxygen in the Brewery, MBAA TQ, Vol 57 (1), 2020.
⁵ Huige, N., Evaluating Barrier-Enhancing and Scavenger Technologies for Plastic Beer Bottles, MBAA TQ, Vol 39 (4) 2002.
⁶ Teumac, F., Ross, B., Rassouli, M., Air Ingress through Bottle Crowns, MBAA TQ, Vol 27, 1990.
⁷ Bamforth, C., Muller, R., Walker, M., Oxygen and Oxygen Radicals in Malting and Brewing: A Review, JASBC, Vol. 51 (3), 1993.

Written by John Palmer