Oxygen and Mashing

Hot side aeration (HSA) is a myth, right? Or is it? I'm sure by now many of you have read the PDF put out by the German Brewing Forum that has caused a bit of a stir. The reactions I've read have either been "This changes everything!" or "This is complete rubbish!" with little in between. I don't want an Apple v. Android style fanboy dogfight. I want to talk facts and figures, science and chemistry. Let's eliminate the hyperbole and see if we can dig into what is and isn't really going on here. If you haven't read the PDF, it basically advocates using sodium metabisulfite (SMB) in the strike water and working towards lowering oxygen ingress at all stages of brewing.

What This Gets Wrong

“If oxygen were really that reactive we would have all gone stale by now"

Dr. Charles Bamforth [1]

Although the PDF itself points out "malts are already oxidized before the
experiment takes place", let's examine for a minute what malt goes through before it gets to you. Barley is germinated by steeping it in water. The water is then drained off and the grain is typically held at 70C (~158F) to kill the embryo and dry out the barley corn. The temperature is then ramped up to 90C or more for kilning. So, already we're talking about barley being exposed to mash like temperatures while wet and with air constantly being pumped over it to help dry it out. When the product is finished, it's packed up into breathable sacks, palletized, and loaded up for shipping. Assuming you're buying German malts because you're interested in making German beer, that means they go into a shipping container and spend several weeks at sea. According to one study by the ISTA[2], the inside of a shipping container can be as much as 30C above ambient. Assuming you have an average of about 30C out at sea, your malt could be sitting at 60C in a humid shipping crate for weeks before it's even unloaded in the US. Once here, who knows how domestic distribution or your LHBS is going to treat that sack. My point is if there was something magical about oxygen, moisture, and malt that combined to drive this ultra delicate "it" off into the ether, it would have happened a long time before it ever hit your mash tun. If anyone even for a second assumed some flavor positive volatile were to exist, malt would be shipping in vacuum sealed bags, much the same way hops are. No one would want to lose this flavor. I can not find one single example of a volatile reaction that would account for the "loss" people think they're tasting. Everything about this premise that you're "losing something" by oxidation in the mash is extremely unlikely and I think that's where the PDF loses credibility and interest with most people.

Another point that seems to get glazed over is that oxygen is key in getting your polyphenols and proteins to coagulate into hot break. Not to say they can't coagulate without O2, but they do so much better when the polyphenols are oxidized. The removal of HMW protein through hot break is important downstream for yeast health, removal of harsh protein derived bitterness, and improved colloidal stability [3]. This is mentioned in both Kunze[4] and Narziss[5], so I'm not sure why a methodology developed around their publications wouldn't take this into consideration. When you pile on the fact there are no objective comparisons to wort produced with a similar mineral profile of that which the SMB would produce or really any form of objective data, it's a tough pill to swallow. The methodology proposed would be difficult for novice or even intermediate brewers to implement. The fundamental argument boils down to "it works, trust us" with no supporting evidence. I can see why people would blow this off. But why would so many people be drinking the Kool-Aid of "it" then?

Why They Still Might Be Right

"There is the 'herbstoffe effect' ... In this mechanism, malt constituents, most notably phenols, are oxidized during wort production."

Dr. George Fix[6]

So, I'm sure there's a faction of people out there high fiving and thinking that was an epic burn. Not so fast there. There was one thing the GBF PDF mentions briefly, but I think is the key to the results some of the proponents might be seeing in practice. On page 4, lipoxygenase (LOX) and peroxidase enzymes are mentioned. I find this interesting because this is a class of enzymes we as brewers will typically glaze over. Along with superoxide dismutase (SOD) and polyphenol oxidase (PPO), they comprise most of what we'd think of as enzymatic browning reactions. You'd commonly see this happen in apples, potatoes, avocados, and just about any other fruit or vegetable after they've been cut and exposed to air. What effect do these enzymes have on beer? Actually, it isn't well studied. It is known that there's a correlation to LOX activity and trans-2-nonenal formation which would be the cardboard staling flavor beer can get[7], but that's about it. With the exception of PPO, these enzymes actually increase in concentration during the malting process, but most of them are inactivated early on in the mash[8]. However, there are two interesting pieces of information to be had: peroxidase survives in all but the highest mash temperatures and PPO is present in low kiln malt, but not higher kilned malts. I find that last point especially interesting. PPO can act on polyphenols to produce benzoquinones[9], which then react to form other flavor active quinones. The reason that's interesting is that benzoquinone can take a form that tastes dirty, like pencil shavings[10]. Engage in a bit of a thought experiment with me for a second. Imagine what you and I normally think of as an all-pils wort straight out of the mash tun. It has a sweet grainy flavor, but can sometimes take on a husk forward, earthy note. If you subtracted the flavor of pencil shavings from the flavor of pilsner wort, how would it taste? Most of the descriptors I come up with seem to fall in line with the way people from the GBF describe their wort. It's my hypothesis that it isn't something being lost, but rather additional flavor being gained by the oxidation of polyphenols due to the action of PPO and possibly the action of peroxidase. This might explain why GBF members detect a difference in some of their beer while a vast majority of brewers have never noticed oxidation effects in their beer; your average American 2-row is kilned and processed to the point where there is no PPO left as are most other malts. Continental pilsner, which would be used in most German recipes, is not processed that highly and would contain some PPO. Step mashing would also mean the temperature is kept lower for longer periods of time, which allows PPO and peroxidase to act longer before denaturing. Some of trials done by forum members indicated a reddish tone to non-SMB'ed controls suggesting that catalase might also play a role in mash oxidation [8:1].

To find out if there was any legitimacy to the claims, I ran a mini-mash test similar to the procedure outlined on the GBF. I was able to use the lab of a couple of friends to measure any actual change with a spectrophotometer to gauge the SRM of the solution in an objective way. Three trials of (2) 500mL samples were run. Both samples used distilled water and one sample was dosed with 50mg of SMB. 240mg of milled Weyermann floor malted pilsner was added to each beaker. Lactic acid was used for both samples to adjust the pH down to 5.2. Each trial was run with the beakers in a 150F water bath for 30 minutes and then set in a 170F water bath for 10 minutes. Once completed, samples were measured in the spectrophotometer for absorbance at 430nm to determine SRM. The remaining portion of each beaker was then tasted. The results from the spectrophotometer are listed below.

TrialSMB SampleControl Sample
12.1 SRM3.1 SRM
22.2 SRM3.0 SRM
32.1 SRM3.0 SRM

Although I wasn't allowed to take pictures in the lab, the control sample was noticeably a shade darker than the SMB. Not extreme by any stretch, but the samples could definitely be differentiated. However, the story was a bit different on the flavor front. Between myself and the two scientists who graciously lent me the use of their facilities, we tasted all samples and couldn't reliably taste a difference among any of them in regards to levels of bitterness or astringency. There was a bit of a sulphur note to the SMB samples, but that was to be expected. To see what the end wort product might resemble, one of the trials had both samples strained and boiled for 10 minutes. The sulphur note was less pronounced, but there was still no perceived difference in the quality of the malt sweetness, bitterness, or astringency. From this small sample set, I can say that SMB positively impacted color but seemingly had no effect on flavor. That does indicate enzymatic browning did occur, but it did not occur in levels higher than the threshold of flavor.

Why It Might Not Matter Either Way

So what does this all mean? Is "it" real or not. I would first ask if you feel there is anything wrong with the beer you're currently producing. Does it's flavor meet your expectations? Can you detect any strange bitterness or astringency? If you think your beer is currently turning out to your expectations, I see no reason to stress over mash oxidation at this point. Perfecting low oxidation throughout the brewing process will also require you to re-evaluate every step of your current system. It isn't sufficient to simply dump SMB in your strike water and call it good. Are you willing to alter your setup and possibly invest in new gear? If the answer is no, then I'd also think it's best to put all this out of mind.

Have you currently perfected your cold side techniques to minimize oxygen uptake? Unless you're doing closed transfers between fermenters, kegging, or bottle filling with minimal O2 uptake, I don't see any point in worrying about hot side oxidation. Flavor and colloidal stability can be hugely impacted by oxygen ingress on the cold side. If your beer is oxidized by the time it hits your lips, does it matter if it happened at the first part of creation of that beer or near the end? If you're not perfect on cold side oxygen ingress, I would think working your way backwards from the glass to the mill would be the most prudent approach. At that rate, it would be a long time before you'd ever worry about oxidation in the mash.

There's also the consideration of the downside of minimizing oxidation in the mash. Oxidation of polyphenols in the mash helps them to bind to proteins and precipitate out of solution. This increases turbidity in the short term, but that turbidity filters out in the mash bed, hot break, and cold break[11]. By not allowing this to happen, that excess protein and polyphenol load is passed onto the cold side. So, somewhat ironically, preventing oxidative flavors early in the process, you increase the risk of oxidative flavors and staling later on in storage. You'll have to have a strategy to manage it.

Another point to consider is that antioxidants don't only reduce oxygen. Antioxidants, including SO2, can also reduce metal ions [12]. The presence of iron or copper ions can actually increase oxidation via the Fenton and Harber-Weiss reactions. If your water contains any measure of iron or copper, it's conceivable that you could actually accelerate oxidation with the addition of SMB. While the use of a chelating agent such as ascorbic acid could mitigate this, in the wrong ratio, ascorbic acid can reduce the effectiveness of SO2 [12:1]. To surpass the reversal point and start to have a positive influence, experimental data indicates that you need approximately 20x the amount of ascorbic acid to SO2. At that rate, you'd need ascorbic acid in the 1500 ppm+ range at the current proposed SMB dosing rate. That would obviously have a negative impact on mash performance and flavor.

There Is ... Another ...

Perhaps after all that you're still interested in adjusting your mash oxygenation levels. There are actually several ways to lower O2 levels and many don't involve additional chemicals. First is actually mentioned in the GBF PDF and has been utilized by brewers for hundreds of years; pre-boiling the strike water. This method was originally used to lower water hardness and get carbonate to precipitate out as chalk, but has the added advantage of lowering the solubility of oxygen to near zero. I'd try this first and see if you notice any improvement in your beer.

In keeping with the traditional aspect of brewing, you might want to revive the acid rest. When you start a mash at between 86F and 122F (30 - 50C), phytic acid forms and naturally lowers the mash pH. Phytic acid has been shown to inhibit the action of PPO and significantly lower enzymatic browning reactions[13]. Maybe those old brewers knew more than they let on?

Not so much a fan of the acid rest, but still looking for a good way to adjust your pH while still reducing O2? Try using an active sauergut. A sauergut is wort or mash intentionally soured with lactobacillus and other organisms. You'd add it in place of lactic acid or acid malt to do your pH adjustment. If your sauergut is a mixed culture containing yeast and bacteria, you can also keep O2 in check because few things reduce oxygen as well as active yeast. The bonus is once you go to boil, the yeast die and become nutrients for the next round of yeast you pitch for fermentation.

Are there any other methods that lean on tradition over science that could lead you to lower O2 in your beer? Actually, hops high in humulone and lupulone serve as powerful antioxidants[12:2]. At first you might not see how that would have an impact on mash chemistry, but it could make itself valuable if you practice the long-thought-useless art of mash hopping. If nothing else, it would have a slightly positive influence on mash pH and lautering, so I'd encourage experimentation here as well.

Another simple adjustment comes even before the mash in. Most of the enzymes that I've proposed as the source of potential issues are concentrated in the acrospire of the malt. To limit their permeation into the mash and prevent increased shredding of husk which provides more surface area for the enzymes to interact with, consider milling your malts a bit more coarsely than you would normally.

So let's say you're going to use SMB to drive off chlorine or chloramine. What's the harm in adding a bit more to get some LODO benefits? Well, you might not have to. SMB actually does deactivate the action of PPO[14] in addition scavenging oxygen and driving off chlorine/chloramine. That means the level of SMB addition needed could be much lower than initially thought to prevent color and perceived flavor differences. That could mean that the metabisulfite some brewers have been adding to remove chlorine or chloramine from their water could already have enough residual effect to deactivate problematic enzymes. It presents another possible reason why some brewers haven't noticed any harsh or astringent flavors in their wort even though they don't practice low O2 brewing. It would certainly be worthy of further investigation.

Want more "better brewing through science" thoughts? Ok! First up would be ascorbic acid. We already touched on that a bit and it seems impractical to implement. That, and the fact that Dr. Bamforth does not seemed terribly impressed with its performance in the brewhouse[1:1] probably means it's safe to skip. Another promising antioxidant gaining interest from homebrewers is Brewtan B. This is a powdered form of HMW gallotannin. The idea is that the tannin binds to the oxygen and becomes insoluble or complexes with protein and falls out as break material. While the science is sound here, I see potential pitfalls around dumping tannins into your beer. There needs to be enough oxygen and protein present for the Brewtan to fall out of solution and not carry over to the fermenter or packaging. Since most homebrewers seems to think more is more, I can see unintentionally astringent, hazy beer in many people's future. Definitely take care to explicitly follow the manufacturer's recommended dosing and perhaps skip this stuff all together if you're going for a very delicately flavored beer. There are many other products with funny names out there, mostly used in winemaking for long term preservation. You'll find they're a blend of tannins, ascorbic acid, sulfur compounds, and fining agents. Since I've touched on those elements individually, I won't go into the action of each here. If you happen to try one of these, please post the results.

So Where Does That Leave Us?

I'd like to bring it back to a question I posed earlier ... do you currently like the beer you're making? If so, don't worry about it. If not, I'm only prepared to say that the methodology outlined by the GBF can help keep your wort lighter in color. The chemistry suggests there could be a flavor improvement but that still seems objectively inconclusive. It is important to realize that any possible flavor impact would more than likely be the addition of unwanted flavors and not the loss of wanted flavors, so that might change how a brewer approaches the problem. Since professional flavor scientists still seem undecided on the matter, I don't think it's going to be solved any time soon. Good thing Matt over at Accidentalis Brewing is going to share some real world results with you ... now! Part 1 of his LODO experiments Part 2 is coming. Stay tuned! In the mean time, I would encourage you to work on process improvement first, then try some of the more traditional methods of O2 reduction before you get into dumping additional chemicals into your beer.

Special thanks do go out to the folks at the GBF. Whether or not you agree with their procedure or the concept of "it", I think it's great that they've brought the topic of HSA back to life. It's sparked fresh debate in the homebrewing community and inspired new experimentation and research. That makes us all better brewers. Cheers!


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Also reviewed for this article but not directly cited are...


  1. Bamforth, Charles (2009) Brew Strong podcast ↩︎ ↩︎

  2. Leinberger, David (2006) Temperature & Humidity in Ocean Containers ↩︎

  3. Sinha, Nirmal (2007) Handbook of Food Products Manufacturing ↩︎

  4. Kunze, Wolfgang (2014) Technology Brewing and Malting 5th English Edition ↩︎

  5. Narziss, Ludwig (2005) Abriss der Bierbrauerei ↩︎

  6. Fix, George (1999) Principles of Brewing Science: A Study of Serious Brewing Issues ↩︎

  7. Drost el al (1990) Flavor Stability Journal of the American Society of Brewing Chemists ↩︎

  8. Clarkson, Simon; Large, Peter; Bamforth, Charles (1991) Oxygen-Scavenging Enzymes In Barley And Malt And Their Effects During Mashing Journal of the American Society of Brewing Chemists ↩︎ ↩︎

  9. Matheis, Gunter and Whitaker, John R. (1984) Peroxidase-catalyzed cross linking of proteins Journal of Protein Chemistry ↩︎

  10. http://www.aroxa.com/catalogsearch/result/?q=2-tert-butyl-1,4-benzoquinone ↩︎

  11. Stephenson, W.H.; Biawa, J.-P.; Miracle, R.E.; Bamforth, C.W. (2003) Laboratory-Scale Studies on the Impact of Oxygen on Mashing Journal of the American Society of Brewing Chemists ↩︎

  12. Kunz, Thomas; Straheml, Arno; Cortes, Natalia; Kroh, Lothar W.; Methner, Frank-Jurgen (2013) Influence of Intermediate Maillard Reaction Products with Enediol Structure on the Oxidative Stability of Beverages Journal of the American Society of Brewing Chemists ↩︎ ↩︎ ↩︎

  13. Du, Y.; Dou, S.; Wu, S.(2012) Efficacy of phytic acid as an inhibitor of enzymatic and non-enzymatic browning in apple juice Food Chemistry ↩︎

  14. Valero, Edelmira; Varon, Ramon; Garcia-Carmona, Francisco (1992) Kinetic study of the effect of metabisulfite on polyphenol oxidase Journal of Agricultural and Food Chemistry ↩︎