Talking Oxidation and Hop Fade

Follow-up to previous answer: I answered a question from Scot in Chicago, Illinois that was published in the November 2019 edition of BYO. His question was about hop fade and why some of his recent New England IPAs lost hop aroma quickly after packaging. Scot let me know in his question that he was a very experienced brewer. Knowing that biased my answer because I did not think that he suddenly had a rash of oxidation problems. His question had me publicly scratching my head. Thanks to emails from Karl Helmstetter and Joe Walts (below), I now offer some more insight into the possible causes of hop fade.

Q
I really enjoyed your answer to Scot’s question about hop fade, especially your honesty as an author and brewing expert. It’s always easier for me to believe and accept people’s answers when they acknowledge their own bias or limitations, even when those come from the questioner!

I had a thought regarding Scot’s situation; he said he cold crashes in the carboy, hops are bright when added, faded a week later. If he is cold crashing without a CO₂ source, suck-back could easily add more than enough O₂ to dull the hops, and hazy IPAs are especially sensitive to this. No way to know without more information, but does this seem possible?

Karl Helmstetter
Charlottesville, Virginia

Q
I was catching up on brewing magazines this morning, and I came across your Mr. Wizard response to a question about fading hop character — which got me thinking: Could the problem be related to either oxygen pickup during kegging, or migration of hop aroma into a too-large (and increasing over time) headspace in the keg?

On the oxidation front, I would not expect this to be an issue for two experienced brewers. However, maybe something unexpected happened such as a loose hose/tube connection or even a low-purity CO₂ cylinder. And the fact that they cold crash prior to packaging could cause all kinds of problems if they don’t apply headspace pressure beforehand.

On the headspace front, it would be interesting for the brewers to package one batch in a keg and one batch in bottles to see how they compare over time — but the addition of bottle conditioning could create an unfair advantage in terms of oxygen reduction and possibly even biotransformation. Even without this investigation, maybe the brewers simply had a few low-yielding batches that accelerated their normal hop fade.

Joe Walts
Octopi Brewing Company
Madison, Wisconsin

A
These emails are great and give me a little more fuel to expend on the topic of hop fade. For starters, as someone who really dislikes oxidized beer, why did I not consider oxidation as a possible cause of the problem? I dismissed oxidation as the probable cause because of Scot’s experience. That is what is called a really bad assumption.

In the world of commercial brewing, brewers are obsessive about minimizing oxygen pick-up because oxygen quickly ruins beer. Even low concentrations of this potent molecule have a marked effect on reducing beer shelf life. Over the last 20 years or so, two technological changes have really been a game-changer when it comes to the general topic of oxidized beer. The first is the development of low-speed packaging lines that have dramatically reduced total package oxygen (TPO) in cans and bottles to levels that are on par with the fanciest, high-speed lines used by the largest brewers. The second real game changer is with dissolved oxygen (DO) meters. Today, DO meters are reliable, accurate, and widely used by brewers of all sizes (although not all breweries own these spendy instruments). Joe’s reply about leaks and gas purity comes from his experience in commercial brewing where brewing problems almost always include a question about equipment or raw materials . . . as in, what mechanical failure or out-of-spec [insert ingredient here] caused this problem? And Karl’s question about suck-back is in the same vein of the original inquiry.

Let’s start with suck-back. This happens when liquid from an airlock is sucked into the container it is intended to protect from the environment when the container headspace is cooled and a vacuum is formed. That sucks! And for a couple of reasons. One reason is that the airlock may contain “stuff” in the liquid barrier, such as microbes, cheap vodka, or bleach, that does not belong in beer. Even if the airlock contains nothing but pristine water, it offers no protection from the environment when empty. Picture a carboy that was bubbling slowly after primary fermentation. If a sample could be taken from the beer and the headspace and measured, there would be very little if any DO in the beer and in the headspace.

The second reason that suck-back sucks is air. Once the airlock has been gulped into the carboy, the headspace picks up oxygen from the environment, and headspace oxygen then enters the beer. A fallacy about carbon dioxide headspaces is that the density of the carbon dioxide protects the beer beneath. While a carbon dioxide gas blanket, especially during fermentation when the blanket is constantly flowing up and out of the fermenter, do help protect beer from oxygen, the blanket is not immune from gas mixing. Pesky principles like gas diffusion, convection, and Brownian motion work 24-7. An empty airlock needs to be re-filled pronto, and a full airlock is preferably never sucked into a carboy!

Joe’s approach to this problem is open-ended and questions everything involving beer handling that can result in an increase in DO. A DO-meter-wielding commercial brewery experiencing hop fade and suspecting oxygen would be on this by methodically measuring beer DO before and after transfers, during storage, and right before packaging. After packaging, quality control (QC) pros vigorously shake packages to equilibrate sample headspaces with the beer before measuring TPO because inquiring minds want to know!

The point is that there are numerous, and often times invisible, sources of DO. Leaky gaskets, pump seals, and valve seats, DO in push water, low-purity carbon dioxide, vacuum pump failures (used on package lines for pre-evacuation before filling), inconsistent fobbing before capping, poorly purged bright beer tanks/kegs, below normal fill levels in tanks, and loose hose clamps can all lead to increases in DO.

The below-normal fill level is an interesting problem that can affect oxidation and/or aroma partitioning. Large headspaces contain more gas, and a headspace of a given oxygen content is less problematic when the headspace is small, for example in a normally-filled bottle or can. But large headspaces also provide a big, aroma-devoid gas space where aromas in beer can equilibrate.

Response by Ashton Lewis.