How “Dry Hop Creep” Causes Diacetyl In Beer and How Brewers Can Minimise The Risk

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Have you ever wondered why your perfectly good, diacetyl free IPA left your brewery in a good state only to find that it picked up diacetyl some time later?

In this article, I’m going to break down what may be happening in your beer.

Each month, Rockstar Brewer Academy members get together online for our Mastermind webinar where we discuss pertinent topics of the day.  They’re pretty awesome!

In August 2018, we discussed “Dry Hop Creep”, a phenomenon brought to my attention during Brewcon 2018 in Sydney, Australia.

Caolan Vaughan of Stone & Wood Brewing did a presentation on “Dry Hop Creep” and how it was dealt with in his brewery. If you want to download his slide show, click here (password required).

While Caolan discussed what Dry Hop Creep was and how Stone & Wood dealt with it, it didn’t delve sufficiently into the secondary consequence of how dry hopped beers can cause the formation of diacetyl in package when they were in perfectly good condition when they left the brewery.

This blog post draws on information from The Master Brewers Association of The Americas Podcast, Brewcon Australia 2018 and Google Books and is intended as a quick reference guide to this phenomenon and focuses in particular on how dry-hopping can cause diacetyl in your beer.

What Is “Dry Hop Creep?”

“Dry Hop Creep” is a phenomenon where over-attenuation and over-carbonation of beer that has been dry-hopped can occur in package.

This was observed as far back as 1893 with an article in The Brewer’s Guardian by Horace T Brown and G Harris Morris called “On Certain Functions of Hops Used In The Dry-Hopping of Beers.”

Knowledge of dry hopping and it’s enzymatic effect on beer has been known since 1893. How did this information get lost to history!? (Image:  Google Books)

In the article, the authors postulated the following theories:

  1. That hops themselves contain fermentable sugars that the yeast consume.

  2. The hops have “wild” yeast attached to them that re-ferment the beer.

  3. That hops contain a “diastase” which means that they have an enzyme which breaks down non-fermentable sugars into fermentable sugars.

Their conclusion is that it is indeed an enzyme in hops that causes the break-down of dextrins into fermentable sugars.

Subsequent studies by Janicki et al in 1941 further reinforced that hops do indeed contain diastase enzymes.

The enzymes that are in hops are a-amylase and b-amylase (yes, the same enzymes found in malt that do the heavy lifting during mashing!), limit dextrinase and amyloglucosidase.

But it’s limit-dextrinase and amyloglucosidase that are the most interesting since they break down non-fermentables found in beer into fermentable sugars for the yeast to act upon and cause “Dry Hop Creep.”

But How Did The Diacetyl Get Back Into My Perfectly Good Beer?

It’s crazy right?

You did all the diacetyl testing in the brewery and you were certain that your IPA was perfectly fine.

So how did the beer see the re-appearance of diacetyl after packaging?

This can be attributed to “Dry Hop Creep”.

What has happened is this:

  • The enzymes in your dry hops have acted upon the starches and dextrins in your beer, breaking them down into fermentable sugars.

  • The yeast that you have left in your beer has refermented those sugars. Yes, it’s a secondary fermentation.

  • While additional alcohol and CO2 have been produced, a natural by-product of fermentation called a-acetolactate has been produced by the yeast and has gone through it’s normal oxidative decarboxylase reaction to form diacetyl. I discuss how this happens in a recent Youtube video below.

  • Because your packaged beer is probably refrigerated, the yeast cannot adequately clean up the diacetyl as per normal during a diacetyl rest thus leaving it in your beer after its formation.


What Are The 3 Key Metrics at Play With “Dry Hop Creep?”

While we now know what “Dry Hop Creep” is and how hop enzymes and yeast play their respective part, there are a number of inter-related metrics at play here that can cause or exacerbate the problem.

They are:

  • Temperature. Hop enzymes are active from around 1-2C/34-36F up to about 60C/140F at which point they denature and become inactive.

  • pH. Hop enzymes are most active at around a pH of 4.0 to 5.0.

  • Yeast Activity. Naturally, it takes active yeast to consume the newly-formed fermentable sugars.

Even If You Refrigerate Your Packaged Beer, You’re Still Not Safe From “Dry Hop Creep”

So here’s where it can get complicated so let’s pull it all together.

Clean beer pH is usually in the 4.0 – 4.5 pH range. This is ideal conditions for hop enzymes.

Enzyme activity is able to break down dextrins into fermentable sugars even while your beer is refrigerated.

This enzyme activity is independent of yeast activity and, at the same time, the yeast left in your beer can’t begin to re-ferment at cold temperatures.

But – if your beer were to come back up into a temperature range- let’s say when it’s a bottle or can sitting on a warm shelf in a bottle shop or a keg that’s being shipped in the back of a warm truck – where the yeast can become active again, fermentation will begin.

Some American ale strains can start to become active at as low as 9C/48F.

Heck, I’ve even heard of some breweries using an American Ale yeast strain to make pseudo-lagers!

And when fermentation begins, a-acetolactate is produced and will likely go through its oxidative decarboxylation to form diacetyl.

But because the beer never goes up in sufficient temperature to undergo a second “diacetyl rest”, then the diacetyl remains present in the beer.

And that is game over.

Great in a candy. In an IPA, not so much.

How To Reduce The Risk “Dry Hop Creep” and the Resulting Diacetyl

There are a number of ways you can prevent “Dry Hop Creep” and the resulting diacetyl formation.

The advice here is broad and you can use one or more of the below options, but it’ll give you some ideas as to how to tackle the issue:

  • Option 1 – Remove all yeast from your product. This can be done by centrifugation or filtration and to be absolutely sure that no yeast is left in your beer, sterile filtration is the way to go. Sterile filtration will deal with the yeast but not the enzyme. Left to it’s own devices, the enzyme will break down dextrins and your beer make pick up a sweet taste over time as the enzymes do their work.

  • Option 2 – Pasteurisation. By no means am I anti-pasteurisation. It’s an awesome tool available at the brewer’s disposal that will not only kill any active yeast but will denature the hop enzymes this completely eliminating the risk of “Dry Hop Creep”. Done properly, you’d barely even notice the difference – even in the hoppiest of IPAs. It is, however, expensive to implement.

  • Option 3 – Coarse filtration to remove hop particles (but not yeast). Horace T Brown in 1893 deduced that the diastase enzymes in hops are largely contained in the hop material itself. Hop flecks being present in packaged beer are definitely a beer quality no-no. Here’s another reason why coarse filtration may be an option.

  • Option 4 – Dry hop during your diacetyl rest. Instead of trying to fight the enzymes, why not try to work with them? Dry hop at diacetyl rest temperatures and when the beer has about 2-3 Plato of gravity shift to go before final gravity. Don’t forget how wort and beer pH can effect yeast’s ability to clean up diacetyl so make sure you have your formulations and ability to measure pH accurately in place.  You will also need to adjust your recipe’s Original Gravity to counter the extra alcohol from the higher attenuation.  Give your beer a good 3-4 days of free rise after dry hop to clean up any diacetyl.

  • Option 5 – The more dry hop – the higher the concentration of enzyme that can potentially damage your beer. Consider the reducing returns on increasingly higher dry-hop rates on flavour and aroma and it’s added risk of more “Dry Hop Creep” occurring.  Less is sometimes more in the brewing world.

  • Option 6 – Measure and reduce Dissolved Oxygen (DO) pick up.  Dissolved Oxygen will exacerbate and accelerate the AAL to diacetyl reaction in cases where Dry Hop Creep has played its part.  It’s 2018 and you should have a low-range DO Meter in your brewery.

You should check each of your beers and how the variables of temperature, pH and yeast activity differ in your brewery.

Also don’t forget that different hop varieties will have different levels of enzymes so some in-house forced fermentations with your favourite hop varieties should be on your lab analysis program.


Diacetyl in hoppy beer is never a good thing unless you’re brewing English style beer – and even then, it should help balance the beer and not over power it.

Knowing that there are a number of seemingly independent factors working together – enzymes, yeast, pH and temperature – will help you to manage “Dry Hop Creep”.

Conduct your own in-house lab work and dedicate yourself to understanding your own brewery, your ingredients and your processes to help you manage any potential problems.

Throw back to 2017 when I brewed the AIBA collaboration beer (a delicious Citrus East Coast IPA) with Stone & Wood, Two Birds and Pirate Life. There was no diacetyl in that IPA.

Talk through your particular issues with other brewers. The Rockstar Brewer Academy’s monthly Mastermind sessions are a great platform to get real solutions to real problems.

As always, my door is open, leave a comment below or get in touch.

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14 thoughts on “How “Dry Hop Creep” Causes Diacetyl In Beer and How Brewers Can Minimise The Risk”

  1. I don’t quite understand how the yeast can ferment the sugars created from hop enzymes, but they can’t they clean up AAL? How is the temp sufficient to ferment but not to consume AAL?

    1. Thanks for your comment.

      It’s diacetyl that’s consumed by the yeast – not AAL. Think of it like a referment without a diacetyl rest. The AAL needs to change to diacetyl as well as force the yeast to consume that diacetyl.

      Yeast tend to not have a propensity to consume diacetyl at lower temperatures but you’d be surprised how low a temperature yeast can consume sugars and secrete AAL.

      So while you may have refrigerated beer, the enzyme will still break down unfermentable sugars into fermentables. Then the beer may just get warm enough for a slight amount of fermentation which creates AAL. That AAL converts to diacetyl via oxidative decarboxylation but the beer may not be warm enough for the yeast to consume that diacetyl because it’s too cold.

      Raising the temperature of packaged beer is counter-productive as that will only kick off further fermentation and overcarbonate the product in pack.

      I hope that clarifies things. Let me know if you have a follow up question.


  2. Could you keg the beer then take the beer out of your keezer and rest it at room temp for a few days to let the yeast eat the diacetyl? I still don’t quite understand how a nowmal d rest doesn’t work.

    1. Thanks for your comment 🙂 Theoretically, this would work but then there would also be sugars for the yeast to consume (thanks to the enzyme) which would cause overcarbonation at the same time. There’s also lots of research on how fluctuating beer storage temperature is bad for the product.

  3. Hendo, nice post, very useful, I got one personal question, do you leave your tank open during Diacetyl Rest, or you keep it shut for a natural carbonation? Specially if we dry hop during the rest, we will get a lot of CO2 produced, which could be useful, is there any issues with it? I mean on the sensorial side of the beer, either aroma and flavour. thanks for now.

    1. There are a lot of positive benefits of at least some natural carbonation as hop aroma compounds can be scrubbed from the beer during forced carbonation. If you can smell hops in the cellar then they’re not in your beer. I have no preference to either, actually so it’s best to do what works best for your brewery and your desired product outcomes. Thanks for your comment 🙂

  4. Hendo,

    Has this recent rise in hop creep related incidents been to the change in technological approach in the processing of hops specifically the t-90’s? There is a movement to pelletise at cooler temperature and which in turn is not denaturing hop derived enzymes – Is there a benefit to pelletising colder or is it an energy saving means by Yakima, Ellerslie and the likes … Or is there positives to the product with new methods ?


    1. Short answer: Absolutely!
      I was fortunate enough to have a conversation with Dr Tom Shellhammer a while back and he highlighted that the lower kilning temperatures may be causing enzymes to remain active in hops although this is an area he said requires more study. If true, it’s basically brewers who are requesting lower and lower kilning temperatures in the quest for aroma making for a causation loop where we’re effectively causing our own Dry Hop Creep! Think about it…. a brewer at a hop farm smells the kiln and goes “hey, that aroma should be in my beer! LOWER THE KILN TEMPERATURE!!!!” though it’s not proven that that specific aroma even winds up in beer. This whole area around kilning requires more study.

  5. If the enzymes responsible for creep denature at 140 Fahrenheit, could you “sous vide” your hops destined for dry hop at that temperature? My thinking being, immerse them in a vacuum sealed bag for 5 minutes to eliminate the enzymes then let the heated hops rest until cool, and add according to your dry hop schedule. If this is feasible, I’d imagine the process and usage happen same day.

    1. If you were a home brewer, this would be a reasonable approach. In fact, this is quite innovative! It wouldn’t be practical for pro brewers though with the amount of hops that would need to be treated.

    2. Hi Nathan, have you tried that approach – that is denaturing the enzyme sous-vide at 140 Fahrenheit and was it successful

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