Hop Creep: The Ghost of Brewing's Past

Ever had a wonderful, hop-forward beer at a brewery only to wonder why it tastes a bit dry, or over-carbonated, or perhaps developed some unpleasant Diacetyl in the can or bottle you brought home? You just had it a few days ago from the source, yet now there’s something not right about it. What’s scary is a can or bottle becoming over-carbonated and leaving you with a puddle to mop up when it once had appropriate levels of CO2. Almost as if these changes seemingly crept up on the beer. Well, they kind of did.

The Past

There’s a phenomenon in the brewing industry that was seemingly forgotten or at least put on the sidelines for over 100 years. Dry Hop Creep (Hop Creep for short) is very interesting and is important for brewers to understand, especially if they want to make exceptional IPAs. First mentioned in an article in The Brewers Guardian in 1893 by Horace T. Brown and G. Harris Morris, it was known that adding hops to beer at the end or near the end of fermentation helped dry out, or “freshen” the beer by fermenting some leftover carbohydrates the initial fermentation may have left behind. The article makes a few logical hypotheses on what causes refermentation after Dry Hopping.

Let’s first address what dry hopping is for those who may now be aware. Dry hopping is the addition of dried hops into the fermenter either at the tail end of fermentation or when it has been completed. Hops contain, among other things, acids & oils that are essential to the finished beer’s flavor. The acids require to be boiled to impart their bitterness to balance out the beer’s natural sweetness. The oils, by contrast, contribute much of the hop flavor (piny, herbal, and/or fruity aromas) and are volatile so they benefit from late additions in the boil or after fermentation. Dry hopping contributes to the iconic flavors and aromas we associate with the modern IPA (clear or otherwise).

For Brown and Morris, the question they had wasn’t about flavor, it was about dryness. They knew adding hops at the end of fermentation would induce a secondary fermentation, but they didn’t know why. They hypothesized that within hops there must be either 1) Fermentable sugars, 2) wild yeast or bacteria, or 3) enzymes. After data collection from experimentation, the evidence suggested to Brown and Morris the presence of a diastase, enzymes, in this case, are indeed found in hops.

Quick aside: The term Dry Hop comes from this drying out of the beer (or freshening), not from adding dried hops to the fermented beer, as is most often used as the reason for the term. However, the modern-day use of the term renders both explanations acceptable.

What does this all mean? Put on your beer goggles, er, I mean, put your glasses on and grab an IPA, because this is going to get frothy and nerdy. Let’s talk about some beer science!

The Nerdy Talk

With current research, we now are aware of four enzymes found in hops: Alpha (α) Amylase, Bata (β) Amylase, Limit Dextrinase, and Amyloglucosidase. (It must be noted that the direct presence of Amyloglucosidase hasn’t been detected, but the activity is similar to its end product.) Some of these are found in malted barley, as well. Enzymes by themselves do not ferment beer. That’s the yeast’s job. What the enzymes do, in this case, is break down complex carbohydrates into simple sugars.

In brewing, enzymes are proteins that catalyze to accelerate a biological chemical reaction.
Enzymes are found naturally in malted barley and are heat-activated during the mash (steeped in hot water). Of the many enzymes found in malted barley, α-Amylase, and β-Amylase are the ones that brewers rely on to extract carbohydrates. This is done by the enzymes breaking down complex carbohydrates, or starches, into simpler sugars. Starch conversion happens around 131°F to about 162°F, with α-Amylase most active between about 154 – 162°F with some activity noticed as low as 149°F, while β-Amylase is most active around 140 - 149°F. Limit Dextrinase, an enzyme found in raw barley, but is almost completely denatured by the malting process, and Amyloglucosidase, the enzyme used in Brut IPA’s, do the same but more methodically. Both have been known to be active to around 140°F and have shown signs of movement at much lower temperatures. It should be noted that enzyme activity seen at very low temperatures is minimal, and very little data is out there, but the evidence is there nonetheless. The pH of the mash plays a part in the big picture with the optimum for the enzymes being between about 4 and 5. As luck would have it, that’s the pH of a typical IPA.

Long-chain carbohydrates from malted barley are made up of Glucose (a monosaccharide when isolated by itself) molecules linked together by various linkages. Two linked Glucose molecules form the disaccharide Maltose. The trisaccharide Maltotriose has three Glucose molecules. Longer chains of glucose form the polysaccharide called Amylose (image above*). These are all linked together by an α(1-4) bond.

Amylopectin (image above*) contains multiple branches of Amylose chains bonded by an α(1-6) linkage that flare out to create a large complex carbohydrate. With that in mind, α-Amylase catalyzes the α(1-4) bonds seemingly at random to release Maltose while β-Amylase is more methodical at snipping away at the same type of linkages releasing even more Maltose. As proficient as they are, α and β-Amylase, still can’t break down the complex carbohydrates completely and they leave behind some Dextrins, or unfermentable sugars, particularly those with an α(1-6) linkage. Limit Dextrinase and Amyloglucosidase can snap the α(1-6) bonds.

Is the issue becoming a bit more apparent now?

The Creep

If a packaged beer that has been highly dry-hopped and those hops contained large amounts of diastase, we could have a major problem. A beer stored cold could still show some Hop Creep by the enzymatic activity producing some extra sugars. It won’t ferment out since any yeast that may be present is inactive and will remain in the finished beer. There could be some refermentation happening in the can or bottle if it is not stored cold and there is some yeast present. Any residual sugars could be broken down and the remaining yeast could begin to ferment it. This would be nice if it was meant to be a bottle-conditioned beer. If it wasn’t, however, there could be additional ethanol production that might knock the beer out of compliance.

Should any refermentation occur in the packed product, an increase in CO2 levels will be noticeable because the gas can not escape and will be reabsorbed into the beer. It could create some geysers when opened for a cool show and a bit of a mess if CO2 levels reach excessive amounts. It was thought that Hop Creep was responsible for some beer grenades, but the wild yeast Saccharomyces Cerevisiae var. Diastaticus is the more likely culprit. That's not to say that Hop Creep alone can't cause some exploding cans and bottles. Also, fermentation by-products end up in the packaged product that will be impossible to get out of the beer. In particular...

Diacetyl = Butter. Movie Theater Popcorn in beer? No Thanks.
Photo by Jessica Rice McNew of Beer and Baking

...Diacetyl, the dreaded D-word in brewing. This compound causes beer off-flavors best described as butter, movie theater popcorn, or butterscotch candy. Diacetyl is produced by yeast during the initial stages of fermentation. While the yeast cell creates the amino acid Valine, this part of its metabolism cycle produces the compound α-Acetolactate along this pathway. A non-enzymatic oxidative decarboxylation of α-Acetolactate outside the yeast cell converts it to 2,3-Butanedione, the compound we know as Diacetyl. As fermentation wraps up, most of the Diacetyl is reabsorbed into the yeast cell. Through some more enzymatic activity, Diacetyl is converted to 2,3-Butanediol, which has a much higher threshold for us humans so we can't taste it. Brewers refer to this as the “Diacetyl Rest” and the reduction is done in the fermenter, usually by letting the temperature rise slightly.

This poses a predicament when an IPA leaves the brewery without any Diacetyl notes and reputable reviewers or customers keep insisting the packed product tasted like butter. Once any Hop Creep happens and there is any viable yeast left in the beer, the yeast will consume the sugars but will not be active enough and/or warm enough for a proper Diacetyl Rest. The once great-tasting IPA is now a butter bomb.

Acetaldehyde will also be produced along. If the yeast is healthy, fermentation will run its course as best it can creating Ethanol from Acetaldehyde. Although it may not be a big jump, it may be enough to place a beer out of spec.

The Future

Dry Hop Creep isn’t a universal problem, however. Recent evidence suggests it may be dependent on many factors. Research shows that hop varietals differ in diastatic power. For example, the preliminary findings indicate Citra and Noble Hops may have very little enzymatic power present while Cascade and Centennial have shown a noticeable amount. A cultivar is not the only factor to consider; year, yield, farm location (terroir), and harvesting process may also be a factor.

It turns out one of the most interesting areas for future study could be the drying process after hops are cultivated. A common practice is to have hops dried from 80%(+/-) moisture down to around 8 to 10% moisture and pelletized at a much lower temperature as opposed to the higher heat in years past. The temperature for drying high alpha acid fresh wet hops off the bine is anywhere from 130°F to 140°F, with some farms as high as 145°F.

The Hop Quality Group conducted research and found that lower-temperature kilning preserved delicate thiols and oils. Aroma hops, those typically used in Dry Hopping, were already dried at lower temperatures. Today, it is getting down to only about 100°F or even lower. It is thought lower temperature drying improves the aromatic properties of the hops.

This, unwillingly, may have left much of the enzymes intact since the heat does not reach high enough to denature them. More research needs to be done on this aspect, but the correlation between when this process in hop processing and Hop Creep's jump-scare into the modern-day beer scene can't be ignored.

Fresh Hops on a Bine

Playing "Creep" Away

It’s not all doom and gloom. There are plenty of ways to avoid any possible Hop Creep in one's brewery. Aside from the obvious solution of adding less or no Dry Hop to your IPA (not going to happen!), here are a few things to consider along with their possible drawbacks.

1) Sterile Filter, Centrifuge, and/or Coarse Filter

A good sterile filter or centrifuge removes any possible refermentation that could cause access carbonation and/or Diacetyl by removing the yeast, but it will not prevent any added breakdown of sugars by the enzymes that could end up adding a sweeter taste as time passes. A coarse filter can remove any vegetal matter that contains the enzymes to prevent any further breakdown of sugars.

2) Pasteurise the Beer Before Packaging

This may be an off-putting option for many brewers, and not just because of the added expense of new equipment. Although pasteurizing will kill off any yeast present and denature the enzymes, the high heat will alter the intended flavor of a highly hopped beer.

3) Dry Hop at the Tail End of Fermentation

By Dry Hopping when primary fermentation is close to wrapping up or during the Diacetyl Rest, any added enzymatic activity can occur when there is yeast present to consume any additional fermentable sugar. This may add some extra attenuation (percentage of extract available consumed by yeast) to the end product, but that can be accounted for with some experience. Embrace the creep.

4) Invest in a Dissolved Oxygen Meter

As noted above, α-Acetolactate requires oxygen to decarboxylate into Diacetyl. Although dissolved oxygen is required for proper yeast metabolism in its early stages, any dissolved oxygen left over can aid in the creation of the unwanted buttery compound.

5) ALDC (Alpha Acetolactate Decarboxylase)

ALDC is an added enzyme brewers can use to control Diacetyl production. It catches the α-Acetolactate and turns it directly into Acetoin, bypassing the Diacetyl stage in the cycle. Although this does not deal with any of the hop enzymatic activity, it can be a solution to prevent movie theater popcorn aromas in the finished beer. This only helps with Diacetyl and does not fix Hop Creep.

This subject has become a recent topic of great interest to brewing chemists and brewers of all experience levels. An idea proposed over a century ago is finally, with the great minds applying modern technology and chemistry knowledge, at a point where we can begin to understand what is happening at a molecular level. As more data is collected by research and experimentation, the knowledge surrounding Dry Hop Creep will only continue to increase. For now, there is enough information for the talented IPA producers to embrace “the freshening power of the hop” to keep crafting quality highly hopped beers. Nothing creepy about that.

Cheers!

Gilbert “Charlie” Perez, Advanced Cicerone®

References/Resources:

Brown, H. T., & Morris, G. Ha. (1893). On Certain Functions of Hops Used In The Dry-Hopping Of Beers. The Brewer’s Guardian. (Google Books)

Janicki, J., Kotasthane, W. V., Parker, A., & Walker, T. K. (1941). The Diastatic Activity of Hops, Together With a Note on Maltase in Hops. Institute of Brewing Research Scheme. (Online Library)

Hieronymus, S. (2012). For the Love of Hops: The Practical Guide to Aroma, Bitterness and the Culture of Hops. Brewers Publications (Brewers Association).

Kirkendall, J. A., Mitchell, C. A., & Chadwick, L. R. (2018). The Freshening Power of Centennial Hops. Journal of the American Society of Brewing Chemists, 76(3)

Kirkpatrick, K., & Shellhammer, T. (2018). A Cultivar-Based Screening of Hops for Dextrin Degrading Enzymatic Potential. Journal of the American Society of Brewing Chemists. 76(4)

Kirkpatrick, K., & Shellhammer, T. (2018). Evidence of Dextrin Hydrolyzing Enzymes in Cascade Hops (Humulus Lupulus). Journal of Agriculture and Food Chemistry, 66

Viva. (Accessed 2020). 10 Difference Between Amylose And Amylopectin (With Pictures & Structure). Viva Differences. (*Image Source).

https://vivadifferences.com/difference-between-amylose-and-amylopectin/

Hop Creep Technical Brief 

https://cdn.brewersassociation.org/wp-content/uploads/2020/05/Hop-Creep-%E2%80%93-Technical-Brief.pdf

Shellhammer, T. (2019). Hop Creep Derived Enzymes and Hop Creep. Master Brewers Association of the Americas - Presentation Slides.
https://www.mbaa.com/meetings/districtpresentations/DistrictPresentations/SHELLHAMMER,%20Hop-derived%20enzymes%20and%20hop%20creep.pdf

Hieronymus, S. (2020).Brewing with Hops: Don’t Be Creeped Out. Craft Beer and Brewing.

https://beerandbrewing.com/brewing-with-hops-dont-be-creeped-out/

Henderson, S. (2018). How “Dry Hop Creep” Causes Diacetyl In Beer and How Brewers Can Minimise The Risk. Rockstar Brewer

https://rockstarbrewer.com/how-dry-hop-creep-causes-diacetyl-in-beer-and-how-brewers-can-minimise-the-risk/#:~:text=%E2%80%9CDry%20Hop%20Creep%E2%80%9D%20is%20a,hopped%20can%20occur%20in%20package.&text=That%20hops%20contain%20a%20%E2%80%9Cdiastase,fermentable%20sugars%20into%20fermentable%20sugars.

Kirkpatrick, K. (2019). An Attempt at Understanding Hop Enzymes and Their Action on Dry-Hopped Beers, Q&A PDF. American Society of Brewing Chemists.

https://www.asbcnet.org/lab/webinars/webinars/Documents/20190418_QandA_HopEnzymesWebinar.pdf

White Labs, (Accessed 2020). Saccharomyces Cerevisiae Var. Diastaticus.
https://www.whitelabs.com/diastaticus

Rockstar Brewer Youtube Channel. (2019). How Dry Hop Creep Causes Diacetyl in Beer Plus 7 Ways You Can Manage it.

https://www.youtube.com/watch?v=uVFPnD5saoE

Master Brewers Podcast - Youtube Channel. (2018). 098: Dry Hop Creep
https://www.youtube.com/watch?v=mNcrJexo1pI&t=22s

Master Brewers Podcast - Youtube Channel. (2019). 127: Dry Hop Creep Continued - Does Variety Matter? (Live Episode).
https://www.youtube.com/watch?v=G3xWyM1nGb8&t=1372s

University of Oklahoma - Janux Youtube Channel. (2014). Chemistry of Beer Playlist.
https://www.youtube.com/playlist?list=PLTve54sz-eh-I4N1VmmskFrW9CS65iUiM

Photos by me unless noted otherwise.