Hop Creep

During fermentation, beer typically reaches final attenuation or terminal gravity once the fermentable extract has been consumed by yeast, leaving behind unfermentable sugars, or real extract. Hop creep occurs when beer attenuation continues due to a dry-hop addition. Because most of the literature and research conducted over the past century focuses on lager and ale production of lowly hopped beers, the potential of hop creep in highly hopped beers was not fully realized until recently. 

The hop creep phenomenon is not new, in fact earliest reports go back over 130 years. However, its effects began to gain attention over the past decade when higher dry-hop loads and hazy beers started creeping up in popularity. In 2016 Allagash Brewing was experimenting with dry hopping an existing beer recipe to make a hoppy table beer. The brewers reported an unanticipated jump of CO₂ volumes from 2.6 to 4.5 in three weeks from the dry-hop addition. The brewery dumped its first 60-barrel test batch due to this deviance. Their story was told at the Craft Brewers Conference in 2017 in the seminar titled “Unintended Over-Attenuation from Dry Hopping Beers.” A follow-up question from a brewer in the audience began with, “We are all slaves to the creep,” in reference to the effect of dry hopping on final gravity reduction. The name hop creep stuck.¹ 

Hop creep reduces the final gravity of beer and can lead to a beer that is drier than desired. More problematic than the shift in desired mouthfeel are the subsequent increases in ethanol, which can lead beer to be out of spec and exceed predicted labeling percentages. The other main by-product of fermentation is also problematic; the increased volumes of CO₂ produced can result in gushers or devastating safety concerns around exploding cans and bottle cap issues. In addition to this, the refermentation that occurs during dry hopping also creates a diacetyl spike. In order to mitigate the potential for the buttery off-flavor, brewers must extend tank residency time during cellaring, which further increases production time as well as costs for pro brewers. 

What We’ve Learned about Hop Creep Over the Years

One of the earliest records we have evidencing hop creep is from a paper written in 1893 by Horace T. Brown and G. Harris Morris titled “On Certain Functions of Hops Used in the Dry-Hopping of Beers.” While the addition of a certain amount of dry hops to finished beer in cask had been widely practiced and generally recognized by the beer industry, “certain fundamental scientific principles” of the practice were not well understood: The distinct conditioning or “freshening” power of the hops to cask beer. While the clarifying and antiseptic properties of the “hopping down hop” (dry hopping) were well accepted, the freshening power (attenuation power) of the hop remained a mystery. 

The authors performed a simple experiment by treating beer from primary fermentation with dry hopping and a control without dry hopping.  The control showed little to no signs of fermentation for many days, however the dry hopped beer entered into a brisk “after-fermentation” in a very short time and attenuated rapidly.² The authors postulated three potential causes for this activity. The first was that hops contain fermentable sugar. The second that wild yeast were present (that could degrade dextrins), and the third that diastase (diastatic enzymes) could be present in the hops. While it was known that hops contain sugars of around 3.65%, at typical dry-hop loads for the day (½ to ¾ lb./barrel, or ~0.25–0.4 oz. per gallon), this amount of sugar would not produce the observed attenuation. The authors also ruled out the effect of wild yeast. Although known and proven to be present, studies conducted on the effects of wild yeast inherent to hops did not produce the same freshening power. These experiments left no doubt regarding the hydrolytic power of hops on starch. The hops act in the same way as malt-flour by hydrolyzing maltodextrins and converting them into fermentable sugar, which the yeast can then feed upon to make ethanol and CO₂. One of the questions that came up in this work was the source of the diastase. In 1893 hops typically contained higher seed content than they do today. Brown and Morris tested the diastase potential of hop seeds as well as bract. Their work revealed that seeds contributed largely to the diastatic activity of the hops, however, the bracts of the strobiles also had diastase activity. 

The diastase activity of hops depended on variety. As for why some hops were less active than others? Brown and Morris suggested that perhaps hop tannins of some varieties were prohibitive of the diastase. More details about this study are in the sidebar at the end of this story.

Despite the many questions remaining on this dry hopping freshening power phenomenon, it was nearly half a century before findings from another study would be published. 

In 1941, Janicki et al. published their findings entitled, “The diastatic activity of hops together with a note on maltase in hops.”³ The authors aimed to determine the relative diastatic power of hops by variety, origin (geography), and age. The examination of 33 samples of seeded hops of different country, age (up to 3.5 years), and exposure to cold or warehouse ambient storage did not reveal much difference in diastatic power. However, their work corroborated the work of Brown and Morris in that seedless hops showed less diastatic power than seeded hops. However, whole seeds did not express diastatic activity, only crushed seeds displayed activity (one variety had up to 20% seeds!). The authors further confirmed that polyphenols were the likely inhibitors of hop diastase. 

More recently, brewers at Russian River Brewing re-investigated the effect of seeds on hop creep. In a conversation with Luke Holderfield, Lab Manager at Russian River, Luke revealed that the investigation into seeds stemmed from their hop creep issues impacting tank residence time. Their tank residence time had increased by 8–10 days due to diacetyl uptick from 60–70 ppb at the day of dry hopping to 150–250 ppb in three days. While final gravity could be anticipated by playing with various levers in the brewhouse, such as mash time and temperature and liquor-to-grist ratio, the diacetyl uptick was beginning to create a bottleneck in production. In their studies, seeds were tediously extracted from whole cones by tearing open cones by hand and removing the seeds with tweezers. Seed ranges of their whole cone lots of two varieties, Simcoe^® and Amarillo^®, had varying seed content, 0.05% to 5.50% by weight respectively. The brewers conducted their experiments on Pliny the Elder prior to dry hopping and monitored changes in extract and alcohol from treatments. While their benchtop attenuation rates were less than what they typically saw at production scale, the results gave them some directional information. The presence of whole seeds does not seem to affect attenuation significantly while the presence of crushed seeds, as would be found in pellet hops, does influence hop creep attenuation, as was presented by Vinnie Cilurzo at the 2022 Craft Brewers Conference. This finding counters the results of Brown and Morris’s tests on hop seed diastase activity that show whole seeds do contribute to diastase. 

So how does hop creep happen? To start, you need residual dextrins in beer, which are the main component of real extract. (Dextrins + protein + minerals + ash = real extract.) Dextrins survive the brewing process to contribute sweetness, body, and mouthfeel, making them desirable in heavily hopped, bitter beers as they help balance out high bittering loads. 

Real extract (RE), which is the calibrated amount of residual sugars, dextrins, proteins, and other components  resultant from the mashing process typically remains steady during fermentation unless exogenous enzymes or amylolytic (starch degrading) yeast are present. Examples of such yeast are Saccharomyces cerevisiae var. diastaticus and Brettanomyces. While it has been postulated that the presence of amylolytic yeast in hops could be the culprit of hop creep, we have enough data from the literature to indicate that hops indeed possess their own diastatic power to break longer chain carbohydrates into maltose and glucose.⁴ Hops possess amyloglucosidase, alpha-amylase, beta-amylase, as well as limit dextrinase.⁵

While different hop varieties possess similar diastase activity in a soluble starch solution, they behave differently in real-life dry-hopping trials.^4,6 Data suggests that amylase activity of varieties is due to genetics. Of 30 commercially grown and kilned hop cultivars tested in pellet form, Amarillo^®, Cluster, Fuggle, Nugget, and Perle displayed high activity. Azacca^® and Cascade altered the alcohol and gravity levels more than Simcoe^®, Centennial, and Citra^®. Some varieties such as Crystal, Centennial, El Dorado, Hersbrucker, Summit, and Saaz seem to have lower diastatic activity. That said, the maturity (but not age) of the plant or cone, farming conditions, and handling and processing during and after harvest heavily impacts hop diastase activity. Brown and Morris noted that hops lost freshening power when subjected to steam for a very short time. The authors also warned that high kiln temperature may destroy the freshening power of hops. Researchers at Oregon State University recently looked at the effect of higher kiln temperatures on hop creep potential to confirm Brown and Morris’s suspicion.⁷ They found that higher hop kiln temperatures can reduce amylase enzyme activity, however increasing kiln temperature can be more costly and potentially impact overall aroma quality of hops. Researchers at OSU also looked at autoclaving hops to reconfirm the findings of Brown and Morris on the effect of steam or high heat on diastase reduction in hops. With so many factors at play from genetics to harvest, more data is needed to fully vet the most impactful cause of hop diastase activity. 

Applying this Information in the Brewery

While it has been suggested that hop creep is unanticipated, it seems clear that brewers can now at the very least anticipate its occurrence. Which levers are the biggest drivers and how can a brewer manipulate these to target predictable and consistent dry hop creep is, however, still a bit of a guessing game. 

Unfiltered, highly hopped beers become problematic when three conditions occur simultaneously: The presence of unfermentable real extract (which is typical for most beers excluding extremely light lagers), active yeast, and hops of high diastase activity are used for dry hopping. To start, brewers can begin to dial in real extract in the front-side of the process by keeping the grain bill simple and limiting the amount of unfermentable sugars to achieve a consistent extract yield. 

Jake Kirkendall, Hop Scientist and co-author of The Freshening Power of Centennial Hops recalls his experience at Bell’s Brewery when variability in hop creep was a nagging issue that caused production headaches and would occasionally require blending batches to achieve consistency. Even within the same variety (Centennial), differences in diastatic activity resulted in varying degrees of hop creep and thus final ABV aberrations.⁸ At Bell’s, hop creep potential was found to be related to yeast (presence of), dry-hop timing, and dry-hop temperature. Early additions of hops on active yeast (day 1) resulted in an ABV% shift of 0.6% vs. late additions (day 9) augmenting ABV% by only 0.15%. Jake recommends dry hopping early to get the full effect of biotransformation, release of flavor volatiles from hops by yeast, and keeping methods consistent so that you can predict the dry-hopping effect. In this way, brewers can use the anticipated attenuation as a tool. Increasing dry hop temperature from 50 to 68 °F (10 to 20 °C) will also increase the effects of hop diastase. So combining temperature and duration on dry hopping will significantly increase hop creep, resulting in a much greater ABV% variance. 

Overall, dry hopping practice is tied to hop creep. Hop cones and T90 pellets will augment hop creep, while the use of hop extracts, T45 pellets, or Cryo^® hop pellets can be used to reduce hop creep.⁹ The use of extracts and Cryo^® pellets will also assist in diacetyl clearing. Lowering the vegetative material as well as the total dry-hop load will also assist in reducing hop creep. 

Because the presence of yeast is necessary for the conversion of maltose and glucose to alcohol, dry hopping post yeast crash is also an option. However, this will also limit the amount of biotransformation that could be achieved from early dry hopping. For brewers looking to bottle beer, the only way to ensure that your packaged product is 100% safe is to filter and pasteurize. However, as filtration and pasteurization are not typical practices in a homebrew setting, your best bet at home is to monitor your fermentation by measuring changes in residual extract post dry hopping. In this way you can anticipate hop creep on the finishing side. If you miss your target the first time, you can try adjusting the hopping type, time, amount, and duration for your next brew.  

Summarizing the Data from Brown and Morris (Sidebar)

1. In their studies published in 1893, the authors found total sugars to be 3.65% (dextrose 1.55 + Levulose 2.10)

a. This was determined by optical and reducing properties, 20 g of hops were extracted with ethanol (80%), and the distillate of this made up to 100 cc that was examined for sugars by polariscope and cupric reduction. Invertase and acid hydrolysis were used to test for other carbohydrates. 

2. The complete fermentability of these sugars was shown by using yeast. 

3. A large number of experiments were conducted to test hypotheses: 

a. Wild yeast are present but conditioning properties of hops is “long antecedent to the growth and development of these wild yeast forms and is quite independent of them.”

b. Initial extracts failed to show diastase.

c. 5 grams of whole hops into 250 cc of a solution of soluble starch containing 2.5 g/100 cc digested at 86 °F (30 °C), with addition of chloroform (antiseptic) against a control (same preparation but boiled) = three days increased reducing power was found in test solution: 5 g of hops in three days had produced 1.640 g of sugar (as maltose) – 1⁄3 of the weight of the hops employed. This test extract was then fermented with yeast to yield alcohol equivalent to that produced from 0.475 g of maltose. The authors repeated this experiment using amyloins and monitored maltose production over 3, 11, and 27 days; revealing that more maltose was released over time — up to 2.539 g maltose per 100 mL of solution after 27 days. 

d. Follow-up studies looked at the ability of three U.K. and Bavarian hop varieties to produce maltose from soluble starch or amyloin solutions over 48 hours. These results confirmed that, under the right conditions, hops can produce up to 91% of their own weight of fermentable sugar. 

References: 

¹ Hieronymus, S. Brewing with Hops: Don’t Be Creeped Out. Craft Beer & Brewing. 2020.

² The Brewers’ Guardian, (Vol. 27, 1893). Northwood Publications. https://books.google.com/books/about/Brewers_Guardian.html?id=MsI9AQAAMAAJ 

³ Janicki, J. et al. The Diastatic Activity of Hops, Together with a Note on Maltase in Hops. J. Inst. Brew. 1941.

⁴ Pierre-Yves Werrie, Sylvie Deckers, Marie-Laure Fauconnier. Brief Insight into the Underestimated Role of Hop Amylases on Beer Aroma Profiles. Journal of the American Society of Brewing Chemists. 2021.

⁵ Kirkpatrick, K; Shellhammer, T. Investigating Enzymatic Power of Hops Poster 35. 2017 ASBC. https://www.asbcnet.org/events/archives/2017ASBCMeeting/proceedings/Pages/35.aspx 

⁶ Kirkpatrick, K.; Shellhammer, T. A Cultivar-Based Screening of Hops for Dextrin Degrading Enzymatic Potential. J. Am. Soc. Brew. Chem. 2018.

⁷ Rubottom, L. et al. Hop Kilning Temperature Sensitivity of Dextrin-Reducing Enzymes in Hops. J. Am. Soc. Brew. Chem. 2021.

⁸ Kirkendall, J.; Mitchell, C.; Chadwick, L. “The Freshening Power of Centennial Hops.” J. Am. Soc. Brew.
Chem. 2018. 

⁹ Stokholm, A.; Shellhammer, T. Hop Creep – Technical Brief Brewers Association. https://www.brewersassociation.org/educational-publications/hop-creep-technical-brief

Written by Dr. Pattie Aron