Hop Creep: History and Puzzle Pieces

Digging into the research like it's 1899

By Keith Lacy

Apr 25, 2022


Not long ago, a topic appeared suddenly and quite uninvited in brewing circles: well-planned and executed beers were showing a tendency to over-attenuate as well as experience issues with diacetyl production. To a casual beer drinker, higher attenuation than expected may not sound all that bad, but things like regulatory oversight, packaging safety, production times, inconsistency with the final product, off flavors, and other potential issues are extremely important considerations for breweries. In the relatively short time since this phenomenon became widespread there have been a flurry of studies performed and solutions have been proposed for what we now refer to as hop creep.

Hop creep: a slow secondary fermentation brought about by the addition of hops post-fermentation.

Fortunately, there are ways to mitigate the effects of hop-creep for all types of brewers, ranging from regional brewers with millions of dollars of equipment to homebrewers with a propane burner and plastic buckets for fermentors. As is often the case with processes that rely on living organisms, full answers require a fair amount of context. By understanding the root causes of hop creep, breweries and brewers can mostly avoid the hang ups in production or out of spec ABVs. The bulk of this article will address two of the greater process issues caused by hop creep, and conclude with actionable recommendations to avoid it. Okay, let’s dive in!


Many discussions about hop creep were happening across the United States and one of the breweries leading the charge was Allagash in Portland, ME. Allagash, who makes legendary Belgian-style beers, naturally carbonates their beers through bottle conditioning. They had brewed what was intended to be a hoppy table beer and found that it was finishing at a lower than expected gravity and was over-carbonating in the bottles, leading to an internal study. Additionally, in an experiment carried out by Oregon State University’s Fermentation Sciences department in collaboration with Allagash, they found a 4.75% increase in CO2 production and a 1.3% (v/v) increase in alcohol as a result of hop creep.

Nearly 1,000 miles away, Bell’s Brewery in Michigan was another larger craft brewery experiencing this phenomenon and had been performing a battery of tests to find the root cause of the problem. After ruling out an infection, differences in ingredients, and production methods, it was determined that the main difference was the hop schedule. It seemed that dry hopping could lead to a beer finishing at a lower gravity than the brewers had planned for. 

In addition to over-attenuation there was a noticeable increase in diacetyl production. Diacetyl is often described as tasting like butterscotch or popcorn butter flavoring. One of my coworkers described it in a way that really pinpointed the flavor for me: a popcorn-flavored jelly bean. Diacetyl is also a common off flavor in beer and it is relatively easy to control once you discover it — diacetyl rests are a normal step in lager production. But clearing diacetyl can take from several days up to weeks and an unanticipated rest is a serious logistics and production headache when you run a tight, well-structured schedule. So here we have two problems: 1. Over-attenuation and 2. buttery beer, each brought on by the same cause. 

Okay, sure, but why? Where do we go from here? Wait, did you just say late-Victorian England?


Section 1. Over Attenuation

Horace T. Brown. Mustachioed and a badass scientist.
Horace T. Brown. Mustachioed and a badass scientist.

Yeast Loves Sugar a Little Too Much”

Several studies on hop creep from the past five years reference a paper from 1893 called On Certain Functions of Hops Used in the Dry-Hopping of Beer” from the publication The Brewers Guardian. One of the writers, Horace Tabberer Brown (and yes, with a name like that he DID have a mustache) was an absolutely brilliant and fascinating scientist. After expounding upon the virtues of dry hopping for its conditioning or freshening” effect on beer, the article speaks to an observation: after dry hopping, casked beers suddenly showed signs of further fermentation. They performed a study where they explored three possible explanations: 1. Fermentable sugars were introduced by hops, 2. Wild yeasts rode into the beer on the backs of hops, and 3. Hops contain a diastase” which releases sugars.

Their study found that with a 0.5 – 0.75lb of hops added to a barrel, only about 8 – 12 grams of sugar would be introduced. Not enough to account for the amount of fermentation seen. Wild yeast cultures could not be isolated from the hops themselves, and there was a lack of spoilage, signaling that the second explanation was unlikely. The third hypothesis, however…. They found that if they steamed the hops they suddenly lost the ability to trigger this after fermentation” that freshened” the cask beers. With that in mind, let us move forward to the modern day again. 

In 2017, Allagash partnered with OSU’s Thomas H. Shellhammer and Kaylyn Kirkpatrick. What they found is that hops contain enzymes that hydrolyze dextrins. The main enzymes we are concerned with are alpha-amylase, beta-amylase, amyloglucosidase, and limit dextrinase.

Enzymes are proteins that have a job to do: regulate the speed of chemical reactions without disrupting the reaction. To give a brief overview, dextrins are fragments of long chains of non-fermentable sugars that result from the brewing process and partly why we have to mash malt in the first place. During the mash, starch in malted grain is broken down into amylose (straight chains of glucose) and amylopectin (branched chains of glucose). The enzymes we are concerned with here each attack the dextrins in different ways during the mash, ultimately resulting in amylose and amylopectin being broken down into fermentable sugars: maltotriose, maltose, and glucose.

That is a huge piece of the hop creep puzzle. 

Enzymes have an optimal temperature where they work the most efficiently, but it is also close to the temperature that inactivates them, often with only a difference of 5 – 10°F (2 – 5°C). Limit dextrinase is inactivated at 145°F (63°C), beta amylase at 158°F (70°C), and alpha amylase at 176°F (80°C). Once the mash is done and the wort is heated to a boil the enzymes are completely inactivated. Because of this, exactly like H.T. Brown found when he steamed them, any hops that are added on the hot side have their enzymes denatured and will not cause hop creep. This is why over a century later a study on refermentation in cask ales in England became relevant after being almost entirely forgotten.

During the 20th century, the act of dry hopping at the tail end of, or after fermentation, virtually disappeared. The industrialization of brewing, the ability to manufacture and ship beer globally, the desire for beer that tasted the same no matter where it was made, and the need to make it shelf-stable all had a part in the knowledge of refermentation from late dry hopping being lost. Enter craft brewing.

Since about 2017 or so there has been an explosion in post-fermentation dry-hopping. New England IPAs, double dry hopped, and juicy” beers generally use hops at rates from two to four pounds per barrel, and certainly more. There has been a shift from craft beer pushing the envelope of tolerable bitterness (double and triple IPAs) to beers that have very low bitterness but a huge sensory impact, taste- and aroma-wise from hop additions. My first hazy IPA was Crushed Velvet from Marz Community Brewing and it was revelatory. The haze craze” is still going strong. The people have spoken, and they want ALL THE HOPS.

Marz Crushed Velvet, like in a fancy painting.
Marz Crushed Velvet, like in a fancy painting.

This is another huge piece of the hop creep puzzle.

With these hop-forward styles, the beers are designed to have more dextrins left in the beer after fermentation. This gives them a fuller mouthfeel and helps balance the potential bitterness of so much hop matter. When hops are added during or post-fermentation their endogenous enzymes break down dextrins into more maltotriose, maltose, and glucose. With fermentable sugars available once more, and often with agitation during the additions, yeast renews fermentation. If a brewery briefly dry hops before packaging their beer to get that last hoppy kick, the yeast could cause the bottles and cans to over-pressurize, creating the possibility of a dangerous situation — exploding packages. This additional fermentation could potentially cause the ABV to be labeled incorrectly on packaged products, so the brewers might have to have a few words with the Alcohol and Tobacco Tax and Trade Bureau (TTB) or Food and Drug Administration (FDA) as well.


Section 2. Diacetyl

Everything Tastes Better with Butter Unless It’s Beer”

Another huge issue caused by hop creep is a bump in diacetyl. Also called 2,3‑butanedione, if we’re talking nerdy, it belongs to a group of molecules called vicinal diketones, or VDKs. Diacetyl produces a taste and aroma similar to butter if it exceeds the sensory threshold (10 – 40 ppb). It can be a sign of bacterial infection, such as Pediococcus damnosus, but is also a natural byproduct of yeast metabolism. When yeast are synthesizing the amino acids valine, leucine, and isoleucine they produce an intermediate molecule, alpha-acetolactate. The conversion of this intermediate to valine requires a few more steps, one of which is a relatively slower process, what we call a rate-limiting reaction. Excess alpha-acetolactate builds up and some is excreted by the cell into the surrounding beer where it spontaneously reacts to make diacetyl.

Diacetyl formation branching off of valine synthesis.
Diacetyl formation branching off of valine synthesis.

While yeast are happy and healthy they will reabsorb the diacetyl and convert it to another molecule that doesn’t have a sensory impact. However, when dry hopping is performed late- or post-fermentation the yeast are no longer in an ideal environment (e.g. fresh, sweet, delicious wort made by a brewer). At the end of fermentation the beer lacks nitrogen and sugars so fermentation slows down and will essentially stop once terminal gravity is reached. The hops introduce enzymes, which liberate more fermentable sugars. The yeast metabolisms kick back into gear to consume the sugar, but they still don’t have all the nutrients they need. Consequently the yeast start to manufacture their own amino acids, including valine, leucine, and isoleucine and once again diacetyl is produced. But with their metabolism slowed by a nutrient-depleted and stressful environment of finished beer, the reabsorption of diacetyl is much slower.

This is another piece of the hop creep puzzle.

Thankfully diacetyl will eventually drop below the sensory threshold as the remaining yeast reabsorbs it, but at a much slower and indeterminate rate. Whereas a lager has a scheduled diacetyl rest as part of the production process, hop-forward ales typically do not. Though less spectacular than over carbonated cans and bottles exploding, a long resting period after a secondary diacetyl spike can be a significant process issue. The fermentor has to stay full until diacetyl drops below sensory threshold, holding the vessel up for cleaning, sanitizing, and refilling with the next batch. Staff has to be rescheduled, yeast has to be kept healthy and stored until it is time to be pitched. Literally every production step after complete fermentation is thrown off. 

Recommendations for mitigating hop creep

To recap, adding hops at the tail end of fermentation or even post-fermentation will introduce enzymes that liberate more sugars for fermentation. The beer drops below what was originally the terminal gravity while the yeast produces excess alcohol and carbon dioxide, the latter potentially a safety issue if the beer has already been packaged. Diacetyl can also be produced and is very slowly reabsorbed, delaying production or even just releasing the beer. Ultimately what must be remembered is that it comes down to yeast. When we brew we make wort, the yeast makes beer. The better the wort, the happier the yeast, the better the beer.

If you provide yeast with a food source they will try to consume it. It’s what we’ve asked them to do! To mitigate creation of more fermentables, studies in academia, in breweries, and in our own lab have shown that dry-hop timing choices make a significant difference. Dry hopping during active fermentation is one solution. By boiling wort we completely deactivate the enzymes that came from the malt and reduced starch to dextrins and sugars. Those dextrins that were not broken down during the mash go through the boil and into the fermentor. By adding hops during fermentation you are adding more enzymes, and they can act to break down any remaining dextrins while yeast metabolism is still active as opposed to after the yeast have mostly finished fermenting. This also moves up the process of diacetyl formation, absorption, and conversion. The best timing for dry hopping is right after you are able to harvest yeast for the next fermentation. This harvest window occurs right as the yeast is flocculating and collecting in the cone, but just before it is completely finished fermenting. The downside is that vigorous fermentation could potentially scrub some desired and more subtle aromatic hop compounds from the beer. 

It has also been shown that dry hopping at lower temperatures can decrease the extent of refermentation and the size of another diacetyl bump, though not prevent it completely. Remembering that enzymes have optimal functional temperatures, we can aim for a cooler, sub-optimal temperature, which has the potential to slow the chemical reaction of breaking down dextrins as well as slowing the metabolism of the yeast. With fewer dextrins and lower yeast activity in a cooler environment, the danger of hop creep is reduced. If a brewer is able to remove yeast, via centrifuge or by dumping the yeast cake, it can make this even more effective, as there is a reduced chance of refermentation with less yeast available. 

Another approach is to have a small sample of the beer undergo a forced fermentation. If the smaller sample gets the same treatment (e.g. pitching rate, hopping rate, temperature control) then it is reasonable to assume that the final gravity of both the full batch and smaller forced fermentation will attenuate to roughly the same terminal gravity. This approach is essentially acknowledging that hop creep is just going to happen but you can at least try to predict and manage it.

In the end, there are several ways to approach the same problem, but clearly what works for a regional brewery might not be an option for a smaller brewery, or even on a homebrew scale. The inherent variability of the craft beer brewing process is key to what makes it an artform, and so these different solutions are not meant to be prescriptive. With these alternative strategies for breweries of varying size in mind, ultimately the most important recommendation is this: be consistent with your process. When you find a method that works for you or your brewery, when you make the juiciest, haziest brews that the discerning public is clamoring for, make sure that process is repeatable. Document hop timings, temperatures, weights, volumes, pH, fermentation rates, just all the things. When you have your brewing process dialed in you will have more control over how your yeast behave and, by extension, peace of mind that your beer will be in spec and tasty.

1 Brown HT, Morris GH. 1893. On Certain Functions of Hops Used in the Dry-Hopping of Beers. The Brewers Guardian XXIII: 93, 107 – 109.

2 Bailo A. 2017. Dry-Hopping and Stirring Pellets Increases Vicinal Diketones and Lowers Apparent Extract. 2017 ASBC Annual Meeting; 2017 June 4 – 7. Sanibel Harbour Marriott, Fort Myers (FL): U.S.A.

Janish S. 2019. The New IPA: A Scientific Guide to Hop Aroma and Flavor. 

Kravitz M. Master Brewers Association of the Americas. Insight into the Relationship Between Dry-Hopping, Hyperattenuation, and VDK. 

Stokholm A, Shellhammer TH. Brewers Association. Hop Creep-Technical Brief.

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