Dry-hopping is a common technique used in the brewing process. It is a post fermentation hop addition that adds an intense hop aroma character to beer. With this hop addition, brewers have experienced over attenuation in their beers. This over attenuation is more commonly referred to as “hop creep” in the brewing community.
Hop creep is essentially the refermentation experienced after dry-hopping a fully fermented beer.
The refermentation can be easily observed by monitoring the increase in alcohol and CO2 with a subsequent decrease in specific gravity. Brewers that are dry-hopping should be aware of and monitor the over attenuation experienced with this hop addition. Over attenuation causes issues for brewers that are looking to meet brand specifications such as flavor profile, specific gravity and alcohol content. In addition, refermentation has been known to cause over pressurization in packaged bottled beers due to the increase in CO2, causing bottle caps to explode, which is a safety concern for distributors, retailers and customers.
The current research available on dry-hopping and over attenuation of beer seems to conclude that hops contain starch degrading enzymes (Diastase: mixture of amylase enzymes) that break down unfermentable carbohydrates into fermentable sugars. These fermentable sugars can then be metabolized by yeast and produce ethanol and CO2. Unfermentable carbohydrates in conjunction with hop starch degrading enzymes and active yeast causes over attenuation. A summary of this research is provided along with suggested ways to help control hop creep is discussed.
The Early Years:
The first published research on the effects of dry-hopping producing a refermentation is by Brown and Morris in 1893 (1). They stated that the refermentation could be caused by several sources like wild yeast, fermentable carbohydrates in hops and the presence of diastase in hops. They ruled out the wild yeast because refermentation started before the development of wild yeast from hops. They also stated that hops do not contain enough fermentable carbohydrates to account for the amount of refermentation observed. It was concluded that the diastase enzymes in hops is the cause of refermentation. To test for enzyme activity, aqueous hop solutions were made with a tannin binding agent called hide-filings, and when these solutions were added into a soluble starch solution, the starch was converted into maltose.
The next published research comes about 50 years later. In 1941 Janicki, Kotasthane, Parker and Walker wrote an article titled “The Diastatic Activity of Hops Together With a Note on Maltase in Hops” (2). They continued with the previous research by making aqueous hop solutions and removing tannins with a binding agent and concluded that peptone was a better binding agent than hide-filings (1). Their data showed that, without removal of the tannins, there was no diastase activity. They determined the diastase was not extracted in the solution in the presence of tannins.
Today one would may conclude that the tannins inhibit the diastase activity (4-6). Tannins are a class of astringent polyphenolic compounds, and polyphenol molecules have shown to inhibit amylase enzymes (7). Their data also supported that hop seeds had a higher diastatic activity than that of a hop cone. The rate of conversion of starch to maltose was greater for seeds than hop cones, but if given enough contact time, the amount soluble starch converted to maltose was equal.
They also concluded that the diastatic activity of hops was equal among varieties, storage conditions and growing regions (Europe and United States) and a pH range of 4.1 to 4.8 did not seem to have any affect on diastase activity. They also showed that the higher amount of diastase in solution equated to higher diastase activity. They continued with actual dry-hopping experiments which showed that, although hop varieties may have the same diastase activity in soluble starch solutions, they did not have the same activity in actual dry-hop trials. They attributed this difference to the presence of diastase and/or yeast activators in hops.
Their dry-hop trials also showed the amount of refermentation depended on active yeast, since no change was seen in pasteurized beers, and the amount of unfermentable carbohydrates present after fermentation (higher final gravity beers have a higher amount of refermentation).
After 1941 until 2015 there does not appear to be any published research on the over attenuation of dry-hopped beers. This is probably due to the craft brewery boom we are currently experiencing with the increase in dry-hopped beers in the market place. It appears that the refermentation from dry-hopped beers was all but forgotten.
The Rediscovery of Refermentations Caused by Dry-Hopping:
Dry-hopped beers are a style of beer that nearly all craft breweries produce and, with this production, most have experienced refermentation after dry-hopping. They may not have known what the cause was, but they reported:
decreases in gravity
increases in alcohol
increases in diacetyl (buttery off flavor in beer)
exploding bottles in the warehouse in their dry-hopped brands.
All of this is evidence of over attenuation caused by dry-hopping in the presence of active yeast and unfermentable carbohydrates, which is now named “hop creep”. Although presentations at industry conferences in 2015 (3) would discuss this phenomenon, no new research would be published until 2018.
In June 2018 Kirkendall, Mitchell and Chadwick published an article titled “The Freshening Power of Centennial Hops” (4). Their research was completely focused around dry-hop trials and monitored the over attenuation of the trials by measuring specific gravity and alcohol. They used a series of dry-hop experiments that included:
multiple hop varieties
timing of hop addition
The non-dry-hopped controls showed no change in alcohol or gravity, and only dry-hopped beers showed evidence of refermentation which they called the Freshening Power of Hops (FPH), a term that was actually coined by Morris and Brown et al. The dry-hop trials with Centennial hops of different crop years which had been stored up to 2 years confirmed the findings of Janick et al.; the age of hops had no difference in enzyme activity.
However, the varietal trials showed differences amongst the varieties tested. Amarillo® and Cascade had the greatest change in alcohol and gravity (highest enzyme activity), followed by Simcoe®, Centennial and Citra®, which had the lowest. They explained that this could be a varietal attribute and/or an agronomic, postharvest or processing factor. They found in the temperature-dependent dry-hop trial that if a brewer dry-hops at a temperature where yeast have no activity, then no change in gravity or alcohol would occur. The major contributors of FPH were hop contact time and yeast activity. This study in the end provides brewers with a good experimental model as well as simple monitoring techniques (alcohol and gravity) to investigate hop creep within their breweries.
Two more recent articles from Kirkpatrick and Shellhammer have also been published in 2018 and 2019, entitled “Evidence of Dextrin Hydrolyzing Enzymes in Cascade Hops” and “A Culitvar-Based Screening of Hops for Dextrin Degrading Enzymatic Potential”, respectively. These papers provide methods to actually screen hop cultivars for specific dextrin degrading enzymes using enzyme assay kits. Starch degrading enzymes amylogucosidase, α-amylase and β-amylase were measured in different hop cultivars including Cascade. During the dry hop trial, besides measuring gravity and alcohol, they also include HPLC analysis of small molecular carbohydrates like glucose, fructose and maltose combined with short chain carbohydrates (DP4-DP7) to measure the carbohydrate changes throughout their trials.
HPLC analysis of the carbohydrates during dry-hop trials concluded that the level of increase in small carbohydrates was a result of the starch degrading enzymes of hops reducing the unfermentable carbohydrates left in the fermented beer, just as Brown and Morris concluded in 1843 (1). In the first article, they investigated the enzyme activity of Cascade hops in dry-hop trials over several sets of conditions including temperature, hopping rate and contact time. They found that hopping rate was essentially proportional to the formation of glucose. The hop enzymes could be deactivated when the hops were heat treated prior to dry-hopping (121 degrees C). Warmer dry-hopping temperature and longer contact time increased fermentable sugar formation.
They concluded that over attenuation could be controlled by reducing dry-hopping temperature and/or contact time.
In the 2019 article, although they successfully analyzed for the specific enzymes of 30 varieties and conducted dry hop trials for all the cultivars, they could not conclude which variety had the highest or lowest enzyme activity. They concluded that the actual enzyme activity of a hop cultivar may be genetic (types of inhibitors), but it is heavily influenced by the maturity of the cone and plant, agronomic considerations (nutrients, pest management, water), harvest handling (kiln temperatures) and postharvest processing.
Hop creep, or the over attenuation observed with beers that are dry-hopped, is not a new discovery and has been documented since 1893. The research behind this phenomenon has been reignited with the amount of dry-hopped beers that are now being produced. The cause of this over attenuation is from hop starch degrading enzymes breaking down unfermentable carbohydrates into fermentable carbohydrates in the presence of active yeast. Hop creep can easily be monitored by measuring alcohol and specific gravity.
According to the early research, enzyme activity is highest within the seeds of hop plants, but is definitely in the hop cone as well. The hop enzyme activity of individual hop cultivars does not appear to be distinguishable at this time. Factors that affect the enzyme activity of a hop is not only genetic, but influenced by agronomics, maturity, postharvest handling and processing. One factor that seems to be agreed upon in the research is that the tannins in hops affect the enzyme activity and identifying these specific inhibitory compounds will probably be the focus of future research.
Brewers who have dry-hopped brands of beer must be aware of and monitor the over attenuation that happens with dry-hopping. It is imperative for consumer safety (as it can cause bottles to explode from the pressurization of CO2 increased in closed systems), the regulation of stated alcohol content on labels, as well as maintaining flavor/aroma specifications for a beer brand. Hop creep can be controlled by use of contact time, temperature and wort gravity.
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1. Brown, H. T.; Morris, G. H. on Certain Functions of Hops Used in the Dry-Hopping of Beers (Abridged version accessible via https:..books.google.com/books?id=MsI9AQAAMAAj&pg = PA93 as two excerpts from The Brewers Guardian, 1893; part1. March 28, 1893 (no. 586, pp 93-94); part 2. April 11, 1893 (no. 587, pp. 107-109). Trans. Inst. Brew. 1893, VI(4).
2. Janicki, J.; Kotasthane, W. V.; Parker, A.; Walker, T. K. The diastatic Actiity of Hops, Together with a Note on Maltase in Hops. J. Inst. Brew. 1941, 47(1), 24-36. DOI:10.1002/j.2050-0416.1941.tb06070.x
3. Bailo, A. Dry Hopping and Stirring Pellets Increases Vicinal Diketones and Lowers Apparent Extract. Presentation at the ASBC Annual Meeting, Fort Myers, FL, 2017.
4. Jacob A. Kirkendall, Carter A. Mitchell & Lucas R. Chadwick (2018) The Freshening Power of Centennial Hops, Journal of the American Society of Brewing Chemists, 76:3, 178-184, DOI 10.1080/0.3610470.2018.1469081
5. Kaylyn R. Kirkpatrick and Thomas H. Shellhammer Evidence of Dextrin Hydrolyzing Enzymes in Cascade Hops, J. Agric. Food Chem. 2018, 66, 9121-9126
6. Kaylyn R. Kirkpatrick and Thomas H. Shellhammer (2018) A Cultivar-Based Screening of Hops for Dextrin degrading Enzymatic Potential, Journal of the American Society of Brewing Chemists, 76:4, 247-256, DOI: 10.1080/0.3610470.2018.1546091
7. Gordon J. Mcdougall, Faina Shpiro, Patricia Dobson, Pauline Smith, Alison Blake and Derek Stewart Different Polyphenolic Components of Soft Fruits Inhibit α-Amylase and α-Glucosidase, J. Agric. Food Chem. 2005, 53, 7, 2760-2766.