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hen researching for a book I self-published in May of 2019 called The New IPA: A Scientific Guide to Hop Aroma and Flavor, I was struck how the collection of hop science was leading me to consider both shorter and cooler dry hopping than I was currently employing. Combining the collection of science with my own trials and discussions with other breweries, I’m becoming more of a fan of this short and cool approach. The research seems to indicate that longer contact and warmer dry hopping may not be improving hop extraction and may even have some negative impacts. So, using some of the research and language from my book and looking at new research that has come out this year, here is my case for considering short and cool dry hop durations.

Quick Dry Hop Extraction

The whole point of dry hopping is to extract the aromatic compounds from the hops, so the biggest question I have is how long does it actually take to get peak extraction? A study authored by Peter Wolf, Michael Qian, and Thomas H. Shellhammer focused on how extraction of hop compounds during dry hopping can be impacted by the duration of the dry hop. Here, three separate lots of pelletized Cascade hops were dry hopped for a week in a beer-like solution at 1/3 pound per barrel in a flushed and sealed stainless keg with no agitation. Samples were taken and analyzed on day one, four, and seven.

The results of the week-long dry hopping showed that for both linalool (monoterpene alcohol) and myrcene (hydrocarbon), day seven concentrations weren’t higher than day one concentrations. In fact, most of the results showed a decrease at day seven than the first day of dry hopping, which suggests that for these two compounds, 24 hours might be enough to get full extraction. In fact, terpenes like linalool and myrcene may reach solubility threshold in a matter of hours. Even more surprising, due to the hydrophobicity of some hop aroma compounds, extended dry hopping can cause removal out of the beer and back into the spent hops. Keep in mind this experiment was tested in a keg without agitation or special procedures to try to speed up or increase extraction.

Looking closer at short dry durations, one paper tested dry hopping at 1/3 lbs./bbl at 68°F (20°C) in both a beer-like solution as well as fermented beer (1.044 original gravity fermented with an ale yeast strain and hopped to 12 ppm iso-α-acid). The biggest difference in this test was that the beer was agitated with a shaker table during dry hopping to maximize extraction. Again, quick extraction was shown with the hydrocarbon compounds, which were extracted in just four hours (greener tasting compounds). The monoterpene alcohols also fully extracted in about four hours (fruiter and more polar compounds), peaking and then gradually reducing, especially linalool. Again, the tests were done while the beer was agitated via the shaker table, so it seems safe to assume typical idle dry hopping would experience slower extraction times.

Wolfe’s results indicate that dry hop extraction times are much quicker than most of us originally thought. Even at colder temperatures of 34-39°F (1°C-4°C), Wolfe suggests that extraction could still occur in less than three days. Looking closer at temperatures during dry hopping, a paper tested dry hopping at 39°F (4°C) and 68°F (20°C). Tested linalool solubility over the course of two weeks, the authors found the lower temperature resulted in slightly faster extraction, which peaked around day three, but was near maximum extraction on day two. The warmer temperature followed a similar extraction pattern but was just under the lower temperature’s linalool concentrations. After the full fourteen days, they were both at same levels.

In a follow-up study, Wolfe again looked at dry hop extraction, but this time adjusted the test to better replicate brewing conditions and brewed a pale ale bittered with α-acid extract, resulting in 21 ppm iso-α-acid. The beer was fermented with Wyeast 1056 and filtered prior to dry hopping. Dry hopping was done in 3-bbl (352 liter) stainless steel tanks with bagged Cascade pellets dosed at 1 lbs./bbl (3.8 grams/liter) The tank was either stirred (by pumping the beer in and out of the tank at a constant rate of 1,000 rpm) or held passive. Samples were then tested with instruments (SPME and GC-FID) and sensory testing.

The findings of the experiment showed that stirring the beer during dry hopping significantly yielded more extraction, which resulted in more aroma intensity, but at the expense of increased in astringency and bitterness. The increased astringency from recirculating dry hops was also something that came up during my interviews for the book as a complaint of recirculating dry hops, almost as if the hops are getting over extracted.

The increase in bitterness and astringency found in the experiment above correlated to the total polyphenol content in the beers. As the extraction rate increased with the dry hop duration (six hours to 12 days) so too did the polyphenol content and perceived bitterness. Pellet hops had an overall greater extraction of compounds than leaf hops and with that, a much greater total polyphenol content.

Bitterness levels also increased with extended extraction time, despite reduced levels of iso-α-acids, which agrees with findings in more recent work showing that leaf material in hops absorbs and reduces the concentration of iso-α-acids.

Short One-Day Dry Hopping Scored Higher for Fruity Flavors in Sensory Tests

In terms of extraction time, the pellet hops were nearly fully extracted in just 24 hours and the leaf hops took a bit longer but stirring of the hops helped more with leaf hops extraction than with pellet hops. Looking at sensory results rather than just measured hop compounds, the non-stirred pellet hops at just six hours had essentially the same aroma intensity scores compared to day four, again suggesting a short contact time with dry hopping may be enough to get extraction, but perhaps with less polyphenol bitterness.

A more recent 2019 study looking specifically at the hop Eureka! tested compounds daily during an unagitated dry hop. Using a 5% German Pilsner as the base, the authors dry hopped in 10 hL tanks (8.5 bbl) at -1°C (30°F) at a rate of 250 g/hL (~0.6 pounds per bbl) dropped in from the top of the tank. Even at cold dry hop temperatures, the authors found quick extraction, the monoterpene alcohols (linalool and geraniol) did nearly all their transfer of compounds within the first two days. Interestingly, a tasting panel scored the beer with just one day of dry hopping the highest fruity, citrus, and black-currant flavors. The beers with longer dry hop contact time scored higher in herbal and spicier characteristics. The authors attributed the beer with just one day of dry hopping as scoring higher in fruity-forward flavors because of the quick extraction of the monoterpene alcohols compared to the greener tasting hydrocarbon myrcene. The green and resinous myrcene saw most of it’s extraction between days 1-2, and it’s likely the more this compound extracted the more it may mask the fruitier flavors.

This study can be particularly important for hazy IPAs where the more viscous protein and carbohydrate rich beers can hold onto hydrocarbons like myrcene in larger amounts than a lighter-bodied West Coast IPA. If the viscosity increases and fewer volatile compounds (like myrcene) can diffuse out of the beer, you can see why more viscous beers could become more astringent if over hopped or over extracted with long contact times. For example, a West Coast IPA was tested for <0.3 ppm of myrcene and commercial hazy beers averaged 1.4 ppm myrcene (the highest was 2.5 ppm). So, although hydrocarbons like myrcene are more volatile than fruitier monoterpene alcohols, the more viscous hazy beer is encouraging these compounds to stay in solution, potentially with more hop bite and astringency.

Homebrewers May See Quicker Extraction

The size of a dry hopping vessel might also play a role in the extraction rates. For example, Peter Wolfe suggested in an online Q&A session that as the tank size increases, dry hop extraction efficiency decreases. So, potentially, you would need more hops when brewing on a larger scale to get the same results as you would on the homebrew scale.

Wolfe also suggests that extraction times will be faster on the homebrew scale compared to commercial size tanks. For example, it might take three to five days in a 500 BBL tank to get full extraction without re-circulation or agitation. Homebrewers would get faster extraction because of the smaller volume, even quicker if you swirl the carboy or keg a few times during the dry hop.

Wolfe explained to me that dry hopping is entirely dependent on local diffusion speeds, not reaction rates (in contrast to say, hop acid isomerization in the kettle). Thus, anything that speeds up diffusion (stirring, temperature, etc.) will speed up extraction rates, and all of those things are generally slower as you scale up tank size – there will be less liquid in direct contact with hop material and it’s harder to move that liquid around.

Hop Thiol Extraction

Polyfunctional thiols are the portion of a hop’s sulfur compounds that can be desirable in beer because of their potential for intense fruity flavors because of low taste thresholds. Some of the more researched fruity thiols are 4MMP (also known as 4MSP) which has descriptors of box tree, ribes, and black currents. The second is 3MHA and has descriptors of passion fruit, and guava and is converted from the third thiol often studied in the literature which is 3MH (grapefruit, gooseberry, guava characteristics). Just like with the desirable fruity monoterpene alcohols discussed above, thiols appear to extract quickly in beer during dry hopping.

How quickly is the transfer rate from 4MMP-rich hops into beer during dry hopping? An experiment with Eureka hops into a Pilsner style beer found quick extraction. When testing for 4MMP on different days of the dry hop, the authors determined that the majority of the 4MMP extracted into the beer the first two days of dry hopping and very little was extracted between days 2-8. Sensory evaluations of the beer agreed with the paper results as the panelist scores showed an increase of 4MMP-like note during days 1-2 of dry hopping and little further flavors for the later dry hop contact times.

As a quick reference, here are some hops with high levels of 4MMP levels: Citra, Simcoe, Eureka!, Summit, Apollo, Topaz, Mosaic, Ekuanot, Galaxy, Nelson Sauvin.

Head Retention and Dry Hopping

A paper that looked at how dry hopping impacts head retention found that as the pH increased in beer the head retention decreased. In the paper, Cascade hops were used for dry hopping at dosage rates of 0.5, 1, 1.5, 2, 2.5 lbs./bbl and measured for foam stability. As the dosage of Cascade increased, foam stability decreased as the pH increased. As a general rule, dry hopping will increase the pH by approximately 0.14 units per pounds of hops used per barrel. In addition, the longer Cascade hops sat in the beer during dry hopping, the more the foam stability was reduced. This decrease in stability was slight after two days of dry hopping, then accelerated on day three, and continued to slowly decrease until day eight. Ultimately, long term dry hopping can have a negative impact on head retention. Another point in favor of shorter short dry hop times.

The same paper makes a case for reduced dry hopping temperatures during dry hopping to improve beer foam. Here, alpha-acids were added to beer and head retention was measured at a temperature range of 57°F to 69°F (14°C-21°C), the authors found alpha-acids had a greater impact on improving beer foam at the lower temperatures.

Dry Hopping and Polyphenols

One of my biggest complaints with heavily-hopped hazy IPAs is an aggressive vegetal bitterness, sometimes referred to as “hop bite.” Part of this could be explained by a large portion of polyphenols making their way into beer via dry hopping. In a study examining the transfer rates of hop substances during dry hopping, researchers found the rate of total polyphenols pickup is about 50–60%.

A paper looking at dry hop temperature, dry hop duration, and the impact on polyphenol extraction also suggests that their may be benefits to shorter and cooler dry hop durations to reduce some hop bite. The authors found that the temperature of the dry hop significantly increased polyphenol extraction. When dry hopping at 66°F (19°C) compared to 39°F (4°C), there was an increase in polyphenol concentrations of nearly two-fold for the low-alpha hop and nearly 2.5-fold for the high alpha hop. In general, the higher the alpha-acid concentration in a hop, the lower the polyphenols. Likewise, the lower the alpha-acid hop, the higher the polyphenol content (pretty much the opposite of what I would have guessed).

How quickly do polyphenols get into beer during the dry hop? The same study above suggests that peak concentrations are around three days of dry hopping. The peak concentration of polyphenols remains consistent when tested up to a 14-days. So, extended time isn’t gradually increasing polyphenol concentration or helping to pull them out. In this case, the beer was dry hopped in 30 L (approximately 8-gallons) of wort. With what we know about extraction times and tank size, it’s possible that polyphenol extraction time on the commercial level could be slightly longer than three days. Small batch homebrewing may see full polyphenol extraction even sooner.

Combining temperature and dry hop contact time research, if we want to try and reduce the total polyphenol content in our dry hopped beers to reduce astringency, it’s best to dry hop at temperatures below typical ale fermentations and for a short duration. Experimenting with dry hopping while crashing is one way try this. Another is to dry hop during the cold carbonation period, especially since certain hop compounds like linalool were tested to still extract at colder temperatures. At Sapwood Cellars, we typically start our post-fermentation dry hopping around 58°F.

Hop Creep

Although the topic of hop creep deserves its own post (it has it’s own chapter in my book), dry hopping can produce significant quantities of fermentable sugar in beer due to enzymatic activity from the hops, especially if there are an abundance of dextrins in the wort. This re-fermentation is now commonly referred to as hop creep. In particular, four different enzymes have found in hops (Amyloglucosidase, α-amylase, β-amylase, and limit dextrinase) and when active in beer with dextrins available can lead to re-fermentation. Why is hop creep a problem? If this re-fermentation happens after the yeast has been removed from the beer you can produce diacetyl from an unhealthy re-ferment. In addition, re-fermentation after packaging can lead to over carbonation and potential bottle grenades!

Looking at the research on the best ways to prevent hop creep also plays into this concept of shorter and cooler dry hopping. Oregon State research concluded that shorter (one or two day) dry hopping at a lower temperature of 50°F (10°C) compared to 68°F (20°C), shorter durations (reduced contact time), and lower dry hopping rate created fewer fermentables. Essentially, there is less chance of hop creep when dry hopping was done cooler and quicker.

Hazy IPA Stability and Dry Hopping

Although I go into much greater detail in my book on stability issues and the science around the sensitivity of hazy IPAs, here I’ll explain how research has shown how hop duration can impact beer stability. When it comes to transition metal ions and their impact on oxidation in beer, the focus is primarily on iron and copper. But like those, manganese can also promote the staling of beer by converting ground state oxygen to reactive oxygen species (ROS). When it comes to dry hoping, it’s the manganese that is the most efficient at extracting compared to the other natural metals found in hops. How much manganese is typically in beer? A study looking at manganese concentrations in fifteen different commercial beers found relatively low levels for most of the styles, but dry hopped styles had higher levels. For example, an Imperial IPA had 0.21 ppm of Manganese and two different pale ales had 0.23 ppm and 0.15 ppm, compared to an American light lager with 0.05 ppm.

Because the beers with dry hop additions had higher manganese concentration, the paper then looked at the manganese content in hops themselves and found they contain a high amount of manganese. Of the hops tested, Columbus, had nearly twice the amount as the others. Pacific Jade and Galaxy had two of the lowest levels of manganese.

Hops Manganese (ppm) Columbus 101.9 Cluster 61.6 Fuggle 59 Cascade 56.3 Centennial 55.9 Tettnang 54.9 Citra 54.5 Saaz 54.2 Mosaic 52 Pacific Jade 38.1 Galaxy 33.1

One way to reduce the pickup of Manganese into beer during dry hopping with some of the varieties in the chart appears to be by reducing the contact time and the temperature. The same study measured the manganese level in beers over the course of a fifteen-day dry hop and found that manganese levels peak after about five to seven days. Interestingly, the warmer the dry hop temperature, the more manganese pickup into the beer. The test showed that dry hopping at 68°F (20°C) had 0.10 mg/L more manganese than the beer dry hopped cold at 38°F (3°C).

So there you have it, a brief look into dry hop research suggesting the potential benefits of short and cool dry hop durations. As always, I’m not trying to suggest this is the only or best way to approach dry hopping as many people have success doing the opposite. However, if there are multiple studies putting points in favor of one processes over another and sensory results from my own brewing trials shows great results, that’s when the beer science is the most helpful and lends confidence to incorporating into future brews.

Cheers!

Key Findings