What We Know About Dry Hopping
Free sample chapter from my book titled, “The New IPA: Scientific Guide to Hop Aroma and Flavor.”
About the Book: In The NEW IPA, Scott Janish scours through hundreds of academic studies, collecting and translating the relevant hop science into one easily digestible book. Through experiments, lab tests, discussions with researchers, and interviews with renowned and award-winning commercial brewers, The NEW IPA will get you to think differently about brewing processes and ingredient selection that define today’s hop-forward beers. It’s a must-have book for those that love to brew hoppy hazy beer and a scientific guide for those who want to push the limits of hop flavor and aroma!
Dry hopping regimes among homebrewers and commercial breweries can vary drastically and these differences (like tank sizes for example) can alter extraction rates and ultimately the final flavor and aromas of beer. In this chapter, I analyze research that has been conducted on dry hopping to help brewers maximize these important aroma additions. Unfortunately, even after reading hundreds of pages of studies on the topic, I can’t with any certainty suggest the best way to approach dry hopping. However, the research can help guide brewers into experimentation with different methods, recipe designs, and equipment to boost results.
Oxygen is the number one enemy of dry hopped beers. It can quickly transform a fresh and fruity hoppy beer to a sweeter cherry cardboard darker version of itself quickly. In my experience, this can happen even faster in the hazy hoppy style. For example, I’ve had a hazy IPA oxidize in primary after just two weeks because of a leaky lid! I would avoid using buckets on the homebrew scale for hazy hoppy beers for this very reason, they seem to be notorious for leaking, which is particularly concerning post-fermentation.
Dry hopping is often overlooked as a potential source of oxygen introduction, but a thesis by Peter Harold Wolfe, titled, “A Study of Factors Affecting the Extraction of Flavor When Dry Hopping Beer,” notes that the introduction of dissolved oxygen is “inevitably introduced” when dry hops are added to beer, “resulting from the multitude of crevices inherent to their anatomy.”
Because of this inevitable oxygen introduction, Wolfe suggests that dry hopping early in fermentation while the yeast is still present and active would allow oxygen to be metabolized by yeast before it can oxidize the beer.[i] We know that active fermentation dry hopping may reduce oxygen introduction, but may also impact final hop compound levels C02 production and removal and absorption from yeast cells.
Considering most ale fermentations, when adequately pitched and oxygenated, are typically done fermenting around five days or so, it would seem then that dry hopping for the purposes of eliminating oxygen should be done around day 3-5 of fermentation. What if you want the post-fermentation dry hop profile? A few brewers I spoke to who like to dry hop after fermentation will add a small amount of sugar with the dry hops to encourage quick re-fermentation in hopes of scrubbing some oxygen introduced with the dry hops.
Specific to homebrewers, another way to avoid oxygen pickup during dry hopping is to add dry hop additions to an empty keg and purge the entire keg and the dry hops with CO2 (filling to 15-20 psi multiple times). You can then transfer the beer into the keg through the liquid keg post. If transferring into the keg with an auto-siphon, be sure to keep releasing pressure from the keg to ensure flow.
If you are fermenting in a keg or another vessel hooked to CO2, you can use a spunding valve when making the transfer to the serving keg. The spunding valve helps maintain a nice slow transfer by slowly releasing pressure from the receiving vessel which helps prevent foaming. Ideally, you want the serving keg’s PSI to be slightly under the PSI of the source keg. If the serving keg’s pressure is too high, there won’t be flow into the keg. On the other hand, if the pressure from the pushing keg or vessel is much higher that the receiving keg it will cause the beer to transfer too quickly.
Commercial brewers can do a version of this as well, just expect to use a lot more CO2! In my discussions with commercial brewers, most are doing CO2 transfers of beer from tank-to-tank (for example, fermentation tank to brite tank) but also first purging the receiving tank with CO2 (like purging a keg for homebrewers). Some brewers have reported lower dissolved oxygen readings when using CO2 to transfer beer between vessels compared to using a pump and a balancing line. I outline how Sapwood Cellars does this with hoppy beers in the last chapter.
Another option to keep oxygen levels low is to fill a receiving vessel with water or sanitizer and push it out with CO2. When the vessel is filled with the liquid and pushed out with C02, it requires less C02 than purging and can remove even more of the oxygen. This can be done with kegs on a homebrew scale, but I wouldn’t recommend doing this if you cut your dip tubes because sanitizer or water will be left in the bottom of the receiving keg and will mix with the beer.
Another method to reduce oxygen exposure during dry hopping is by continually flushing the headspace with CO2 while adding the hops. For homebrewers, if fermenting in a keg, simply hook up the CO2 to the gas side of the keg and flush the headspace while opening the lid and adding the hops (5-10 psi). If fermenting in a carboy, you can place a hose hooked to your C02 above the beer to fill the headspace dropping in the hops. Commercial brewers might have CO2 go through the spray ball while pouring in dry hops. This method of flushing the headspace with 15-20 PSI of CO2 has been tested and shown to reduce variability in the aroma intensity of dry hopped beers caused by oxygen.[ii]
Homebrewers are unfortunately at a disadvantage when it comes to oxygen exposure potential during dry hopping. The smaller the batch size, the more the total quantity of beer exposed to oxygen introduced during dry hopping. With larger commercial brewing systems, the massive amount of beer being produced acts to dilute the oxygen to a much greater extent. It’s similar to why a 5-gallon barrel has the potential for faster oxygen intake compared to a standard sized barrel. In smaller barrels (like with smaller fermenters), there is a greater surface area to volume ratio, which means more of the beer can be in contact with oxygen.
Because smaller batches can be impacted more through oxygen exposure during dry hopping, I would consider the purging of CO2 for homebrewers during dry hoping an important step. I’m not even sure making extremely small batches of hazy hoppy beer is worth the effort as the study mention above found that dry hopping should be done at volumes at least equal to 20 L (5.3 gallons), suggesting oxygen issues with small batches especially at risk to oxidation.
Commercial brewers can dry hop using a hop doser, which is a device that attaches to the dry hop port on the top of the tank via a tri clamp fitting and a butterfly valve. The hop doser allows you to open the butterfly valve that is connected to the dry hop port on the tank and drop in the hops without exposing the beer to oxygen. An added benefit of the dry hop doser is the ability to purge the hops with C02 prior to dropping them into the tank. The hop doser is generally big enough for an 11-pound bag of hops and is especially great for dry hopping after primary fermentation has completed. When dry hopping late, brewers will often put a few pounds of pressure on the tank (since fermentation is complete) to help trap in volatiles and the dry hop doser allows you to match the tank head pressure ensuring no oxygen pickup when opening the butterfly valve to drop in the hops.
How long to dry hop is a common discussion among brewers. Some prefer hops on beer for weeks, while others prefer 24 hours or less. Sometimes contact time is just a matter of practicality. But luckily, there has been research that may influence how you approach dry hop durations going forward.
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 and myrcene, 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. [iii] Keep in mind this experiment was tested in a keg without agitation or special procedures to try to speed up or increase 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 recirculation 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.[iv]
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.
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 compounds). Terpene alcohols also fully extracted in about four hours (fruiter 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.[v]
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.[vi]
Homebrewers could use this data to experiment with dry hop and carbonating at the same time. If colder temperatures still achieves extraction, adding hops to a keg at the same time it goes into the fridge for carbonating could help speed up your grain-to-glass times without having a negative impact on extraction rates from the dry hops. You could even accelerate extraction by gently swirling the keg.
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-barrel stainless steel tanks with bagged Cascade pellets dosed at 1 lbs./bbl. 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 this 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.[vii]
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.
Overall, pellet hops have been found to extract at higher rates in multiple studies. For example, Hopsteiner found that linalool, for example, was nearly 50% greater after two days of testing compared to whole leaf.[viii] So, you may need to increase the quantity of leaf hops used to get the same intensity you’d get with pellets. As a starting point, I’d suggest increasing the whole leaf additions by about 25% compared to what you would dry hop with in pellets.
At the commercial level, one way to experiment with recirculating while also attempting to keep astringency down is to do short recirculation periods with lower dry hop amounts. For example, one experiment showed that after just two hours of recirculating (via a pump) immediately after adding the dry hops and then allowing the hops to sit idle in the tank, resulted in increased aroma compounds compared to a non-circulated beer. Specifically, in the test above, linalool averaged an increase of about 58% when recirculated for the two hours. [ix] I would like to see this experiment repeated at even shorter recirculation periods.
To expand on polyphenols and dry hopping, a study looked at how polyphenolic bitterness of hops could influence the harshness of beer when combined with iso-α-acids. The authors found that when polyphenols increased along with iso-α-acids, bitterness was described as harsh, medicinal, and metallic. Interestingly, not only did this character increase with more polyphenols and iso-α-acids, but so did the duration of the sensation on the palate. This suggests that beers with high levels of polyphenols that you might get from heavy dry hopping (especially when combined with a heavy protein grist) might induce a lingering harshness The authors didn’t rule out the possibility that humulinones could contribute to this along with polyphenols. As we’ll learn later, is likely.[x]
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%.[xi] In terms of a hop’s potential for polyphenols (although they vary by hop variety) hops generally have a total polyphenol content of around 2-6%. Obviously, as the dry hopping rates increase, so would the overall polyphenol concentration.
A 2018 study by Hopsteiner found that a unique polyphenol found only in hops (xanthohumol) is in significantly higher rates in hazy beers than other styles. Compared to a West Coast IPA for example, which had 0.7 ppm of this polyphenol, the highest commercial hazy IPA was tested at 3.5 ppm and the average of all the hazy IPAs tested in the study was 2 ppm of xanthohumol (more than double a West Coast IPA.[xii]
The same body of research also tested other non-polar compounds (like myrcene) in commercial hazy IPAs and found they are also much higher than in the West Coast IPA tested. The West Coast IPA tested for <0.3 ppm of Myrcene and the hazy beers averaged 1.4 ppm and the highest was 2.5 ppm. So, although myrcene is non-polar, the more viscous protein-rich base is encouraging these compounds to stay in solution, potentially with more hop bite.
This same Hopsteiner study looked at how putting a hazy beer through a centrifuge might impact some of these compounds and found that centrifuging removed half of the myrcene and the half the measured polyphenol (xanthohumol) content. Although incredibly expensive devices, centrifuging hazy IPAs may be a way to decrease the conditioning time required to reduce hop bite by removing some of the harsh tasting polyphenols by as much as half their levels almost immediately.
There is even more evidence suggesting that dry hopping can impact the perceived bitterness in beers, likely from polyphenols. Here, a paper considered the phenolic acid and total polyphenol contents of 34 commercial lager beers brewed in different countries, combined with sensory testing of the same beers. The authors found that beers that were dry hopped had the highest phenolic, polyphenol, and humulinone concentration. These same beers were also perceived as having a harsh and progressive bitterness.
In the same study, researchers found that beers bittered with a blend of tetra and pre-isomerized iso-α-acids products were rated as having a smooth and diminishing bitterness compared to more traditionally hot-side hopped beers.[xiii] This could potentially be an option for the bittering addition for a beer that will be heavily dry hopped. If you know that lots of dry hopping may contribute to some polyphenol and humulinone bitterness, then replacing the hot-side bitterness with a pre-isomerized iso-alpha product could be something to consider. Especially if your system is limited to a maximum amount of whirlpool hopping.
Although also impact other hop compounds, one method to try and minimize polyphenol bitterness from dry hopping (other than running the beer through a centrifuge) is to dry hop while the yeast is active. Polyphenols can interact with yeast and drop out of solution when yeast flocculates, which should, in turn, reduce overall polyphenol content.[xiv] For some hops that have higher polyphenol contents (generally higher α-acid hops), early dry hopping could be worth an experiment. Leave the lower polyphenol varieties for late hopping when fermentation is done or nearly done.
I would like to see experiments done testing the various hop compounds and polyphenol content when fining agents like Biofine® Clear are used. Advertised to formulate the rapid sedimentation of yeast and other hazy forming particles, I wonder if it would help drop out some of the harsher astringent qualities of a hazy beer, like centrifuging, but at a much lower price point. But won’t that make the beer clear?! It hasn’t in my experience on the commercial level anyways!
|Hop Variety||Polyphenols (%w/w)||α-acids|
Another study looked at two varieties and tested them for α-acids and polyphenols. The authors found that a 17% alpha variety imparted fewer polyphenols into a tested beer during dry hopping compared to a much lower 3% alpha variety.[xvi] So, although this is a relatively small sample size of hops tested for polyphenols, from the information I could gather, it seems that brewers can likely assume higher polyphenol concentrations in lower α-acid hops and higher concentrations of polyphenols in lower α-acid varieties.
In conversations with the researcher who authored the study above, it was mentioned that the highest polyphenol content hops (averages around 5%) were classic aroma varieties with greener aroma and flavors. The medium polyphenol-content hops (averages around 4.4%) were the aroma varieties and medium alpha flavor hops. The lowest polyphenol-hops (averages around 3.6%) are the bittering varieties.
Moving to other variables that can impact the total polyphenol content of dry hopped beers, a paper looking at dry hop temperature, dry hop duration, and the impact on polyphenol extraction had some interesting results. 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
How quickly do polyphenols get into beer during the dry hop? One study suggests that peak concentrations are around three days of dry hopping, which is about where the research suggest most hop compound extraction peaks. The peak concentration of polyphenols remains consistent when tested up to a 14-days.[xvii] 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 (14°C) and have had success as low as 40°F (4°C).
Although I don’t have any direct experience with the fining polyvinylpolypyrrolidone (PVPP), it’s another method to try and reduce polyphenol content of beer and reduce astringency. As it has been tested, beers with reduced polyphenol content scored lower on harsh character.[xviii] It’s possible, however, that removing polyphenols might also reduce the mouthfeel fullness of beer. [xix] Trevor Fisher, a good friend and accomplished homebrewer did a hop experiment with PVPP as I was doing this research and we concluded in relaxed sensory tastings that the beer made with PVPP had reduced hop flavors along with reduced astringency. Neither of us were particularly impressed overall with the PVPP beer, but this is just one example.
Pellet Disintegration and Sedimentation
When dropped into beer, pellets simultaneously begin to swell and disintegrate into primary particles. During this time, the particles break off and will drop or stay suspended. Eventually, all the hop particles will settle to the bottom of the vessel, but the velocity of the settlement depends on the size and density. In other words, the larger the particle size that breaks away from the pellet during swelling, the faster it will drop out of the beer and extraction stops or slows. On the other hand, the smaller the particle size, the longer the particle will stay in suspension and contribute to the extraction of volatile compounds.
Since sedimentation of the hop particles, especially large flocs, can happen rather quickly, it’s essential to keep hops in suspension to continue the extraction process. This is likely why some studies found better or faster extraction when using stirring or pumping techniques to keep the hops in contact with the beer.
In addition to suspension times and particle size, swelling process can influence hop compound extraction, because the surface area depends on the particle size and fluid uptake of the primary particles. Small particles have a large specific surface, but fluid uptake (swelling) increases the surface area. So, it’s ideal to have fast and efficient swelling of the hops and small particle size distribution. One way to encourage speedier swelling is by increased temperatures. Swelling velocities increase as the dry hopping temperature increases.
Interestingly, the higher the α-acid of a hop, the less swelling because α-acids don’t isomerize at dry hopping temperatures: they stay hydrophobic (repel or fail to mix with liquid) and don’t uptake the surrounding beer. This means you might get better extraction when dry hopping with low α-acid hops because they will have a higher swelling volume, leading to a larger surface area for extraction of the hop compounds.[xx]
The above information on how pellets behave during dry hopping helps explain the results of a Hopsteiner study on the extraction of linalool when dry hopping with loose pellets vs. hops contained in a sack. Here, the beer dry hopped loosely had nearly 50% more extraction than the beer dry hopped with the use of a hop sack. Getting reduced extraction when using a hop sack makes sense because keeping the hops contained reduces the surface area of the hops, leading to less extraction. Dry hopping loose could then increase extraction, reduce hop usage (which might also reduce harsh polyphenols), and speed up dry hopping time required.
I wouldn’t think that containing hops (like in a hop-spider) on the hot-side of brewing would have as much of a negative impact a was tested on the cold side above with the hop sack. This is because elevated temperatures and constant agitation from the boil should increase extraction.
As a homebrewer, I know that dry hopping in kegs and dealing with clogged poppets can also be a problem. Because of this, I worked with a company (Utah Biodiesel Supply) to create a stainless 300-micron filter designed to go around the keg’s dip tube. You can then insert a #6 or #6.5 pre-drilled silicone stopper into the open end of the filter and slide the dip tube through the stopper and into the filter, which seals up the filter from rogue hops. The 300-micron filters are intended to be used with hop pellets and the 400-micron filters with whole leaf hops, I would recommend going with the 300-micron filters to use for both. You can also use a filter setup like this if you choose to ferment in kegs, which allows for pressurized hop-free transfers out of the primary fermenting keg.
Other homebrewers have also had success dry hopping loose in kegs with a “Clear Beer Draught System,” which forces the keg to pull beer from the top of the keg rather than the bottom. At colder serving temperatures, most of the hops in the keg will sink allowing for clear hoppy beer to be pulled from the top via the floating device.
With so many factors that can impact variables like extraction rates, using a standard dry hop amount figure might do more harm than good. However, there is research into how different dry hop volumes might impact the final flavor and aroma.
A 2017 study looking at dry hop amounts tested Cascade hops from the same 2015 hop crop on a pale ale dry hopped at a range of 200-1,600 g/HL (0.5-4.1 lbs./bbl). The beer was then analyzed for oil concentration and sensory testing was also performed. Tested with 40L of beer (10.5 gallons), dry hopping was done for 24 hours of contact time at 68°F (20°C).
Not surprisingly, the higher the concentration of Cascade hops, the higher sensory scores for hop intensity. However, the gap between 8 g/L (2 lbs./bbl) and 16 g/L (4 lbs./bbl) was relatively small. Hop intensity may not always be the goal, however, as results also found that as the rate increased (with hop intensity), so did scores for herbal and tea characteristics. Cascade hops scored the highest for citrus marks when used at 4 g/L (2 lbs./bbl), but at higher rates, herbal and tea descriptors were cited more often. The conclusion (for Cascade hops, anyways) is that dry hopping at a rate of 4-6 g/L may be the best way to maintain the citrus quality of the hop.
Another key takeaway from this study is a look at the extraction percentage of terpene alcohols at the different dry hopping rates. In other words, how much of the possible fruity monoterpene alcohols, like linalool, geraniol, and nerol, were extracted at the different dry hop amounts? They found that as the dry hopping percentage increased, the efficiency of the extraction decreased. In other words, these monoterpene alcohols are extracted more efficiently when dry hopped in smaller doses. Specifically, when the beers were dry hopped at just 2 g/L, they had higher extraction percentage increases compared to dry hopping at 4 g/L (about 8% increase in linalool, 2% for geraniol and approximately 4% for nerol).
Looking at even bigger dry hop volumes, the authors found that dry hopping at 2 g/L compared to 16 g/L had increase in efficiency was about 17% for linalool, 4% for geraniol, and 4% for nerol in the smaller dry hop charge. [xxi] Although not massive differences, this suggests that dry hopping in smaller stages with fewer hops (double or triple dry hopping) may be better for getting more flavor compared to one big dry hop charge.
Another study examining extraction of hop compounds confirmed that as an amount of dry hops increases, the extraction percentage decreases. Here, the authors dry hopped a 5% IPA using a recirculating filter and T90 Citra pellets at either 250 g/hl, 500 g/hl, 1,000 g/hl, and 2,000 g/hl. Measurements of linalool showed a roughly 20% decrease in extraction from the 250 g/hl to the highest dry hop charge of 2,000 g/hl. For homebrewers, this would mean a drop of about 20% extraction when dry hopping around 5-gallons of beer at a rate of 400 grams vs. 50 grams. Considering the highest total extraction achieved for linalool was only around 38% of it’s potential, reductions from large dry hop additions could be significant.
The data makes a case for experimenting with low dry hop dosages (maybe 1.5-3 ounces at a time for homebrewers), but two to three different times throughout the ferment to increase extraction rates. Increasing these rates with fewer hops could potentially mean less extraction of harsh polyphenols and bitter humulinones that could come with higher hopping amounts.
For homebrewers, in regard to the Cascade study anyways, it would mean that for a 5.5-gallon batch (leaving room for a half gallon of loss), it might be best to use a modest dry hop charge of just 124 grams (about 4.5 ounces) of Cascade to keep the citrus profile of the hop. It might be even better to split up the 124 grams into 41-gram increments to try and boost extraction potential, being sure to purge the headspace with C02 with each dry hop addition to prevent oxygen exposure post-fermentation.
Another factor to consider with dry hopping is how batch size can play a role in extraction. One paper tested the same dry hopping rates (100 grams per 100 l (26.4 gallons) of beer) in three different batches ranging from large industrial-scale, semi-industrial, and laboratory scale (done in 5-gallon kegs). The authors found that at the same dry hopping rate, the smaller 5-gallon keg had substantially higher concentrations of hop compounds than the two larger batches. Sensory testing also revealed that the reduced batch size resulted in higher intensities in smell and taste.
However, the smaller batch size had hop characteristics that were defined as less fruity than the larger batches with more of a raw hop and herbal character.[xxii] The sensory information would appear to agree with previous research in that greater extraction in smaller batches means less is probably more in terms of how much dry hops to use.
Ultimately, the authors concluded that the composition of hop compounds from dry hopping depends on scale because the mass transfer rates of hop aromatics are not comparable for different batch sizes. This means that trying to scale up a homebrew recipe to a large commercial size may come up short regarding desired hop aromatics. On the other hand, trying to emulate the same dry hopping rates from commercial breweries on the homebrew scale would also not likely get the same results.
One of the biggest downsides to extreme dry hopping rates are beer losses. This may not be as big of a deal to the homebrewer who can brew slightly over 5 gallons to accommodate for the loss. However, professional breweries need to make money, so losses from excessive dry hopping can add up. The study above looking at batch sizes and dry hopping rates looked directly at beer losses at different dry hoping rates and found that absorption of beer from hops ranged from less than 2% at 250 g/hl to almost 14% with 2,000 g/hl. Somewhere in the middle at 1,000 g/hl (around 7 ounces for a 5-gallon batch) had losses just shy of 6%.
The authors suggest that to avoid losses and keep a strong aromatic profile in the beer, brewers can supplement dry hopping with hop extracts and oils. Oil products produced from cold CO2 can take the oils of single hop varieties, which still enables flavor control, but at the same time can be used at 100% transfer efficiency with zero beer loss. In other words, dry hopping Citra at 250 g/hl (0.65 lbs./bbl) resulted in about 69 ppm of geraniol (41% extraction efficiency), where just 2 g/hl of hop oil resulted in 84.2 g/hl at (100% efficiency). But an important disclosure: this study was produced by Totally Natural Solutions, who sells hop oils.[xxiii]
I always assumed that if a particular hop variety was high in total oil content, then it had a better chance of producing a beer with bigger flavor and aroma. But is this the case? In 2016, researchers at Oregon State University put this concept to the test by evaluating beers dry hopped with Cascade hops from 29 different lots. Total oil content from each lot ranged from 0.60 ml/100g to 2.10 ml/100g. A trained panel evaluated the beers five times, each in a randomized fashion.
The authors concluded that Cascade dry hopped beers containing higher amounts of total oil did not necessarily result in a beer with higher overall hop aroma intensity. In fact, the Cascade hop with lowest oil content had one of the highest aroma intensity ratings and one of the higher total oil hops had the lowest evaluated aroma intensity.
The authors suggested that one possible reason could be the quality of the oil, which can be affected by a number of different variables like geographic location, climate, irrigation, disease pressure, harvest date, post-harvest processing, storage, transport, and evaporation. It’s also possible that relationship exists within the tested hop compounds (synergy) that certain compounds may work together in a way when in greater amounts to produce hop aroma.[xxiv]
Research conducted in 2018 at Oregon State University also found that a hop’s total oil content is a poor indicator for predicting hop aroma potential from dry hopping.[xxv] In the study, 84 hop samples were tested spanning three different harvest years (2014, 2015, and 2016) within two different hop varieties (Cascade and Centennial). Sensory and hop oil analysis was performed on the dry hopped samples from unhopped beer produced commercially. Dry hopping was done for 24 hours at 59°F (15°C) in half barrel kegs at a rate of 386 grams of hops per HL of beer (1 lbs./bbl).
Using multiple linear regression analysis found that for Cascade one particular compound (geraniol), not total oil, was the most effective at predicting aroma quality and intensity. It should be noted that the dry hopping was done after fermentation was complete and in the absence of yeast, as active fermentation can biotransform geraniol to citronellol (more on this in chapter 10). As it relates to Centennial, the authors again determined the total oil figure not the best indicator for determining aroma quality and intensity, rather, the compound was the better indicator.
Using the results from the study above, it’s interesting to note that Centennial has one of the highest β-pinene concentrations averaging 0.8-1% of the total oil. Other hop varieties with high levels of beta-pinene are Bravo, Tahoma, and Comet.
Why is total oil likely a bad figure to use to determine hop potential? Most of a hop’s total oil is made up of hydrocarbons, which can be less important to hop aroma compared to terpene alcohols and esters from dry hopping (depending on the variety). Using Centennial as an example, myrcene (a hydrocarbon) can be as much as 60% of the total oil, but doesn’t appear to be as important as β-pinene which is only about 1% of Centennial’s total oil.
In a previous section in the book titled, “Oxygen Fraction of Hops,” it was assumed that total oil was an important factor in determining hop intensity when estimating hop usage rates. I still like this logic because when the total oil figure is combined with the percentage of a hop’s oxygenated compound makeup, there’s likely more of the desired fruity monoterpene alcohols to impart flavors and aroma into the beer. Again, those hops with higher oxygen fractions and total oil concentrations could be used in reduced amounts compared to the opposite (lower total oil and lower oxygen containing fraction).
If you filter a beer, is it at the expense of losing the volatile hop compounds? The answer appears to be both yes and no. A study using GC-MS to determine the hop-derived volatile compounds in beer looked at both a filtered pilsner and an unfiltered pale ale. Both beers were made with similar hopping schedules and dry hopped with the same amount of lemon drop post-fermentation. The biggest difference, other than yeast strains, was that the pilsner beer was filtered.
The results of the study revealed that filtering the pilsner had very little to no differences in the more polar fruit forward monoterpene alcohols (linalool, geraniol, and α -terpineol). However, there was a significant difference with the sesquiterpenes (myrcene, β-caryophyllene, α-humulene, β-farnesene, and β-limonene). The pilsner, which was filtered, had lost most of these sesquiterpenes, compared to the unfiltered pale ale. For example, the pale ale had nearly 250 ug/L of myrcene, and the filtered pilsner had slightly over 50 ug/L. Although the point of the paper was not to determine the effect of filtering, the authors still suggest that the filtration probably contributed to the loss of the more volatile sesquiterpenes in the pilsner.[xxvi]
Based on this study, it appears filtering has the biggest impact on removing the woody, spicy, and resinous/green flavors and doesn’t have as much of an impact on the more polar monoterpene alcohols . The results suggest that filtering could potentially give a brewer more control over the final flavor, however, the synergy between various compounds would also be affected. Although I don’t have any direct experience with filtering, it could be a way to speed up the conditioning time some hazy IPAs require by removing some of the greener characteristics, but likely at the expense of some of the haze. The filtering processes appears to be like centrifuging in terms of what compounds are removed.
Not surprisingly, the role of hops and head retention is a complicated matter. Although good head retention has a lot to do with the makeup of the grist, research does show hops also play a role. However, depending on hop selection and timing, outcomes can be either negative or positive for foam.
It’s generally understood that beer foam is the result of a combination of multiple factors, including foam-positive proteins from malts, α-acids, iso-α-acids from hot-side hops, metal cations, alcohol concentration, and the reduction of lipids. In addition to iso-α-acids, one study found a hop acid called dihydroisohumulone can also substantially improve foam stability and lacing.[xxvii]
A study focused on beer foam tested factors that could potentially influence foam quality: protein, iso-α-acids, and ethanol. They found a significant increase in beer foam from IBUs of 0 to 15. However, there was relatively no change when the IBUs increased from 15 to 30, with a foam increase occurring again around 35 IBUs.[xxviii] The paper also found that the higher the pH of the beer, the more it had a negative impact on head retention, which is interesting when considering dry hopping will increase beer pH.
A presentation looking at how different acids in hops affect foam found that α-acids improved beer foam and lacing better than iso-α-acids. Further, α-acids had an even greater effect on improving foam stability on beer at colder temperatures when tested from 57°F to 69°F (14°C-21°C). The presentation concluded that because α-acids are low in bitterness, they could be isolated and added post-fermentation at just 3 to 4 ppm to enhance both beer foam and lacing.[xxix] Dry hopping at cooler temperatures may be a way to introduce foam positive α-acids to your beer.
Looking closer at the research, pellets of Cascade 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. This would agree with the finding above that as pH increases, foam quality decreases, since 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.
Perhaps the loss of iso-α-acids during dry hopping is causing some of the foam reduction. For example, in the study above a beer with 48 ppm of iso-α-acids had better foam stability compared to a beer dry hopped at a rate of 1 lbs./bbl, which reduced the iso-α-acid to 30 ppm. The dry hopped beer also had an increase in humulinones (oxidized α-acids) and α-acids. So, is it the humulinones decreasing the foam stability?
When a beer with 30 ppm of iso-α-acid was dosed with just 17 ppm of humulinones, the stability increased slightly. This suggests that humulinones on their own can help foam, however you can’t isolate humulinones extraction only when dry hopping. When the beer with 30 ppm of iso-α-acid was dosed with only α-acids, the foam retention increased significantly, even higher than the original beer with only 48 ppm of iso-α-acids. This would agree with the study mentioned earlier that isolated α-acids are a great way to increase foam. Isolated hop acids can be added via hop extracts post-fermentation.
One such hop extract product, called Tetra (a reduced iso-α-acid), has been tested with a concentration of just 6 ppm and was found to increase the foam stability of beer from approximately 270 seconds to almost 340 seconds. When using an iso-extract post fermentation for foam retention, it’s important to take into consideration the increase in bitterness the extract will bring, for Tetra you can count on 1-1.7 times the perceived bitterness compared to the same IBU level from traditional hopping. So, although there is a measurable increase in bitterness at low concentrations for foam retention, this increase is marginal. I should note that Tetra is a product of Hopsteiner, who also conducted the analysis.
Hop Variety and Foam Retention
Just to throw brewers a curveball, the same body of research above also found that hop variety can play a role in foam quality. The research tested five different varieties: Apollo, Bravo, Cascade, Centennial, and Eureka. At dosage rates of 1 lbs./bbl there was a reduction of foam stability in Bravo, Centennial, and Cascade, but an increase in foam with Eureka and Apollo. Again, long term dry hopping, even with the foam positive hops, had a negative impact on foam (the foam positive hops began to have a negative impact after three days of contact time). One possible reason for the increase in foam from Eureka and Apollo is that of the tested hops, they had the lowest amount of fatty acids, which is foam negative.[xxx]
Fatty acids are important for yeast growth[xxxi] and help ensure a healthy fermentation.[xxxii] However, hop fatty acids that survive the brewing process are foam-negative, in part by adsorbing into the adsorbed protein layer of beer and breaking interactions between proteins.[xxxiii] Specifically, longer-chain fatty acids (C12-C18) are more effective at destabilizing foam than short chain fatty acids (C6-C10)[xxxiv] because longer-chained fatty acids are more surface active, which can destabilize adsorbed protein film more effectively.[xxxv]
I speculate that adding foam negative hops to the fermenter as early as brew day may be a way to ensure these fatty acids are used by the yeast and not have an impact on fermentation. However, I wouldn’t worry too much about specific hops and their impact on foam when you can add extracts post-fermentation to boost the foam. You can plan for the slight increase in bitterness by using slightly less hot-side bittering hops.
Although not specific to dry hopping, it’s important to understand the important role malts play in head retention. In general, the higher the protein content of a beer’s grist, the higher the head retention potential. However, the processes of malting and the impact it has on proteins is important to understanding head retention. For example, studies show that foaming proteins primarily form during the germination of barley, which suggests unmalted grains won’t be as foam-positive as malted ones. It’s not just the germinating that matters either, but how long it’s germinated, as beers brewed with under-modified malts (less time germinating) can retain as much as 30% more foaming proteins than beer brewed from typically modified malts.[xxxvi]
One way to know the how long grain has been modified is by looking at the Kolbach index percentage, which is a measure of protein modification. In the study mentioned above, the lower modified malt (Kolbach index of 39.9%) produced more foaming proteins than the control beer malt (Kolbach index of 43.7%). There are certain malts created with the exact purpose of being under-modified to help with head retention and chit malt is a good example of this. Even though the protein percentage of chit malt is like 2-row (because it is 2-row barley) it’s the reduction in the modification of these proteins during malting, not the total percentage of protein, that ultimately helps with head retention. As a bonus, chit is high in starter enzymes and can also increase conversion of starch.
Looking closer at chit malt, a study was done where five different beers were brewed substituting either 10%, 20%, 30%, and 40% of the pilsner grist with chit malt. The authors found that all the beers made with chit malt had higher foam stability (NIBEM values) than the control of 100% barley malt. The best foam stability was tested in beers made with 10% and 30% chit malt beers.
The authors of the above study suggest that chit malt can increase foam-active polypeptides at the same time it reduces the number of negative polypeptides, which are haze-active compounds.[xxxvii] In other words, chit malt can enhance head retention while also slightly increasing clarity. This is the opposite of other protein-rich options like malted wheat, where modified proteins from malting can increase haze when paired with hop polyphenols. Again, I find myself praising chit malt, but it’s an interesting option for hazy IPAs that get too murky or have too much polyphenol vegetative bite. Reducing some of the haze to a “sheen” with chit malt while also lowering protein/polyphenol astringency and boosting head retention are all good reason to experiment with malt. In my experience, using chit malt at around 10% of the grist helps produce dense foam that looks whip crème-like.
- Oxygen will inevitably be introduced to beer when dry hopping. To keep oxygen levels low try adding hops during first 3-5 days of fermentation or continually purging the headspace with CO2 while adding dry hops, especially when post-fermentation.
- Dry hopping for just 24 hours has been tested for nearly complete extraction, however the size of the batch plays a role. Homebrewers will get faster extraction than commercial breweries.
- With agitation or recirculation, extraction can peak as quickly as four hours, but may also enhance the astringency, especially when hop usage rates are high.
- Dry hopping at colder temperatures will still see extraction, likely peaking in three days.
- Hop pellets have higher extraction rates than whole leaf hops.
- As dry hop rates increase, so will the amount of more non-polar compounds like myrcene (green) and aggressive tasting polyphenols. This is particularly true with high-protein and beta-glucan grist.
- In general, hops low in α-acids will have more polyphenols than hops high in α-acids.
- Polyphenols peak after three days of dry hopping and will have higher extraction rates at warmer dry hop temperatures.
- Increased yeast pitching rates may help to remove more non-polar hop compounds and polyphenols.
- Dry hopping too much of a particular variety can alter that hop’s profile, likely going from fruity flavors to more herbal and raw flavors.
- Dry hopping in stages will result in greater extraction than one large dry hop addition.
- Both α-acids and humulinones (oxidized α-acids) will increase beer foam, but higher beer pH from dry hopping can negatively impact beer foam as well as hop varieties high in fatty acids.
- Chit malt can help increase foam stability as well as slightly increase beer clarity.
[i] P. H. (2012, August 7). A Study of Factors Affecting the Extraction of Flavor When Dry Hopping Beer. Retrieved from http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/34093/Wolfe_thesis.pdf
[ii] Dry Hopping on a Small Scale: Considerations for Achieving Reproducibility. (2016). Technical Quarterly. doi:10.1094/tq-53-3-0814-01
[iii] P. H. (2012, August 7). A Study of Factors Affecting the Extraction of Flavor When Dry Hopping Beer. Retrieved from http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/34093/Wolfe_thesis.pdf
[v] Wolfe, P., Qian, M. C., & Shellhammer, T. H. (2012). The Effect of Pellet Processing and Exposure Time on Dry Hop Aroma Extraction. ACS Symposium Series Flavor Chemistry of Wine and Other Alcoholic Beverages, 203-215. doi:10.1021/bk-2012-1104.ch013
[vi] W. M., & S. C. (2013). DRY HOPPING – A STUDY OF VARIOUS PARAMETERS. Retrieved from http://hopsteiner.com/wp-content/uploads/2014/03/Dry-Hopping-A-Study-of-Various-Parameters.pdf
[vii] Humulinone Formation in Hops and Hop Pellets and Its Implications for Dry Hopped Beers. (2016). Technical Quarterly TQ. doi:10.1094/tq-53-1-0227-01
[viii] W. M., & S. C. (2013). DRY HOPPING – A STUDY OF VARIOUS PARAMETERS. Retrieved from http://hopsteiner.com/wp-content/uploads/2014/03/Dry-Hopping-A-Study-of-Various-Parameters.pdf
[ix] Lagemann, A., Dixius, D., Hanke, S., & Stettner, G. (n.d.). Improved HS Trap GC-MS analysis of hop aroma compounds in dry hopped beer. EBC 2017. Retrieved from http://www.ebc2017.com/inhalt/uploads/P003_Lagemann.pdf
[x] S. (2008). Bitterness-Modifying Properties of Hop Polyphenols Extracted from Spent Hop Material. Journal of the American Society of Brewing Chemists. doi:10.1094/asbcj-2008-0619-01
[xi] Forster, A., & Gahr, A. (2013). On the fate of certain hop substances during dry hopping. Brewing Science, 66, 93-103
[xii] Maye, J. P., Ph.D. (2018). Hidden Secrets of The New England IPA a.k.a. Hazy IPA a.k.a Juicy IPA. Lecture presented at Hopsteiner.
[xiii] Oladokun, O., Tarrega, A., James, S., Smart, K., Hort, J., & Cook, D. (2016). The impact of hop bitter acid and polyphenol profiles on the perceived bitterness of beer. Food Chemistry, 205, 212-220. doi:10.1016/j.foodchem.2016.03.023
[xiv] Haslam, E., Lilley, T. H., Warminski, E., Liao, H., Cai, Y., Martin, R., et al. (1992). Polyphenol complexation. In C. T. Ho, C. Y. Lee, & M. T. Huang (Eds.), Phenolic compounds in food and their effects on health (pp. 8–50). Washington, DC: American Chemists Society.)
[xv] Deutscher HOPFEN. (2016). Pocket Guide 2016 [Brochure].
[xvi] Oladokun, O., James, S., Cowley, T., Smart, K., Hort, J., & Cook, D. (2017). Dry-Hopping: the Effects of Temperature and Hop Variety on the Bittering Profiles and Properties of Resultant Beers. BrewingScience, 70, 187-196.
[xvii] Oladokun, O., James, S., Cowley, T., Smart, K., Hort, J., & Cook, D. (2017). Dry-Hopping: the Effects of Temperature and Hop Variety on the Bittering Profiles and Properties of Resultant Beers. BrewingScience, 70, 187-196.
[xviii] Mikyška, A., Hrabák, M., Hašková, D., & Šrogl, J. (2002). The Role of Malt and Hop Polyphenols in Beer Quality, Flavour and Haze Stability. Journal of the Institute of Brewing, 108(1), 78-85. doi:10.1002/j.2050-0416.2002.tb00128.x
[xix] Forster, A., Beck, B., and Schmidt R. Investigation on hop polyphenols. In: Proc. Congr. Eur. Brew. Conv. Brussels, Oxford University Press, Oxford. Pp. 143-150, 1995
[xx] Engstle, J., Kuhn, M., Kohles, M., Briesen, H., & Forst, P. (n.d.). Disintegration of Hop Pellets during Dry Hopping. BrewingScience, 69, 123-127.
[xxi] Lafontaine, S., & Shellhammer, T. (n.d.). Understanding the Impact Hopping Rate Has on the Aroma Quality and Intensity of Dry Hopped Beers. Retrieved from http://www.ebc2017.com/inhalt/uploads/TUEL18-LAFONTAINE.pdf
[xxii] Schnaitter, M., Kell, A., Kollmannsberger, H., Schüll, F., Gastl, M., & Becker, T. (2016). Scale-up of Dry Hopping Trials: Importance of Scale for Aroma and Taste Perceptions. Chemie Ingenieur Technik, 88(12), 1955-1965. doi:10.1002/cite.201600040
[xxiii] Marriott, R., & Wilson, C. (2017). Dry Hopping – a new look at techniques, utilization, and economics. EBC 2017 Presentation.
[xxiv] Vollmer, D. M., & Shellhammer, T. H. (2016). Influence of Hop Oil Content and Composition on Hop Aroma Intensity in Dry-Hopped Beer. Journal of the American Society of Brewing Chemists. doi:10.1094/asbcj-2016-4123-01
[xxv] Lafontaine, S., Pereira, C., Vollmer, D., & Shellhammer, T. (2018). The Effectiveness of Hop Volatile Markers for Forecasting Dry-hop Aroma Intensity and Quality of Cascade and Centennial Hops. BrewingScience, 71, 116-140.
[xxvi] Schmidt, C., & Biendl, M. (2016). Headspace Trap GC-MS analysis of hop aroma compounds in beer. BrewingScience, 69, 9-15.
[xxvii] Smith, R. J.; Davidson, D.; Wilson, R. J. H. Natural foam stabilizing and bittering compounds derived from hops. J. Am. Soc. Brew. Chem. 1998, 56 (2), 52–57.
[xxviii] Siebert, K., Modeling beer foam behavior. Oral presentation at the 73rd Annual Meeting of the ASBC as part of the Brewing Summit, Providence, USA, June, 2010.
[xxix] Wilson, R., Schwarz, H., & Maye, J. (n.d.). A Natural Foam Enhancer From Hops. Speech presented at World Brewing Congress 2012, Portland, OR.
[xxx] Maye, D. P. (n.d.). Dry Hopping And Its Effects On Beer Bitterness, The IBU Test And Beer Foam. Speech presented at 2017 Craft Brewer’s Conference, Washington, DC.
[xxxi] Thurston, P. A., Quain, D. E., and Tubb, R. S. Lipid metabolism and the regulation of volatile ester synthesis in Saccharomyces cerevisiae. J. Inst. Brew. 88:90-94, 1982.
[xxxii] Ohno, T. & Takahashi, R., Journal of tire Institute of Brewing, 1986, 92, 88.
[xxxiii] Clark, D. C., Wilde, P. J., Bergink-Martens, D., Kokelaar, A., and Prins, A. Surface dilational behaviour of aqueous solutions of -lactoglobulin and Tween 20. In: Food Colloids and Polymers: Struc ture and Dynamics. E. Dickinson and P. Walstra, Eds. Royal Society of Chemistry, London. Pp. 354-364, 1993.
[xxxiv] Wolf, P., Husband, F., Cooper, D., & Ridout, M. (2003). Destabilization of Beer Foam by Lipids: Structural and Interfacial Effects. Journal of the American Society of Brewing Chemists. doi:10.1094/asbcj-61-0196
[xxxv] Segawa, S., Yamashita, S., Mitani, Y., and Takashio, M. Analysis of detrimental effect on head retention by low molecular weight surface-active substances using surface excess. J. Am. Soc. Brew. Chem.60:31-36, 2002.
[xxxvi] Isolation and Characterization of Foaming Proteins of Beer. (1980). Journal of the American Society of Brewing Chemists, 38. doi:10.1094/asbj-38-0129
[xxxvii] Buiatti, S., Bertoli, S., & Passaghe, P. (2017). Potential effect of “Chit” malt on beer foam stability. EBC, 36th Congress.