Benefits of Fermenting Under Pressure

A 2001 study tested the influence of temperature and top pressure and the production of kinetics of biomass, higher alcohols and corresponding esters. In the experiments, aerated wort was transferred into stainless fermenters with the combination of two temperatures 50ºF and 60ºF and two top pressures of 15 psi and 26 psi (converted from bars). Each beer was fermented with a lager strain and to prevent premature yeast flocculation, agitation was applied at 100 rmp.

Landaud ultimately concluded that temperature had an accelerating effect on fermentation and dissolved carbon dioxide (increased top pressure) resulted decreased yeast and ester production rates likely by affecting the acetyl CoA production rate. In terms of fermentation time periods, the ester production rate increased slowly during the first hours and then fast at maximum fermentation rate. At the higher end of fermentation temperature (60ºF) no lag time was observed for yeast growth.

It’s interesting to me to see that the Landaud study found fast and increased yeast growth at higher temperatures and less pressure. In addition, based off the visual charts in the study, it appears total ester production starts to really climb around 50 hours (and can still increase after 100 hours) into fermentation and total fusel alcohols around 24 hours into fermentation at the higher fermentation temperatures. This may suggest that if the fermented is uncapped (no top pressure applied) during the first 24 hours of fermentation and then capped with pressure applied, this may allow for more increased and faster yeast growth without experiencing unwanted esters or alcohols.

Learning that esters are mainly formed during the vigorous phase of primary fermentation, it may be wise to not be so quick to the trigger in raising fermentation temperature to help finish out the ferment unless you are trying to get more esters from the beer. All yeast strains can behave differently, but in my experience this ramping of temperatures hasn’t been necessary to reach final gravity. For example, in one post I looked at 25 different beers fermented with WLP002 and ramping the fermentation temperature had a very weak relationship to the final gravity. Although the Landaud study was for beer fermented under pressure, seeing both ester and fusel alcohol formation climbing after 100 hours of fermentation (4+ days) makes me at the very least question this early ramping processes.

Below is a chart I made summarizing their ester production findings against the taste thresholds for each. The chart ultimately shows that increased pressure reduced ester production despite higher fermentation temperatures.

A similar but earlier (1992) study also looked at the formation of esters and higher alcohols during fermentation with top pressure. Here fermentations were carried out at 48ºF with aerated wort and capped. In this case, the wort was not agitated during fermentation like in the previous study as well as the pressure was sustained only by carbon dioxide produced during fermentation. Again, it was found that both esters and fusel alcohol concentrations decreased as a result of increasing pressure, which the authors said was “partially caused by the inhibition of growth of metabolically active biomass.” Specifically, concentrations of ethyl acetate at the end of fermentation were approximately around 30 mg/l @14 psi, 13 mg/l @29 psi, and 9 mg/l @43 psi (converted from bara).

Continuing to work backwards with the studies on the issue, in 1984 it was again found that beer in unagitated ferments under 28 psi (immediately pumped to pressure after filling the fermenter) had slowed ethanol production, reduced final concentration of fusel oils, reduced yeast growth and increased final pH. They also found a distinct increase in the speed of fermentation (so did the Landaud study). Interestingly, they also found that the beer’s fermenting under pressure had less yeast cells in suspension during fermentation, which they attributed to slow absorption of the vicinal diketones towards the end of fermentation in the pressure fermented beers. The increase in the beer’s final pH found in this study is interesting as previous research has shown that higher pH in hoppy/bitter beers can result in an increase in bitterness perception. Less yeast cells in suspension is also interesting, I’m curious how fermenting and dry hopping under pressure will impact the clarity and taste of a New England style IPA with research suggesting that more yeast cells in suspension could result in more hop oils in the finished beer.

The reduction of vicinal diketones (VDK), diacetyl and pentanedione, and their precursors by yeast during fermentation of beer and one of the main considerations that dictate how long a beer should be aged. So you can see the increase in VDK during fermentation in the beers under pressure is not considered a benefit. However, looking at the fermentation temperature, it appears this is more of a concern with lager yeast strains and maybe not as much of an issue with ale ferments.

So the data seems pretty overwhelming to me based on the studies described above as well as other earlier studies that increased carbon dioxide pressure during fermentation results in decreases in higher alcohols and acetate esters. ‘ ‘

Dry Hopping Under Pressure

Some of the great NEIPA breweries are rumored to dry hop their beers under pressure, as the tweet below implies. This is likely because volatile compounds deriving from hop oils face an uphill battle throughout the brewing process when trying to make it into the final beer. Many compounds are evaporated during boiling, some are absorbed during the hot/cold break, some are absorbed by the yeast. Other compounds are metabolized through ester hydrolysis and esterification by yeast.

@kbenoit mash low (~150F ish), add a little british light crystal malt, 90% of hops late/dry, dry hop under pressure in a cornie keg at 60F — Trillium Brewing (@trilliumbrewing) April 2, 2014

Volatile compounds deriving from hop oils face an uphill battle throughout the brewing process when trying to make it into the final beer. Many compounds are evaporated during boiling, some are absorbed during the hot/cold break, some are absorbed by the yeast. Other compounds are metabolized through ester hydrolysis and esterification by yeast.

I like to dry hop early in fermentation while the yeast are still present and active, which allows any oxygen that was introduced by the dry hop process to be metabolized by the yeast before it causing any oxidation the beer. Dry hopping early may also help enhance biotransformation of the hops. The downside however to dry hopping early gets to the last point above, that the active fermentation may be striping out these wanted compounds through our airlock! I can attest to this fact, after adding dry hops around 3-5 days into fermentation, I usually come back a few days later to check on things and as soon as I open the fermentation fridge I get hit with a big tropical fruit aromas. So it seems to make sense to experiment with dry hopping under pressure in an attempt to trap or keep the wanted hop compounds contained in the fermentation vessel rather than allowing them to escape.

Experimental Top Pressure Beer

Using the information from the research above, I brewed a hop forward NEIPA to see how my results would compare to the more traditional methods of fermenting and dry hopping. For the yeast I went with top cropped RVA Manchester Ale, both because I really enjoy the beer it produces, but also because in past brews I have noticed a bit of a vanilla richness in the aroma and taste coming from the strain. I’m interested to see how this yeast derived characteristic comes out in a beer fermented under pressure (which should decrease ester formation). To test out the claims on higher fermentation temperatures allowed when fermenting under pressure, I didn’t give this beer any temperature control. The average temperature this beer sat at during fermentation was around 78F, which is about 10-12 degrees higher than I have gone with the strain in the past.

When I brewed this beer I split the batch into two 5 gallon fermenters and fermented one with London Ale III 1318 at a constant 65F throughout the entire ferment (no ramping of temperatures at all with this beer). The London Ale III beer was a re-brew for the American IPA category for the final round of the National Homebrew Competition so I didn’t really play with the recipe much. Both beers had the same recipe I used prior with the only difference being that the pressure fermented beer only had 10 grams each of Citra/Galaxy in the keg until it kicked versus 40 grams of each in the keg for the competition IPA (slightly different than the original recipe). In addition to the different yeast strains (although both Boddingtonish strains) the London Ale III beer also had a pre-fermentation addition of 15 grams of Galaxy. I was hoping the pressurized dry hop would result in needing less total hops in the keg.

This is what a pressurized ferment looks like with a Boddington strain at 20 PSI. pic.twitter.com/5vrE74eSzr — Scott Janish (@ScottJanish) May 22, 2016

Using a spunding valve, I fermented the beer at 20 PSI, which despite the higher fermentation temperature should reduce the ester formation quite a bit based on the literature above. To try to take advantage the reduced lag time stemming from higher fermentation temperatures (as well as evidence from above that reduced pressure increases yeast growth) I had no top pressure on the keg for the first 24 hours of fermentation. After 24 hours of fermentation, I hooked the 10 gallon keg, fermenting approximately 5.5 gallons of wort, up to my C02 tank and filled to 20 PSI. I then attached the spunding valve and dialed it down until I started to hear a hiss of C02 escaping to keep the keg at a constant 20 PSI.

I chose to cut the diptube in the keg high enough to sit above the cold crashed yeast and hops to then transfer the beer to the serving keg with C02 and without siphoning, which eliminates the chance of oxygen pickup during the transfer (picture below). However, it didn’t work as well as planned. The dry hops didn’t settle out to the bottom like I was hoping after just a few days crashing around 40F. For future beers I ferment in the keg like this, I’m going to add my stainless steel filter to the diptube when I dry hop to help eliminate any floating hop particles and yeast from getting into the serving keg (like the tweet below). This method would have also allowed me to not cut as much from the dip tube.

Keg dip tube filter did it's job. Loose hops = more extraction! #homebrew pic.twitter.com/JeJioEotMy — Scott Janish (@ScottJanish) April 3, 2016

Taking gravity readings during fermentation with no chance of oxygen entering the fermenter is nice. Just hook up a picnic tap to your liquid out post!

On day five of fermentation (gravity at 1.020) I released the pressure from the keg and dumped in the first dose of dry hops (56 grams Citra and 28 grams Galaxy). Rather than immediately filling the keg back up to 20 PSI at this point, I only filled the keg to 5 PSI to seal the lid and cranked up the spunding valve pressure to past 30 PSI. The idea here is during the last bit of fermentation with the dry hops all of the carbon dioxide produced will remain in the keg to try to trap the wanted hop compounds and C02. As the beer goes from 1.020 to its final gravity, all the naturally created carbon dioxide remains in the keg and in the beer for natural carbonation! Using a fermentation chart, I saw that at the fermentation temperature of 78F I would need a PSI of around 30 PSI on the beer for proper carbonation, which is why I had the spunding valve set to 30 PSI. I was hoping not to hit this level (30PSI) because going past it would allow some of the C02 to escape through the valve, along with hop oil compounds. Luckily I succeeded in not capping fermentation too early as the C02 stopped building when the valve reached 28 PSI. It’s easy to see when the beer is completely done fermenting with this method because the pressure gauge will stop increasing, which is also nice perk!

Beer Results

The natural carbonation was just under where I wanted it, but after a few days on gas at serving pressure, it was ready to drink. Despite looking somewhat clear prior to dry hopping (although the shape of the hydrometer helps with this), it definitely ended up being a cloudy typical NEIPA. As you can see from the picture below, the haze really increased after dry hopping. I was curious how the haze would be with this beer because the literature suggested that beers fermented under pressure had less yeast cells in suspension during fermentation. In this case, the clarity for the strain progressed like I’m used to with no pressure ferments and heavy dry hopping.

Comparing this beer to the cooler London Ale III beer, it’s just not at the same level. The London Ale III beer pops with big fresh hop flavors and aroma where the pressurized ferment is less exciting, slightly muted. The vanilla like esters I’ve gotten with this RVA strain in the past are definitely less if not there at all. Likewise, I don’t get any hot alcohols or any noticeable negative flavors from the hot fermentation, which would seem to agree with the research that the top pressure with increased fermentation temperatures can both reduce esters and alcohols. In fact, I can actually detect slightly more alcohol in the aroma of the cooler London Ale III beer (but this is slight).

I was hoping to see a bigger impact in terms of hop aroma with the closed dry hop. Because the hops didn’t settle out during the cold crash period and clogged during the transfer to the serving keg, I did have to release the pressure on the keg and add the sanitized filter then re-pressure and then transfer. This was unfortunate as I was really hoping to keep all the aromas in the keg. However, the London Ale III beer was never under pressure and had a bigger hop aroma (with more hops in the keg) so I’m not sure what sort of impact releasing all the pressure from the keg actually had. Further ferments with this method should help clarify this. On a similar note, I used to carbonate my hoppy beers with the high pressure (30-40 psi) for a few days than release it to serving pressure, but have since moved to carbonation stones to avoid this massive release of pressure. I don’t have any data suggesting that this release is taking hop aroma with it, but if you can smell hops during the release that’s enough for me to avoid it!

Aroma differences between the two beers are fairly drastic the pressure fermented beer has distinct orange juice/peel aroma with and floral characteristic that almost borders a little centennial like. Overall the pressure fermented beer aroma is more subdued in terms of intensity and fades rather quickly. The cooler London Ale III beer is extremely inviting (and big time apparent) with more of a soft sweet overly ripe fruit character of strawberries and raspberries with a bit of peach that all comes together to resemble a handful of sweet jelly beans. The flavor of the pressure fermented beer is lacking, overall it’s kind of boring. Maybe this lack of flavor is in part due to the high final beer pH of the pressure ferment (I couldn’t find any research to explain this).

Another one of the biggest differences between the pressure fermented beer and the London Ale III beer is the mouthfeel despite only being one gravity point higher (1.015 to 1.1014). The pressurized ferment beer has a bigger and creamier mouthfeel that makes the beer feel much bigger than it is. One possible explanation for this difference is that glycerol production can be increased with raising fermentation temperatures In previous research on glycerol production I came across data that suggest increased osmotic stress on yeast might also increase glycerol production (although this was for high gravity beers and not pressurized ferment beers).

Overall, there is no question I prefer the cooler fermented London Ale III beer with no top pressure, but for a beer that fermented near 80 degrees with a yeast strain that (for me at least) usually produces noticeable esters, I’m intrigued by the process! The higher final beer pH, thicker mouthfeel, low alcohols and esters despite the fermentation temperature definitely seem to have potential depending on what your intended results are. I’m anxious to try a combination of a cool non-pressure ferment with capping of the fermenter immediately after adding the first dose of dry hop in a future hop forward beer.

Some Other Thoughts