Fermenting Under Pressure
Pressure fermentation has taken off among homebrewers the past few years, but the effect of pressure suppressing ester formation during fermentation has been known for many years. I first came across a white paper article about ester suppression under pressure in the early 2000s. And it was an old paper at that time! I wish I had saved a copy, but at the time I just found it an interesting tidbit of information. As most brewers know, lager yeasts and cold fermentation temperatures greatly suppresses the formation of the fruity esters that we know and love in ales. Those same flavors can be inappropriate in clean lagers where malt and hops take the front seat in flavor profile.
Esters are made by yeast as they metabolize wort fermentables, amino acids, and lipids during fermentation. As yeast metabolism increases, ester production increases. Lagers have traditionally been fermented at 50–55 °F (10–13 °C). Lager yeast are very unique genetically that they can grow (producing ethanol) at this temperature range. Properly oxygenating wort to 10–15 ppm of dissolved oxygen when using liquid yeast also helps to reduce ester formation (though this oxygenation step is not required if using dry yeast). The result: A clean tasting beer. However, brewers would love to ferment warmer because the fermentation proceeds faster. By applying pressure to the fermentation, yeast metabolism is changed, thereby decreasing ester formation. Now the fermentation temperature can be increased, resulting in a faster fermentation.
In addition to suppressing ester formation, fermenting at room temperature also greatly reduces diacetyl, the butterscotch flavor in some beers. Diacetyl is usually considered a flaw in lagers in detectable concentrations, though in low levels it can add a fullness to the body. Fermenting under pressure, and at room temperature, allows homebrewers to make a clean lager-style beer if temperature control is not available. Since fermenting at room temperature does ferment much faster, and doesn’t create much diacetyl, there is no need to age (lager) the beer to let the yeast clean up those compounds, nor is there a need for a diacetyl rest. Ales, on the other hand, derive their unique flavor profiles from the esters formed during fermentation, so pressure fermentation would be counterproductive to most ale styles.
While fermentation temperature control is known to be vital to producing quality beers, particularly lagers, glycol chillers and temperature-controlled freezers were not common for homebrewers when the paper I referenced earlier was written more than a quarter-century ago, and out of financial reach for most.
Fast forward many years from when I first read about pressure fermentations, as we (Blichmann Engineering) got more and more feedback from homebrewers asking about temperature control products so they could brew lagers, I recalled that white paper and thought perhaps that would be a great solution for brewing lagers at home.
In 2012, Blichmann Engineering had just developed the Cornical conical keg and fermenter product that also allows fermentation pressures up to 30 PSI. While it was originally developed as a convenient way to ferment and dispense out of the same vessel, I also had pressure fermentation in the back of my mind for this product. But we didn’t have hard data on the effects of pressure fermentation and the suppression of esters to make knowledgeable recommendation to homebrewers. So, I reached out to my good friend Chris White of White Labs, who kindly offered to assist in my experimentation as he, too, had an interest in this phenomenon. He sent yeast, helped develop the experiment, and analyzed the beer samples in their lab and using their tasting panel.
The results were quite interesting, to say the least. The added benefits, in addition to ester suppression, is the ability to ferment at room temperature and forego elaborate temperature control, the ability to naturally carbonate the beer during fermentation, utilization of CO2 to pressure transfer to kegs, and the ability to make warm-fermented yet clean lagers in a fraction of the usual time. Of course, today pressure fermentation is a more common technique, with most equipment manufacturers offering pressure-capable fermenters made from stainless steel and/or PET plastic and sized for homebrewers.
Before we get into the details of the experiment, let’s discuss the equipment needed and how to safely operate it. The first thing to discuss is pressure. While 15 PSI (pounds per square inch) may not sound like a lot, the big factor is the surface area on which it is pushing and how much force that generates. My intent isn’t to scare the heck out of you, but it is my intent to help you understand why it is so very important to follow safe practices. Force = Pressure x Area. So, over a 1-inch (2.5-cm) square, 15 PSI will push with 15 lbs. (6.8 kg) of force, a 4-inch (10-cm) diameter lid at 15 psi equates to 189 lbs. (86 kg) of force, and a 16-inch (41-cm) diameter conical lid equates to 3,000 lbs. (1,360 kg) of force! The key to safely fermenting under pressure is to purchase equipment that is designed and rated for pressure and equipped with a safety relief valve.
Safety Rules
Let’s start here. READ and UNDERSTAND the manufacturer’s manual.
NEVER operate a pressure vessel of ANY kind that does not have a separate dedicated over-pressure relief valve (PRV), and a device to release all internal pressure safely such as a pull ring or vent valve so that all pressure can be reduced to zero. A spunding valve (discussed later) is NOT a pressure safety device. In addition to a spunding valve, you will need a separate pressure relief valve. If you try venting during fermentation through a PRV there is a good chance that it may plug with hops or yeast and no longer be able to relieve an overpressure at the design pressure.
ALWAYS be 100% sure before removing any lid or accessory that the pressure in the vessel is fully released and the pressure inside the vessel is zero. Then carefully loosen any clamps keeping your body clear of the path of the lid or device should it be inadvertently under some pressure. The vent should remain open the entire time the tank is being disassembled. In many designs, much like a Corny keg, the pressure relief valve can be unscrewed and removed to ensure the vessel is fully vented.
NEVER exceed the manufacturer’s rated pressure, or tamper with any pressure relief device.
For the fermenter itself: PET fermenters are an economical choice, as are Sankey and Corny kegs. But make sure you use a separate PRV and spunding valve!
The device used to effectively control pressure during fermentation is a spunding valve. These are devices used to vent fermentation gas created during fermentation while maintaining a desired pressure in the vessel. The word “spunding” stems from the German word spund, which means “bung.” Basically, a valve to seal off a barrel or vessel. Most spunding valves will include a pressure gauge or pressure indication so that the desired pressure can be dialed in. Once set at the desired pressure, any pressure exceeding the set pressure will be released through the spunding valve. Some spunding valves have a small bowl containing water to give a visual indication of fermentation activity (as shown in the photo at the top of this story). Others have a means to affix a hose to vent into a bucket of water (as shown in the example to the right), and others simply vent the gas directly to atmosphere.
Spunding valves are adjustable pressure relief valves that allow you to vary the set pressure as desired. Safety valves and spunding valves both utilize a spring pushing a seal into a seat; however, the spunding valve has a means to adjust the pressure to the desired level, commonly a threaded rod to compress the spring more or less, which changes the applied force into the seat. Recall the pressure and force equation discussed earlier. Same physics here. The easiest way to set the spunding valve is to use a CO2 tank and set the regulator at your desired pressure. Connect the tank to the vessel and slowly pressurize. As you hear gas venting or bubbles in the spunding valve, slowly turn the pressure adjustment on the spunding valve until you no longer see or hear venting gas. Check that the gauge on the regulator, or the gauge on the spunding valve if equipped, is at your desired pressure. Disconnect the CO2 supply and you are set. Until fermentation commences, you will likely see the pressure drop as CO2 dissolves into the wort. But as fermentation picks up you will quickly see the pressure rise and stabilize at your set pressure.
What we Learned in our Experiment
The purpose of the test was to quantitively and qualitatively measure the effects of pressure on ester suppression, hop expression, and overall flavor and body. A Munich helles was selected as the test beer so that these characteristics would be easy to detect. One 20-gallon (76-L) batch was split into four identical conical fermenters (the Blichmann Cornical, to be exact). An identical amount of White Labs WLP833 (German Bock Lager) yeast was pitched to 15 million cells/mL, and the wort was oxygenated to a measured 10 ppm dissolved oxygen using pure oxygen and an oxygenation wand.
The control fermenter followed a traditional cold lager fermentation with a diacetyl rest. The other three were fermented at room temperature (68 °F/20 °C) in a temperature-controlled upright freezer. One fermenter was set at 0 PSI (atm pressure), one at 15 PSI (1 BAR), and one at 30 PSI (2 BAR).
The control lager batch finished fermenting in under two weeks and was then put through a diacetyl rest and lagered. The total process took eight weeks. The room temperature-fermented batches were complete within two weeks. After fermentation, several bottles of each beer were shipped to Chris for his lab to analyze with their gas chromatograph and for the White Labs tasting panel to do their qualitative testing. The highlights of these tests are listed in Table 1 and illustrated in the graphs at the end of this story.
Conveniently, the Indiana State Fair beer competition was the following weekend, so I brought bottles for four Beer Judge Certification Program (BJCP) national rank and above judges, including Gordon Strong. They were told that all four beers were from the same wort, but that different fermentation techniques were done on each of the four samples. The bullets below summarize the results of the White Labs and Indiana State Fair judging panels, and as you’ll see they align with the qualitative data:
Traditional Lager: Low ester levels, low to moderate diacetyl, malty, balanced hop character.
0 PSI: High esters for style, very low diacetyl, noticeable fruity esters, mild bitterness.
15 PSI: Lower esters than traditional lager, very low diacetyl. Malt-forward with balanced hop character. Slightly less malty than traditional lager profile.
30 PSI: Very low esters and diacetyl. Muted maltiness, with a somewhat harsh hop bitterness.
Based on the results, the beer that was the maltiest and had the best hop balance true to style was still the traditional lager method, most likely due to the carbohydrate metabolism at that temperature. Sulfur notes were also most appropriate with the traditional lager method. A close second was the 1 BAR (15 PSI) room temperature brew. Above this pressure, maltiness and body are noticeably reduced, and hop expression is notably harsher. Esters were lower in those fermented at both 1 and 2 BAR compared to the traditional lager fermentation. They also fermented a little drier, due to the higher temperature.
So, can you make a true lager substitute using pressure fermentation at warmer temperatures in a fraction of the time? I don’t think so . . . but you can come pretty darn close. The differences were detectable when tasting the traditional lager and lager fermented at 1 PSI side-by-side, though they were quite subtle. I’d venture a guess that tasting one today and the other tomorrow would leave most without serious beer flavor training hard pressed to tell the difference.
Wrapping it all up, pressure fermentation is a simple way to make great tasting lager-style beers without all the cooling equipment and controls normally needed for lagers, or the long lagering time. But keep the pressure to that 10–15 PSI range for the best results. I also recommend running your own side-by-side experiment to see the difference for yourself!
