With new equipment hitting the home brewing market every day, there hasn't been a better time to be a home brewer. Technology is finally catching up to what's available professionally for brewers and the techniques they use can be replicated at home better than ever before. Among these advances is the availability of stainless-steel conical fermenters on the 5-30 gallon scale. These vessels, among other things, allow the home brewer to maintain pressure on their beer. This makes common practices like force carbonation, closed pressure transfers, packaging and serving directly from storage possible. But what about fermentation itself? How can the ability to pressurize a fermenter play into the behavior of primary fermentation? Some information is presented below.

Primary Fermentation and Pressure:

Professionally, the blow off method is predominantly used to allow carbon dioxide to escape during primary fermentation. However, with the market shift to IPA's that are dry hopped earlier and earlier in the process, the question is, how much delicate hop aroma is lost from blow off? How does large tank geometry, with large hydrostatic pressure, affect beers produced commercially, versus those produced on smaller home brewed scales? Not much research exists on the manner so home brewers have been experimenting with pressurized primary fermentation to see if it positively affects their beer. Experimentation is at the core of what home brewing is all about, so have fun, but this approach is not without its risks to the fermentation process and your safety, so here are some things you should know.

By and large, yeast do not like pressure as it inhibits their activity and in extreme conditions, can kill them. In primary, an increase in pressure subdues just about every characteristic the brewer thinks about during fermentation. The table below summarizes how pressure affects some of these activities.


While the reduction in fusel alcohols and esters may be a welcomed positive, the suppression in growth can cause issues with attenuation and stationary phase clean up. For example, a healthy culture of yeast will usually clean up these byproducts themselves after you reach terminal gravity, but if growth is inhibited by pressure, this can stall the process. Moreover, if yeast growth isn’t strong enough, the beer can also end at a higher final gravity, producing an undesired sweetness.

By and large, yeast do not like pressure as it inhibits their activity and in extreme conditions, can kill them.

In the case of New England IPA's, it's also important to point out that the interaction of active yeast with hop oils is part of what produces the juicy flavors now prized by the consumer (known as bio-transformation). Certain esters may also be desired in this style. Therefore, pressurized fermentation may affect this interaction given its effect on yeast activity.

When adjusting pressure

It’s also important to remember that many other variables effect fermentation as well. Factors like temperature, OG, pitching rate and aeration can all create undesired or desired effects, so be sure to isolate pressure as the only experimental variable when conducting pressurized experiments.

Finally, you must ensure your vessel is properly relieved of excess pressure. With the amount of carbon dioxide yeast produce during fermentation, a fully sealed vessel can create a potential explosion hazard. As a brewer, be sure to take the proper precautions with pressure release valves or spunding valves set below the limits of your vessel. It should be noted that aroma will escape at low spunding levels as well, which defeats one of the drivers of fermenting under pressure in the first place.

In summary

Brewing science in this arena is not well documented. To encourage a healthy and complete fermentation, the traditional blow off method remains the gold standard among professionals. Proceed at your own risk!


John Degroote - Head Brewer

New Barons Brewing Coop

Milwaukee, WI