Fermenting Beer Under Pressure

Yes, using gravity to monitor fermentation status is the best method for use at home because observing bubble activity and kräusen appearance are simply not reliable indicators. I am a fan of clear fermenters (like a carboy or FermZilla) because visual observation of movement during fermentation is telling, but that convenience is given up when using stainless steel. Commercial breweries almost exclusively use closed, stainless steel fermenters these days and monitoring activity using instruments is the method in the commercial world of brewing. When monitoring fermentation using density measurement, the end is typically marked by steady readings over 2–3 days. That usually works, until it doesn’t.

Whether fermenting in glass or stainless, and under pressure or not, fermentations sometimes fail to fully attenuate. It’s always a good practice to have an idea of where a fermentation is likely to finish. This depends on grist bill, original gravity (OG), and yeast strain. For the sake of discussion, let’s assume a witbier has an apparent degree of attenuation of 78%. In your case, predicted FG = [66 – (66 x 0.78)] = 14.52 (gravity points) or 1.015 (specific gravity). Your beer finished at 1.018. If you have multiple data points at 1.018, it’s reasonable to conclude that your fermentation finished a bit higher than expected or that your fermentation may be stalled/stuck. Unless you run a forced fermentation (small, over-pitched, fast ferment used to identify the finish line) alongside of your large fermentation, you really don’t know if you are finished or not. Because 1.018 is not much over my estimated FG of 1.015, I would consider this fermentation complete and chalk up the difference to something to do with mashing.

Let’s take this example to an extreme . . . instead of finishing at 1.018, what if the fermentation was sitting at 1.022? Without having any forced fermentation data, my brewing experience is telling me that there is a pretty high likelihood that this fermentation has stalled and is stuck with fermentables still present. Again, this can happen in any type of fermenter and at any over-pressure. The kräusening method is a great tool to have in one’s bag of tricks to address suspected stuck ferments. Simply add very young beer, aka kräusen beer in the high-kräusen stage of fermentation, at a rate of about 10% of the batch volume. Stuck ferments are usually jump-started by the addition of kräusen beer and will finish out fairly quickly. This is an especially effective tool for bigger beers.

Another layer to this onion are changes in turbidity as fermentation moves through the process from wort to beer. Beers like weizens, wits, and hazy IPAs are not the best types to monitor using clarity as a metric tied to finished beer because these styles are all cloudy to varying degrees. Suffice to say, assuming that cloudy styles will go from one degree of cloudiness to a lesser degree of cloudiness is probably realistic because yeast is not the main source of haze in these beers and some clearing is likely to occur at the end of fermentation, but using cloudiness as a performance metric is a pretty dull tool.

Gravity is much more revealing . . . but it does consume beer in the process. That’s one reason in favor of extending the time at fermentation temperature for ~3–5 days after the end of obvious activity before pulling a sample for measurement. I am a pretty frugal homebrewer and am not keen on sacrificing more beer to Ninkasi than required (sorry Ninkasi, goddess of brewing and the baker of the bappir). If I am expecting a FG of 1.015 and measure something near it 3–5 days after the party seemed to end, I am OK with chilling and moving to the next step.

You also mentioned observing kräusen on the top of your fermenter towards the end of the period associated with active fermentation. That’s not uncommon, especially with top-cropping yeast, like many witbier strains.

And now onto the last major layer to your question: Carbon dioxide release and spunding valve set point. When beer is fermented under pressure, carbon dioxide will saturate the beer at the temperature-pressure condition of your fermenter. You fermented at 12 psi and 72 °F (22 °C); beer equilibrated with carbon dioxide under this condition contains about 1.48 volumes of carbon dioxide. That is pretty low in carbon dioxide, so much so that most folks would describe this level of carbonation as “flat.”

Because you are spunding early in fermentation, it’s safe to assume that you are totally equilibrated after a couple days of gas venting. The only reasons for gas venting late in the process are continued fermentation and gas production or increases in beer temperature that will reduce the solubility of carbon dioxide, increase keg pressure, and cause gas release for your relief valve. You may have been observing the effects of slowly fermenting beer.

Pressure fermentations are convenient for a few reasons, including possible changes in ester profile, fermentation rate, and, the big one in my view, carbonated beer at the end of the process. In order to end up with fully carbonated witbier at 72 °F (22 °C), an equilibrium pressure of ~36 psi is needed (note the FermZilla is rated to 34.8 psi). While that pressure is too high for primary fermentation, it is required if you want to end up with fully carbonated beer. The next time you ferment under pressure, you should consider increasing your spunding valve setting with at least 1.5 °Plato residual extract (1.5 °P / 1.006 above predicted FG) to ensure you have enough fermentables to end up with fully carbonated beer. Hope this information is helpful.

Response by Ashton Lewis.