Controlling Fermentation Rate with Pressure
|One thing I've noticed brewing under pressure in a corny keg and using a Tilt hydrometer is that pressure seems to effect fermentation rate. A beer that would ferment in a couple days in a carboy, might take 2-3 times longer under 10+ PSI, most likely this depends on the yeast strain. Further research indicates that the pH of carbonated water is between 3-4 and yeast prefer a pH of 4-4.5 so most likely, the added carbonation is having a real effect of slowing fermentation, even under warmer summer conditions that would make fermentation go faster. I've read about experiments showing that brewing under pressure has no effect on the flavor and in my experience that could be true, but I wanted to see if I left the temperature get a little on the high side due to summer heat and see if the lower pH would counter the effect and the beer still showing no signs of heat related off-flavors, as if fermented slower under lower temperatures. For this experiment I brewed a double batch of Dead Ringer from Northern Brewer.||
Both are under pressure and temperature has been allowed to do what it wants in my garage. I made a big 58 point starter for these batches (since I reduced he volume to 4 gallons for each keg, OG was 1.082 instead of 1.064 target for recipe), fermenting about 30 points in a 2 liter aerated starter before pouring off the top 1.5 liter and then pitching the bottom 0.5 liter, half into each corny keg. It took about 6 hours to show a noticeable drop in SG after pitching, not surprising with a robust aerated starter, WLP001 yeast strain, and pitch temp of 74F. Initially the Google Sheets report showed a fast fermentation until the pressure built up to 12 PSI as visible on the relief gauge. As pressure got higher fermentation rate slowed until a steady fermentation rate as seen in both carboys was established at about 17-18 points per day. I'll be keeping an eye on the fermentation to dry hop at around 30 points and then keep an eye out for an FG of 20 points that is stable before cold crashing. Of course the great part is the beer is already carbonated so am looking to get this on the tap soon. Will follow up on flavor in a couple weeks where I'll be serving at a birthday party. Looking forward to it, cheers!
Update July 6, 2017 : Both fermenter/kegs are showing a leveling off of SG at around 1.019 for Keg 1 and 1.017 for Keg 2. The extra fermentation in Keg 2 led me to cold crash and tap it first. The attenuation was already pretty high, over 80% so wasn't worried about residual sweetness or body. Usually, I would allow more time to let the fermentation fully stop or show a 0 or positive fermenation rate over a 3 day period, however I wanted to compare this somewhat green keg with the 2nd Keg that I plan to fully dry out for serving at the birthday party in a few weeks. I also wanted to check in advance for any high temperature effects during krausen (up to 77F due to a summer heat spell and high fermentation activity). In my taste tests I noted no off flavors I've tasted before in overheated ferments, so it seemed the heat didn't affect it. Due to the beer being 9% abv, I could taste some alcohol heat, but in a pleasant way the mixed well with the malt and hops. The hops were primarily centennial with a wild card African Queen hops to mix things up a bit. The hops have a bit of spiciness to them that give it a nice complexity compare to some of the summertime session IPAs I've been drinking. My wife tested it too and gave me the thumbs up. We'll see if she actually wants more.
Keg 2 (same wort as keg 1)