[TeeDubb]

I know that there was a thread here last month that referenced Tom Shellhammer's work on dry hop rates and also something called Hop Creep, which was new to me. This led me to do a little reading and I found a technical paper entitled, "The freshening power of centennial hops" by Kirkendall, Mitchell, Chadwick.  You can request the full paper here:

https://www.researchgate.net/publication/325704205_The_Freshening_Power_of_Centennial_Hops

This was really fascinating and shows the power of 'after-fermentation' caused by hop enzymes. The mechanism was first noticed and recorded back in 1893! Enzymes in hops seem to interact with carbohydrates in the wort to create/liberate more fermentable sugars. If yeast are still present, after-fermentation starts and metabolizes the liberated sugars. The research also showed that some hop varieties are more prone to this and one varietal (Mosaic) had the opposite effect in related research. Centennial, Citra, Simcoe, Amarillo and Cascade were tested in the paper and all showed hop creep to varying degrees.

As it turned out, I had an IPA on deck this week that was finishing that used a heavy dose of dry hopping (1oz/gal).  After 10 days, primary fermentation finished with a FG of 1.013 (same readings 2 days apart), as expected with WLP002 ale yeast. In the past, I attributed airlock activity during dry hopping to the hop matter providing nucleation sides for CO2 in the beer after primary fermentation. (Out of laziness I only checked gravity after primary fermentation was done, in the past).  This time, I transferred from primary to a corny and added the hops loose to maximize utilization.  I used a spunding valve to control pressure to about 4 psi, just enough to seal the lid well. 1 day after adding the dry hops, the keg started to build pressure and I had the pressure relief set at a high level so I could observe the pressure rise. 2 days in I had to bleed pressure down from about 8-12 psi every 2-4 hours to maintain 4-5 psi. This continued for several days. On day 6 of dry hop I measured the gravity at 1.010 and its still active, may hit 1.009, consistent with what the research paper showed, and perhaps drier than I wanted.

I know there are some aroma and flavor disadvantages of carbonating under pressure but it's quite shocking that some hops alone have the power to create about 3-4 points of gravity change. About the same as enough sugar to prime to ~2.4 vol. Something new to account for in recipe development! Carbonate under pressure with dry hops? Also makes me think about extending the conditioning time to account for the second round of yeast activity and their by-products as well as adjust the temperature during dry hopping to what I like to hold during the first 1-3 days of primary.

[Robert]

The mechanism was first noticed and recorded back in 1893!

The first full blown scientific paper I've seen on this was 1893, but brewers had a practical understanding of this extending back into the mists of time.   It is the very reason dry hopping was performed.  Brewing handbooks mention this function but never seem to consider aroma effects.   Why?  Probably because, as the beer was aged for several months minimum,  there were no lingering aroma effects from dry hopping.  The only other thing they mention about dry hopping is its ability to help with clarifying beer, which we understand to be the result of polyphenols complexing with and precipitating proteins.

Thanks for reporting your practical observations.   Now maybe more brewers will believe this is real.  As a bit of a beer history geek, I like it when it's shown that our forbears actually knew what they were doing, and maybe weren't doing what we assume.