From Vol. 2, No. 6, October 2018
Oregon State University’s Tom Shellhammer, one of the country’s top brewing scientists, says he was introduced to the phenomenon that has become known as “hop creep) a few years ago. “I was giving a talk at the 2015 Craft Brewers Conference, and somebody in the Q & A asked, ‘Hey, do you see people getting diacetyl when they dry hop?’ I was like, ‘No.’”
Diacetyl is one result of hop creep. Beer with more alcohol than a brewery intended is another, as are bottles or cans with dangerously high levels of carbonation. Back in 1893, Horace Brown and G. Harris published research about “the freshening power of dry hops,” claiming that hops contained a “diastate” that was responsible for a second fermentation in the cask due to dry hopping.
More research in 1940 confirmed that Brown and Harris were correct. Yet the matter did not come up again for more than 70 years.
Looking for answers started out as a “hmmmm project,” Shellhammer said. “We opened up this box, and we found all sorts of cool stuff, from a scientific perspective.”
Curiously, it was a question from Allagash Brewing — which at the time dry hopped just 1 percent of its beers and bottle-conditioned 98 percent of them — that led to opening the box. The brewery dumped its first 60-barrel test batch of Hoppy Table Beer in 2016 because instead of finishing with a carbonation level of 2.6 volumes as targeted, the beer reached 4.5 volumes in three weeks. Allagash does not make this sort of conditioning miscalculation.
Zach Bodah, head of quality control, led a series of experiments and treatments, finding a solution to the problem. But then, “we [had] even more questions about why this [was] happening,” Bodah said. “Time to call an expert. And who are you going to call when you have a hop question? Doctor Tom Shellhammer.”
Shellhammer, Bodah, and Allagash Brewmaster Jason Perkins told the story at CBC in 2017. When their presentation, “Unintended Over-Attenuation from Dry Hopping Beers,” was complete, the second brewer to ask a question began, “We are all slaves to the creep.” The phenomenon has been called hop creep from that day forward, “creep” referring to an ongoing and slow reduction in final gravity. (There is not a single word to describe this reduction in German, so German brewers also call this “hop creep.”)
OSU defines hop creep as the refermentation of fully-fermented beer following the addition of hops and that it is observed as a slow reduction of residual extract and steady increase of alcohol and carbon dioxide over time. That refermentation also creates diacetyl, which can be “cleaned up” just like the diacetyl created during primary fermentation. That takes time, time brewers didn’t realize was necessary because they thought fermentation was complete.
Kaylyn Kirkpatrick, who recently completed her masters work at Oregon State, co-authored a paper in the Journal of Agriculture and Food Chemistry with Shellhammer that explains what’s going on. She summarized it, and presented the results of more research, at the MBAA/ASBC Beer Summit in August.
Again, the simplified version: dry hopping liberates fermentable sugars in beer (glucose and maltose, mostly maltose) and hops contribute a small amount of sugar themselves; dry-hopped beer with high residual extract produces more fermentable sugars (it varies by style and is not IPA specific); enzyme activity varies across varieties (cultivars)* and those are further influenced by farming practices; and longer dry hopping time and higher temperatures result in more sugars.
Shellhammer summarized the 2017 research and what has taken place since at the 2018 Craft Brewers Conference. He also suggested things for brewers intent on reducing diacetyl to consider: dry hop timing (both when dry hops are added and for how long), temperature (see above), varieties used (see below) and yeast reduction (via centrifuge and/or filtration). Removing yeast eliminates refermentation, but not production of sugars, so dry-hopped beers beers likely will become sweeter as they age.
I talked to Shellhammer about diacetyl and dry hopping when I was in Oregon, and he suggested I contact Daniel Sharp, the director of brewing operations at Ninkasi Brewing Co. who received his doctorate from OSU (and whose research I’ve quoted many times). He shared what Ninkasi Brewing does, a valuable look into best practices at one brewery.
“For reference, here is our standard dry hopping practice:
1. Ferment (20C/68F) to terminal gravity.
2. Dry hop, cap, hold at 20 C for 48 hours.
3. Pull sample for diacetyl clearance testing 48 hours after cap/dry hop (tested via sensory)
“For beers that use our house yeast, we rarely see anything need extra time to clean up VDKs. We do see a predictable bump in ABV from cap to brite tank (or package). This generally is positively correlated with increased dry-hopping rate. i.e. the higher the dry hop rate the higher the ABV bump. But it could also be confounded by the sugar profile as currently most of our heavily dry hopped beers are also the bigger in ABV and have a bigger RE (residual extract).
“That isn’t to say we don’t see a bump in our dry-hopped pale ales with lower body and RE, just not as big. [Ninkasi removes most of the yeast prior to packaging, and, similar to results at OSU, sees fermentable sugar increase, but not ABV.] In general our house yeast doesn’t seem to throw a lot of diacetyl anyway. Our yeast is of the dry London ale yeast origin. I have heard that most people are struggling with the hop creep diacetyl issue when they use the California Ale/1056 type yeast.
“For other beers, things are a little different. We see that we needed extra time (about 1-2 days) to clean up diacetyl on our dry-hopped beers that use the London III ale yeast. Our non dry- hopped beers we don’t really see a ABV bump.”
Since 2018
Brewers continue to look for ways to mitigate the effects of hop creep. In 2020, Shellhammer headed a CBC panel along with four brewers who talked about the approaches they’ve taken to managing hop creep. Plenty of useful information there.. Stay tuned for reports on more research.