Why do some batches exhibit hop creep?

Let me start this answer off with a brief review of hop creep. This term refers to dry hopped fermentations that slowly ferment to a lower final gravity than they would without the addition of dry hops. Because the process is slow, brewers call it hop creep. Back in 2016, the quality control crew at Allagash Brewing noted something off in one of the first beers they dry hopped when the beer over-attenuated after bottling. Allagash bottle conditions a lot of beer and have a very good idea of what typically occurs during bottle conditioning. Their experience coupled with excellent sleuthing led them to conclude that the root cause of their problem was something to do with hops. See my column from the January-February 2019 issue of BYO for more about the history of hop creep because Allagash and Bell’s Brewery both stumbled on a problem first described in 1893 by Brown and Morris (read it online at www.byo.com/mr-wizard/hop-creep-explained).

Today, the community of brewers and scientists agrees that hop creep is most certainly caused by certain hop enzymes, especially starch debranching enzymes, that survive kilning. These debranching enzymes convert unfermentable dextrins found in beer into fermentable sugars. These enzymatic changes can be problematic for beers during aging because an uptick in fermentation late in the process can lead to increased diacetyl in finished beer. These changes can also be a problem in bottle-conditioned beers when beer carbonation goes above what is expected due to yeast fermenting sugars released from dextrins by hop enzymes. The latter problem can lead to bottle bombs where the former often leads to prolonged aging to wait out the reduction of diacetyl.

The head scratcher for me has always been “why now?” It’s not like dry hopping is new. One of the answers to this question that has often been cited was hopping rate is to blame. In other words, dry hopped beers have always creeped a bit but not significantly until recently because today’s hopping rates are so incredibly high. That was never a strong argument for me because enzymes do their thing independent of concentration, they just do things faster when enzyme concentration is high. More recently, the scientific community has demonstrated that hop kilning temperature is the likely cause. Anecdotal data also suggests that there may be a varietal link with hop creep. Regarding kiln temperature, Dr. Tom Shellhammer’s group have shown that hops that cause hop creep in a lab setting do not cause hop creep when they are heat-treated prior to addition. And this root cause also makes sense in practice because many U.S. hop growers have slowly lowered hop kiln temperatures from about 155 °F (68 °C) to 125–135 °F (52–57 °C) over the last six years or so in response to requests by brewers to improve hop aroma quality. Brewers are now questioning if the lower kiln temperatures provide more headache than they do aroma improvement.

I think what you are likely observing in your homebrew is hop variability. That could be due to the same hop variety coming from different hop farms using different kiln cycles or even the same hop farm kilning hops slightly differently between lots. Whatever the source of variability in hops, what you have observed is not uncommon in commercial breweries; some beers creep and others don’t. What’s a brewer to do?

The most common remedy is to dry hop as usual, be that early or late in fermentation, and let the beer age until changes stop occurring. In practice, early dry hopping is a handy way to add dry hop aroma without extending the process.  And if you don’t want to age your beer on hops for too long, consider tying a string to your hop bag so that you can pull the hops out whenever you like. Two key parameters to track after dry hopping are beer gravity and diacetyl. I personally don’t like over-sampling my homebrews because it wastes beer. Two samples spread apart over 5–10 days are sufficient to let you know if the gravity is steady. If it’s not, keep waiting.

Diacetyl is something to check before cold crashing. Although the method is easy, it does require a special tool. And that is a well-trained and trusty sniffer. Most of us can be trained to detect diacetyl, but some of us simply cannot detect diacetyl, even at levels that most consider off the charts. If you cannot detect diacetyl in beer, find a family member or friend to be your surrogate. When it comes to testing time, take a small beer sample, cover with plastic wrap, hold the sample in a hot water bath at 140 °F (60 °C) for 20 minutes (this forces the conversion of diacetyl precursor to diacetyl), and smell. If you smell butter, keep aging. Be aware that this method can cause beers with certain special malts, like crystal types, to pick up oxidized malt flavors that may be confused with diacetyl and/or make diacetyl detection challenging.

Whether you keg your beers or bottle condition, cold crashing may be a part of your process. When you confirm that gravity is stable, and diacetyl is not a concern, cold crash your brew and package as