Diacetyl and Hop Creep

Diacetyl is an organic compound in a class of vicinal diketones (VDKs), and is a natural byproduct of fermentation. When yeast get to work synthesizing the amino acids valine and leucine they produce an intermediate molecule called alpha-acetolactate. The conversion of this intermediate to valine requires a few more steps, one of which is a relatively slower process. Excess alpha-acetolactate builds up and some is excreted by the cell into the surrounding beer where it spontaneously reacts to make diacetyl. However, this isn’t a problem because healthy cells can reabsorb and convert that diacetyl to a flavorless compound called 2,3‑butanediol, and your sensory panel (and those atop a bar stool) will be none the wiser. 

In a perfect world, this process would run smoothly and efficiently every time. However, low free amino nitrogen (FAN) levels in wort will make yeast cells work harder, as will underpitching. Poorly aerated wort and low ATP levels can also reduce efficiency, and higher fermentation temperatures will speed up all these processes and make it so the yeast cells can’t reabsorb diacetyl quite as readily. So in short: give your yeast the nutrients it needs and give it time and a cozy temperature range to do its thing. Happy, healthy yeast can clean up after itself.

When dry hopping is performed late- or post fermentation, the yeast are no longer in an ideal environment like they were when the wort was fresh and sweet. At the end of fermentation the beer lacks nitrogen and sugars, so fermentation slows down and will essentially stop once terminal gravity is reached. The hops introduce enzymes, which liberate more fermentable sugars. The yeast’s metabolism kicks back into gear to consume the sugar, but they still don’t have all the nutrients they need. Consequently the yeast start to manufacture their own amino acids, including valine and leucine, and once again diacetyl is produced. But with their metabolism slowed by a nutrient-depleted and stressful environment of finished beer, the reabsorption of diacetyl is much slower, and aroma-active diacetyl will remain in the beer, much to the detriment of the drinker and your QA panel.

1. Limit the potential

• Use hops with high diastase activity in the whirlpool, and low diastase activity for the dry hop (hop suppliers may have this info)
• Target more conversion in the mash — don’t leave behind future snacks for enzymes to chomp on
• An early charge of dry hop can minimize creep potential of later additions

2. Ride it out

• Time your dry hop for when the yeast is still active (pre-diacetyl rest)

3. Prevent it with a cold, short dry hop

• Get through fermentation, crash cool, dry hop for a short contact time
• Can be a little risky if enzymes have a chance to convert later in warm storage

4. Pasteurize/inactivate hop enzymes

• Direct inactivation of hops (e.g., sous vide hop trials)
• Very low pasteurization unit targets with a flash pasteurizer can be used to inactivate enzymes

Hop creep takeaways

• Hop creep happens and it is manageable. You can find creative ways to mitigate it while still getting the benefits of yeast repitching, flavor, and haze. 
• Think about changing your yeast strain and/or approach if you continue to see stubborn diacetyl or trailing fermentations in your tanks. 
• Think about what you are asking of your yeast: ​“I’m on my last legs doing my best to clean up your beer, and now you go and stress me out with a dry hop.” — Your Beloved Chico