Reevaluating Dry Hop Techniques

It’s a new year, a new decade, and a great time to reevaluate some of your brewery SOPs. As craft brewers, many of us have adopted a brewing-inspired version of the ”day you stop learning…” mantra: “The day we stop improving, is the day we turn in our boots.” Inspired by that attitude, maybe it’s time to reconsider your dry hopping procedures, or at least take a hard, educated look at them.

With that attitude in mind, here’s a look at some more recent dry hop studies that might help you take a refreshed look at your brewery’s dry hopping methods.

Contact Time and Aroma Extraction

Everyone seems to have their own formula when it comes to the length of time the beer is on hops to control the intensity of dry hop aroma. Often, when it comes to maximizing aroma, the assumption has been the longer the better. Some recent research, however, suggests that may not be the case.

Several studies have shown that concentrations of monoterpene alcohols and hydrocarbon hop fractions reach near-full extraction in beer after being on dry hops for just 24 hrs. Further, some of these constituents can come out of the beer solution and back into hops due to those aroma compounds’ hydrophobic nature. Additionally, hop thiols such as 4MMP, known for their fruity, tropical aroma, are also extracted rather quickly, with most being found in beer within 2 days of dry hopping. ^{1, 2} Considering contact time as a way to mitigate hop creep on beer with higher dry hop loads, it may be time to consider how long you are leaving your beer on hops.

Sensory studies bear out what chemical analysis has shown when it comes to aroma extraction. Aroma intensity was the same for beers dry hopped with pellets for six hours and four days. Shorter dry hop times were also rated with higher fruity characteristics from monoterpene alcohols and thiols while longer dry hopped beers were ranked higher for herbal notes from polyphenols. ³

Considerations for Agitation

Agitating or stirring has been known to shorten dry hop time needed for effective extraction of desired hop aroma in beer. Under laboratory conditions, active mixing of dry hops was shown to maximize both hydrocarbon and monoterpene alcohol extraction in as little as four hours- with subsequent reduction of these compounds over time. Hop astringency and bitterness increased under agitated dry hop routines suggesting it’s possible to “over extract” undesirable hop compounds. This is confirmed when looking at polyphenol levels in long-agitated beers. Brewers should also be aware of how the type of pump they use when recirculating this way can affect their final beer. Shear forces and their effects not only on hops but also how they may act on any yeast left in the tank should be considered. There is evidence that high-shear mixing can lead to the extraction of undesirable flavors in beer from both hops and yeast, especially when both are present. ⁴

The Benefits of a Quick-Turn

While turning out a tank of finished beer more quickly may be a benefit production-wise, shorter hop contact time may also reduce certain unwanted hop flavors and aromas in your beer. Potentially undesirable polyphenol compound levels increase over time, meaning getting the beer off dry hops can reduce overly herbal hop character. Sensory analysis has shown that while perceived bitterness increased, iso-alpha levels decreased over time as hop leaf material absorbed iso-α-acids in beer. ² In some extreme cases, polyphenol extraction has been attributed to what some are calling “hop burn” in dry hopped hazy beers.

Temperature: Aroma Extraction & Mitigating Hop Creep

When considering beer temperature during dry hopping, research has shown that dry hop extraction happens quicker than originally thought, even at 34-39°F (1-4°C). In fact, linalool extraction time was reduced when dry hopping occurred at cooler temperatures (34-39°F, or 1-4°C), finding maximum extraction after two days. Linalool levels were the same in warmer dry hopped beers after two weeks. ² Again, when looking at methods to mitigate hop creep, it should be noted that hop diastase has been shown to be reduced at cooler temperatures.

Tank Volume & Geometry

It may be no surprise that the volume and geometry of the tank you are dry hopping can have an effect of the efficiency of dry hopping extraction. ⁵ That said, how many brewers adjust their dry hop recipe for different tanks?

If you are dry hopping a 100 bbl tank from the top but the original recipe was written for a 20 bbl batch, then simply multiplying the dry hop addition weight by five may not give you the same results. While there is no algorithm we can suggest, simple arithmetic does not take into consideration the contact time of the pellets with beer as they fall through the height of a full tank nor the effects of Fick’s law of diffusion (a favorite of dry hop fans). In this instance, trial-and-error may prove the best method for adjusting your dry hop weights beyond single batches. Agitating the beer/hop solution could help reduce the differences between batch sizes, but that’s not always a viable option and comes with the previously mentioned considerations.

Multiple-stage Dry Hopping

A number of brewers tout the possible dry hop extraction benefits of breaking up larger dry hop additions into smaller quantities- the multiple stage dry hop. While research has shown that there is significant positive sensory affect for multiple smaller additions on the pilot scale, the evidence is not so clear for larger size batches. ⁶ More comparative studies on this dry hop method is needed before any firm recommendations can be made.

Often overlooked: Impacts of Hop Variety & Addition Timing

Though we’ve looked at how hops are used in dry hopping, we’d be remiss if we didn’t look at what hops are used and when they are used in the brewing process. Brewers have often considered a variety’s total oil content when making choices for dry hopping. However, studies have shown that a higher oil content in a given variety is not a good predictor of a higher hop aroma in beer. ⁶ In other words, just because one lot of a hop is higher in oil, that doesn’t mean it will impart more aroma than a lower oil lot.

Though certainly not completely understood at this point, concentrations of hop oil volatiles and precursors (thiol and geraniol) should be considered when evaluating hops for aroma and dry hop additions. When considering a hops contribution to aroma in today’s hazy beers, the relationship between essential oils and precursors and beer aroma is further complicated by yeast enzymatic activity on these hop components. So, despite several yet-unknowns, it’s fair to say it’s the makeup of these essential oils that matters most, not the amount of total oils.

When selecting hops for maximum aroma potential it makes sense to look at a hop’s volatile oil and precursor levels. For example, Lafontaine and Shellhammer suggest that geraniol precursor dominant hops may be better suited for pre-fermentation additions or added during fermentation where yeast can convert the precursors to more potent aroma volatiles. Meanwhile, hops that are rich in geraniol would work better in dry hop additions where they volatiles will not be lost in kettle evaporation and don’t rely on yeast metabolism. ⁷ Therefore, when comparing lots within a variety, higher geraniol hops may be more desirable for dry hopping.

Thiols in hops have become an increasing interest to researchers and brewers of late. These sulphur containing compounds have very low detection-thresholds and thus a large impact on aroma in beer. Unfortunately, thiol concentration data in hops is very hard to come by as they are quite difficult to measure with only a few labs across the globe capable of this analysis. While it may yet be hard to come by thiol numbers, researchers have suggested hops with higher thiol precursors could best be used pre-fermentation to maximize aroma potential. Subsequently, hops with higher free thiol levels could be used for dry hopping. Researchers also point out “the difficulty around trying to define general analytical markers of aroma hop quality… and that the timing of hop additions during the brewing process is a major consideration for determining what constitutes ‘hop quality’ for any given variety or usage.” ⁷

β-pinene was found to be particularly high in Centennial and Citra^TM hops- two varieties favored for dry hopping. Though the oil was not extracted into the beer, it suggests that β-pinene may be a good marker for other compounds that are important to dry hop aroma in beer. No doubt we will see additional research focused on these “dry hop potential” markers in future research. For now, let us all appreciate the brewer’s skill in choosing varieties for dry hopping.

Perhaps more than with any other traditional brewing ingredient, hop research is continually offering new insights into its chemistry and interactions in beer. Dry hopping is an ever-evolving brewery activity that likely will continue to challenge how brewers hop beer. Thought cliché, brewing truly is an art and a science, and brewers are more than happy to blur those lines.

1. Wolfe, P.H. “A Study of Factors Affecting the Extraction of Flavor When Dry Hopping Beer.” unpublished M.S. Thesis, Oregon State University, 2012.
2. Mitter, W., Cocuzza, S. “Dry Hopping- A Study of Various Parameter.” Brewing and Beverage Industry International, March 2016.
3. “Dry hopping potential of Eureka!^TM A New Hop Variety.” BrewingScience, Vol. 72 2019
4. Wolfe, P., Qian, M. C., & Shellhammer, T. H. “The Effect of Pellet Processing and Exposure Time on Dry Hop Aroma Extraction.” ACS Symposium Series Flavor Chemistry of Wine and Other Alcoholic Beverages, 2012.
5. Hauser, D., et al. “A comparison of Single-Stage and Two-Stage Dry-Hopping Regimes.” J. Am. Soc. Brew. Chem 77(4) 251-260, 2019.
6. Vollmer, D., Shellhammer, T. “Influence of Hop Oil Content and Composition on Hop Aroma Intensity in Dry-Hopped Beer.” J. Am. Soc. Brew. Chem 74(4):242-249, 2016.
7. LaFontaine, S., Shellhammer, T. “Investigating the Factors Impacting Aroma, Flavor, and Stability in Dry-Hopped Beers.” MBAA Technical Quarterly 56(1):13-23, 2019.

Submitted by Chad Kennedy, Hop Specialist – RahrBSG Hops