The Effects of Aged Hops on Beer Quality
Hops are considered an essential raw material for beer due to the preservative and functional properties of their chemical constituents. The hop acids (alpha and beta acids) have been well documented to lend antimicrobial and antioxidative protection to beer as well as impart bitterness, foam, and textural quality.
Of course, varieties are also sought after for the distinct flavors/aromas that their essential oils provide. Varieties like Citra® and Mosaic® are well known for their citrus and tropical characters, Lemondrop® is lemony, Saaz and Styrian varieties are floral and herbal, and Cascade reminisces grapefruit. As a result, hops can still be divided into aroma varietals and high-alpha varietals, although many aromatic varietals are also relatively high in bitterness potential (dual-purpose hops).
Historically, hops were harvested, processed, and stored with the intent to maintain alpha acid content. Alpha acids are generally considered most impactful for their contribution to beer bitterness. Once structurally rearranged through boiling, they become isomerized-alpha acids (isomerization is literally a structural rearrangement of the atoms in a molecule), which are more soluble in beer to lend bitterness. Other players, such as beta acids and polyphenols, also affect beer bitterness. Oxidation of beta acids leads to bitter molecules and polyphenols can enhance bitterness due to their astringent nature — and both will augment the analytical bitterness measured in the bitterness unit (BU) by spectrophotometric measurement. Hop chemistry is dynamic, as the accumulation of these secondary metabolites is heavily impacted by hop genetics, growing environment, and harvest timing/practices. Chemical changes such as degradation, oxidation, and rearrangement subsequently occur during post-harvest practices: Kilning, pelletizing, and storage.1 Modern-day practices aim to reduce these impacts and as a result brewers now have some of the freshest and best-preserved hops ever available for brewing.
The question that begs asking is — what makes a hop fresh and how does this translate to quality?
Hop Storage Index
The Hop Storage Index (HSI) is a well-established metric used to assess the relative level of oxidative change that occurs during hop storage. The methodology was developed by Nickersen and Likens in 1979.2 It is a simple analysis to assess how a hop ages from harvest to use. The higher the HSI, the poorer the hop variety fared from harvest to storage and through processing. The measurement is conducted using a spectrophotometer and is non-specific in that it measures the absorbance at 275 nm to 325 nm of an alkaline methanolic extract of hops. A harvest-fresh HSI should be below 0.300.3,4 Harvest HSI is varietal-dependent, with many traditional varieties (Hallertau Mittelfrüh, Hersbrucker, Tradition, Perle, Taurus, and Herkules) scoring less than or equal to 0.275, noble hop varietals like Czech Saaz scoring a bit higher, and some more recent varietals like Celeia scoring above 0.300. Generally, aroma varietals show lower HSI while high bittering varietals like CTZ tend to score higher at harvest.
HSI scores trend upward as hops hang on the vine, particularly for the high alpha acid content hops. Higher kilning temperatures also lead to higher HSI values, for example a shift from 140 °F to 170 °F (60 to 77 °C) can increase HSI by 15%.5 Higher baling pressure and storage temperatures can also augment HSI. However, once pelletized, properly packaged under inert gas, and cold-stored (typically 28 to 39 °F/-2 to 3 °C), HSI should remain stable for several years until use.
While the hop storage index was developed over 40 years ago, it remains the most common tool used to measure alpha and beta acid degradation from harvest to pelletization. HSI is a necessary metric to understand when contracting hops based on alpha acid content.
Interest in Hop Flavor/Aroma Changes During Storage Began More Recently
While a depth of work has been done to study post-harvest practice effects on hop acids, fewer studies have looked at storage effects on the hop volatile oil components and the brewing quality of aged hops from a flavor perspective. We know that as the hop acids break down, the release of their side chains result in organic acids that impart rather undesirable aromas. The cheesy and sweaty-sock notes of iso-valeric acid and pungent, baby vomit notes of butyric acid appear rather readily upon hop oxidation, exposure to heat, and light. As hop acids degrade, their oxidized products still lend bitterness, however the bitterness has typically been deemed less desirable. Despite not smelling fresh, brewing with aged hops can actually lend desirable fruity aromatics to finished beer. Organic acids esterify into much more pleasant stone fruit, cherry, and peach aromas in the presence of ethanol. Historically, aged hops have been used in the production of sour beers such as lambics. With significant aging, the degradation of the hop acids detracts from their preservative effects and allow lactic acid bacteria and non-Saccharomyces yeasts to do their work. The resulting fruity esters also blend nicely with the sour notes of traditional lambics and other fruited beers.
In non-sour beers, these qualities are less desirable, especially for lagers that aim to impart kettle hop aromas and ales that are expected to be accompanied by fresh hop notes. Researchers took an interest in evaluating the effects of hop storage on hop volatiles in the late 1970s and early 1980s. During this time, post-harvest practices were shifting in the hop industry and hop pelletization first became available. Pelletization removed upwards of 10% of the vegetative matter and resulted in a more compact product that could be efficiently shipped and stored. Pellets were also thought to be a more stable form of hops to keep in storage. At this time, refrigeration of bales in storage was still not common practice, however brewers were interested in the use of pellets that had the potential to store longer under refrigeration in the brewery.
I recently spoke with Val Peacock, longtime hop chemist and hop guru at Anheuser-Busch and now a hop consultant, about his experience in the hop industry when these changes were taking place. Conversing with him about “hop quality” really opened my mind. My experience with hops begins after the popularization of both pelletization and extraction capabilities, and so what I know, or we might define collectively, as “hop quality” today is vastly different than how a brewer might have defined hop quality 50 years ago.
In Old Germany the Hop Warehouses Smelled Different: A Conversation with Val Peacock
According to Val, 50 years ago people thought that aging hops was positive. If you would talk to European brewers in the 1970s and earlier, there was a disagreement/confusion around what “noble hop aroma” was. Beers were full-flavored, brewed using 100% malt, and had 20–30 IBUs. He recalls the legendary brewing scientist Morten Meilgaard, most remembered for his sensory work and development of the beer flavor wheel in 1979, referring to noble hop aroma in beer as “reminiscent of hops, but not hops” — an aroma that actually doesn’t last very long in the beer.
Before refrigerated storage in Europe, this would be before the 1980s, bales were not as densely packed as the bales we see today. Bales were more like hop pillows — they were round and stacked rather loosely in piles. Farm bales were later repackaged into higher density bales for shipment to buyers. As Val recalls, “Those warehouses smelled different, more like tobacco or leather and less like the myrcene-driven hop smell we witness today,” when we walk into a hop processing facility. The pillowy farmer bales in Europe pre-1980s were packed at about half the density of today’s bales. Hops were stuffed into a burlap bag (which also gave flavor to the hops). The empty bag was about 2 meters tall and one meter across (6.6 x 3.3 ft). The moisture content was higher, closer to 14% than the 10% it is currently in the U.S. These fluffy bales would sit in warehouses without temperature control where, in a temperate climate, they would not dry out as much as they would in the more arid Yakima Valley of the U.S. These 12–14% moisture bales would sit in farmers’ storage until January, at which time they were delivered or shipped to customers. At such high moisture content, the bales would produce a hop sweat with biological processes going on. If they were too wet they could get very hot. So the bales had to be loosely packed and not as stackable to allow for plenty of circulation. Because of this, flavor components would be altered. He noted that Anheuser-Busch eliminated the use of burlap bale wrap in the 1990s because of the odor it imparted to the hops. What was considered noble hop aroma in beer was different than today, and thus these beers might taste different than beers today.
In the early 1990s the German industry also changed their baling practices. The pillowy bales were replaced by square bales that were 50% more dense. These bales could be stacked much higher and thus would take up less warehouse space. At the time, the industry seemed more interested in economics than hop aroma, and the main goal was to avoid alpha acid losses during storage. These more densely packed bales were more prone to sweat and the heat buildup in the bale could
lend toward spontaneous combustion.
At the time, most warehouses were not refrigerated, and many today globally are still not refrigerated. This is not only a threat to hop aging, it is also a threat to the hop farmer and processor. Spontaneous combustion has been the cause of many warehouse fires. Even as recently as 2006, fires caused from high-moisture bales actually destroyed hop warehouses in Yakima Valley, estimated to cause about $4 million of damage. Approximately 4% of the U.S. hop yield was lost in this one fire.6
As Val recalls, once cold storage was more readily available the nature of the hops’ aroma also changed. Commercial brewers were fine with it as they were largely focused on the bittering resins; refrigerated hops resulted in reduced alpha acid loss from bale-to-beer.
Commercial U.S. brewers 30–40 years ago may have sourced about 1⁄3 of their hops from Europe and 2⁄3 from the U.S. At the time (1980s), beers were becoming increasingly lighter and less bitter. Take for example the original Miller Brewing Company “Lite” beer. Miller Lite was originally in the 20 BU range, whereas today it has only 10 BUs. These lighter beers did not have enough malt backbone to mask the tobacco notes and the lingering harsh bitterness from aging hops was not acceptable with the lower BUs, further pushing the hop industry to invest in refrigerated storage.
In the 1980s, most commercial brewers were using hops in their leaf form. Whole leaf hops would be shipped from Europe or Yakima and be stored up to two years in cold storage at the brewery. From the time hops were harvested, dried, baled, and shipped on a boat across the ocean or via rail in January, hops were already many months old before use.
According to Val, Anheuser-Busch switched to pellets in 2003. Coors was using whole cones up until the 1990s and Miller was using extracts as early as the 1950s. In the 1990s the major lager producers were pressed to switch to hop pellets that took up less space in storage, were more efficient to use, and kept better than whole cone hops under proper packaging and refrigeration.
“Sensory challenges weren’t much of an issue, hops were now less aged and bitterness was perceived to be smoother in quality. The consumers didn’t seem to mind or were not providing a lot of feedback, as no one seemed to be calling in and saying, ‘you changed my beer,’” Val said.
It was a brewing culture shift that did not receive a lot of pushback. Anheuser-Busch had been trialing pellets 10 years before the final switch was made. Coors and Miller had been doing studies on hop pellets and hop products even earlier. In the midst of this culture shift from whole cone to pellet hops seems to be when brewers found a need to investigate the effects of pelletizing and less aging on hop and beer flavor.
Hop Baling Practice Shifts in the U.S.: Conversation with Hop Grower Diane Gooding
Let’s hop back to baling practices for a bit. After learning about some bale history in Europe, I was curious to know more about bale history in the U.S. I reached out to Diane Gooding of Gooding Farms in Idaho. In Idaho, hops used to be put into sacks and a person, generally a really big guy, would literally stamp them down. These sacks would be loosely packed and weighed about 70 lbs. (32 kg) (interestingly, Diane noted that the Spanish hop industry still uses these types of sacks). Hops were dried and stored in the warehouse in piles, baled in winter, and delivered in the spring. Cable balers in the day were pulled by oxen or horses in a circular fashion. The time between picking and baling was extended just as in European practice.
Today, Gooding says from the time her hops are picked from bines in the processing shed it takes five minutes to get them to the dryer. The hops are kilned for an average of 9.5 hours (timing varies based on the hop variety, kiln bed depth, and kiln temperature). Hops are then cooled for 12–24 hours. Sometimes farmers don’t have time to dry them 12 hours due to the demands of harvest. This can lead to inconsistencies in the hops. A modern bale weighs 200 lbs. (91 kg) and is more densely packed than in the sacks so they must go into cold storage until pelletized. While it would be ideal to process into pellets within two months, the reality is that in the U.S., most pellet operations often run until mid-March. Once hops are pelletized, they are generally more stable, depending on the oxygen content of the mylar bags they are packed in. Most producers flush bags with an inert gas and can get the oxygen levels down to 0.05–0.1% oxygen. However, sometimes oxygen can get as high as 1.5–2%, and this can have a big impact on HSI.
It is important to note that hop processing is still not uniform in the U.S. There are different types of pickers, drying systems, kiln bed sizes, pellet types, and packaging practices.
Hop Volatiles Change Due to the Use of Fresher hops
In the 1980s, researchers at Oregon State University and Adolph Coors Brewery conducted research to understand the effect of using fresher hops on beer flavor.
In the early 1980s, Val Peacock and Max Deinzer of Oregon State University (OSU) published their work investigating the effects of refrigeration on hops and the essential oils subsequently transferred to beer.7,8 Six varieties of hops were aged for one year under refrigerated storage (Hallertauer, Perle, Hersbrucker, Tettnanger, Cascade, and Cluster) and evaluated for hop oil and acid changes. The varieties in which alpha acids degraded most also showed the most oxidation of humulene. During storage, geranyl isobutyrate had converted to a more flavor-active form, geraniol. Today, geraniol, its precursors, and related compounds are known to impact beer aroma. This study is one of the earliest works to show the importance of geraniol and its biotransformation during beer production.
In 1985, Foster (Adolph Coors Brewery) and Nickerson (OSU) published their work on hop oil degradation of 20 different hop varieties over six months at ambient temperature. Not surprisingly, most hops lost a significant amount of total oils. However, losses were varietal-dependent ranging from 27.5–90%. A comparison of pellets and cones of three hop varieties also revealed that hop cones lost from 53–62% of total oils and pellets lost from 43–58% of oil after 30 days of storage.9 Oxygen resistance is higher for cones than pellets under aerobic conditions. In the production of pellets, hops are milled to be pressed through a die. Milling of hops into a powder disrupts the lupulin glands and leaves them more accessible to oxygen.10
The 1980s studies on hop aging identified humulene oxidation products as contributors to the traditional “kettle-hop” aroma of beer. Even under one year of refrigerated storage, humulene oxidizes to humulene epoxide II. We don’t find humulene epoxide II in beer produced with fresh hops because wort boiling times do not seem to be long enough to convert measurable amounts. Humulene epoxide II from aged hops does extract into beer and as beer ages it converts to humulenol II (this is seen over a typical beer shelf life). Peacock and Deinzer identified humulenol II as a contributor to “fine hoppy aroma”7 and suggested that its content can affect hoppy beer aroma.
To better understand the impact of fresher hops on beer flavor, researchers at Adolph Coors again teamed up with scientists at OSU using Washington-grown Cascade and Idaho-grown Hallertauer Mittelfrüh hops.11 Beers were brewed in 30-barrel pilot batches with hops of three categories: Fresh hops, hops aged at 90 °F (32 °C) for two weeks (Aged I), and hops aged at 90 °F (32 °C) for nine weeks (Aged II). Flavors of the aged and fresh hop beers were discernably different for both varieties (confidence level 95% and higher).
This early work showed that hop volatile content during the stages of brewing is in fact a very dynamic process. Some compounds decrease from wort production to finished beer, while some compounds increase from wort production to finished beer. Moreover, some go up and down, or vice versa. One such example is citronellol that was only observed post-fermentation. Citronellol is a product of yeast biotransformation on geraniol precursors. Most hop oxidation products of alpha humulene and B-caryophyllene poorly survive fermentation. Some esters such as ethyl octanoate and ethyl decanoate can be detected in samples after fermentation. The floral compounds of linalool, geraniol, and alpha-terpineol generally survive the brewing process and can be found in finished beers from most hop varieties. In the Coors collaboration with OSU, the intensity of kettle aroma (herbal/spicy) decreased with aged Cascade hops, yet increased with aged Hallertauer hops. The highest floral ratings were given to beers with Aged I hops over fresh hops in alignment with geraniol, linalool, and citronellol content. Again, humulenol II was not found in the fresh hop beers, while the herbal spicy note was found in the aged hop beers, and more prominently in the Aged II Hallertauer hopped beers. A grapefruit-like citrus note was also detected in beers brewed with extensively aged (Aged II) Cascade and Hallertauer hops.
The 1980s studies concluded that fresh hops indeed made a difference to beer flavor and that “moderate aging of fresh hops prior to brewing” was positive as it maximized the level of desirable aroma compounds.
Similar studies were recently conducted to assess the impact of aged hops on IPA flavor.12 In beers brewed with Simcoe® and Saaz that had been exposed to five different aging conditions, the authors concluded that moderate aging may make beer more drinkable and that old hops contribute a “unique” flavor (undefined). The results were in agreement with the beers brewed at Adolf Coors Brewery in 1985.
What’s interesting to me about this data is that during my first Cascade hop selection for Coors brands in 2010, I was astonished at the hops we selected as “high quality.” To me, the hops were already a bit oxidized, grassy, and herbal. I was so accustomed to smelling the fresher Cascade hops used in hoppy Pacific Northwest craft beers. It turns out these hops had been selected year over year for the past 25 years in alignment with the 1985 scientific data that Bob Foster had collected during his time as a hop chemist at Coors.
Fresh Hops Today
Today, we are much more accustomed to the smell of myrcene and citrus in our hops. The hops we use today are undoubtedly fresher than they were 40 years ago. The heavy dry hop loads of craft culture depend on the fresh piney and citrus aromas in hops. Some processors today tout the freshness of their pellets. The reality, as Diane Gooding reminded me, is that not all hops are pelletized right away. Only some breweries are fortunate to brew with hops pelletized with minimal storage. During the months between baling and pelletizing, even under refrigeration, HSI increases. And with that, hops will lose alpha and beta acids.
Hop volatiles immediately begin to shift from bale to pellet: Myrcene will undergo auto-oxidation to produce several other attributes such as beta pinene, geraniol, geranial, linalool, and nerol, and humulene will begin to oxidize to its “kettle hop” counterparts.13 The longer a bale sits in cold storage before pelleting, the more they will age. In some hop growing regions such as Slovenia, many growers do not have refrigerated storage,10 which means when in surplus (as in 2020) their hops surely will age if not processed and used quickly. This amount of aging may not be appropriate or acceptable to brewers.
Assessing Hop Quality and Long-Term Hop Storage
What makes a hop high quality? It appears that hop quality is truly in the hands of the brewer. The best way to assess hop quality is through sensory analysis in a good old-fashioned hop rub. The only standard for hop age in the industry we have is HSI, which does not adequately address all metrics of “hop quality.” Research from 2023 suggests that high-HSI hops can absolutely be used effectively to produce high-quality beer — depending on the variety. While beers brewed with hops of an HSI of 0.3 were overall statistically higher in hop aroma quality than beers brewed with hops with HSI 0.5, the researchers suggest that Celeia and Aurora hops, with an HSI of up to 0.5 and Styrian Wolf, with an HSI of up to 0.6, are indeed suitable for beer brewing.12 The authors do not have any justification other than it must be due to the presence of some as-yet-unmeasured compounds or the occurrence of some as-yet-unidentified synergistic effects between compounds in the aged hops.
Typically pellets have a higher starting HSI than fresh hop cones and also have a higher HSI after aging (under aerobic conditions), with some exceptions that seem to be varietal-dependent as volatile profiles tend to reflect genetics. Effectively, both storage conditions and hop variety play a role in hop stability. The range of changes in volatiles appears to be more closely related to the starting essential oil content than to the oxidative changes among volatile compounds.11
Although the project is very much in its infancy, just a few months ago Thomas Shellhammer’s lab at Oregon State University presented preliminary results of their latest studies on hop aging.13 The research is exploring the aging characteristics of American aroma hops and initial results suggest that when stored properly (i.e., satisfactorily sealed, intact, gas-flushed high-barrier packaging with very low oxygen content, and stored frozen) hops show only modest declines in quality over 4 years and in some cases were not significantly different (chemistry and aromatic qualities) from samples that were less than one year old. There was some varietal dependency, but initial experiments show Citra® and Hallertauer Mittelfrüh being quite stable over four years. Centennial displayed modest changes while Cascade showed the most age-dependent changes. Further work is needed to assess the specific chemical and sensory changes in American aroma hops as they age, and work is ongoing.
In my own experience, hops stored properly can last upwards of four years. While at MillerCoors I would sometimes get called over to the brewery to smell a freshly opened bag of hops. My nose was used to determine whether they were good enough for use. Even after three or four years in our coolers, they were almost always still fresh to my nose.
To Recap
The hops of yesterday were not as we know hops today. Post-harvest processing and storage practices lead to herbal and earthy aromas, which are very different than the citrus, floral, and piney aromas today’s consumers are accustomed to. Modern day post-harvest processing and packaging significantly reduces the effects of oxygen and temperature on hop acid degradation during storage, however there is still a window of time where hop cones await pelletization that can stretch to many months. We do not fully understand the changes that these hops undergo, yet we do know that hop variety seems to play a role in how hops “age” and that storage conditions will affect how quickly hops change. Our industry metric for aging, HSI, does not adequately address all the metrics of “hop quality.” The brewing value of high-HSI hops is not necessarily degraded with age, as the data tells us that moderately aged hops can positively impact beer aroma.
References:
1 Identification and Quantification of the Oxidation Products Derived from Alpha-Acids and Beta-Acids During Storage of Hops (Humulus lupulus L.). J. Agric. Food Chem, 2013, 61, 3121-3130.
2 Nickerson, G.B., Likens, S.T. Hop Storage Index. J. Am. Soc. Brew. Chem, 1979;37(4):184–7.
3 Cocuzza, S., Lutz, A., Müller-Affermann, K. Influence of Picking Date on the Initial Hop Storage Index of Freshly Harvested Hops. MBAA Technical Quarterly, 2013;50(2):66-71.
4 Weber, K., Jangaard, N., Foster, R. Effects of Postharvest Handling on Quality of Storage Stability of Cascade Hops. J. Am. Soc. Brew. Chem, 1979;37(2):58-60.
5 Zunkel, From Hop Harvest to the Brewery. www.barthhaas.com/ressources/blog/blog-article/from-hop-harvest-to-the-brewery
6 “Huge Hop Fire” ProBrewer. www.probrewer.com/beverage-industry-news/huge-hop-fire
7 Peacock, V., Deinzer, M. Chemistry of Hop Aroma in Beer:, J. Am. Soc. Brew. Chem, 1981.
8 Peacock, V., et. al. Hop aroma in American beer. J. Agric. Food Chem, 1980, 28, 774-777.
9 Foster, R.T., Nickerson, G.B. Changes in Hop Oil Content and Hoppiness Potential (sigma) during hop aging. J. Am. Soc. Brew. Chem, 1985, 43, 127-135.
10 Rutnik K, Ocvirk M, Košir IJ. Changes in Hop (Humulus lupulus L.) Oil Content and Composition during Long-Term Storage under Different Conditions. Foods, 2022
11 Lam, Kai C., Foster II, R., Deinzer, M. Aging of Hops and Their Contribution to Beer Flavor. J. Agric. Food Chem, 1986, 34, 763-770.
12 Xu, H., et. al. Aging of Hops and Their Effects on India Pale Ale Flavor. BIO Web of Conferences, 2023.
13 Rutnik K, Ocvirk M, Košir IJ. The Impact of Hop Freshness on Kettle-Hopped Beers. Foods, 2023.
