Exploring the History and Mystery of Flavor Hops

All About Beer Magazine - Volume 34, Issue 2
May 1, 2013 By
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Image Courtesy www.BioLib.de – Flora von Deutschland Österreich und der Schweiz (1885)

Writing a manual on hops in 1877, British agricultural authority P.L. Simmonds praised those grown around the town of Spalt in Bavaria. “The products are of a high reputation, and are the Chateau Lafitte, the Clos de Vougent, and the Johannisberg, as it were, of hops of continental growths,” he wrote.

He didn’t offer a long list of adjectives about their flavor, simply stating they were “the finest and most aromatic hops grown.” Spalt Spalter, as they are known today, likely hadn’t changed much since 1511, when the town banned the export of highly sought-after hop cuttings, nor have they since. One difference that Hans Zeiner, manager of the Spalt hop growers association and a farmer himself, has noticed is requests from brewers for hops picked later, so they are richer in essential oils.

“The brewers want different hops. Some want a greener hop; some want a hop that is a bit more mature. It is about the aroma, the aroma they want.” He cautioned against waiting too long. “If it’s too old, the aroma is not so fine anymore. The aroma has changed, but I cannot say how it changed.” In Spalt “fine” seems to be the only adjective necessary.

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A hop-filled landscape near Spalt, Germany

Contrast that with a different sort of description from John Mallett, production manager at Bell’s Brewery in Michigan. Mallett spoke at a seminar during the 2012 Craft Brewers Conference in San Diego, along with other members of the Hop Quality Group, recently formed because these brewers recognize the need for better communication with American hop growers. Mallett quite obviously was referring to a Bell’s own highly hopped Hopslam, a double IPA, when he explained the importance of telling farmers the sort of flavors and aromas brewers want.

“(Imagine) if you’d gone to the hop growers association 20 years ago and said, I’m going to have a beer that we make 4,000 barrels of one time a year,” he said. “It flies off the shelf at damn near $20 a six-pack, and you know what it smells like? It smells like your cat ate your weed and then pissed in the Christmas tree.”

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John Mallett of Bell’s Brewery is a member of the Hop Quality Group, which recognizes the need for better communication with American hop growers.

The adjectives used to characterize hop flavor in beers today obviously include more than “fine.” For instance, there’s catty, polite language for cat piss; and dank, slang for potent marijuana. They reflect that more than a few consumers now embrace pungent, intense flavors considered offensive not long ago. But they also describe aromas such as pine, pineapple, grapefruit, tangerine, melon, mango, lychee, passion fruit, gooseberries, blueberries, stone fruits . . . even Lifesavers and sauvignon blanc.

The Barth-Haas Group, the world’s largest supplier of hop products, recently suggested that hops producing such unique flavors be called, logically enough, “flavor” (or “flavour”) hops. This is a bit confusing for Americans familiar with the brewing process. They understand that brewers boil hops, usually those with a higher percentage of alpha acids, for an extended period to extract bittering iso-alpha acids, and those hops are classified “bittering” or “alpha” hops. Because essential oils are lost during boiling, to preserve them brewers wait until later in the process to introduce “aroma” hops. Sometime between adding the hops for bitterness and those for aroma, they may include a “flavor” addition. This might result in floral and spicy notes found for hundreds of years, or the bolder new flavors of today.

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Peter Darby, who oversees hop breeding in Great Britain, refers to flavor hops as “impact hops.”

To avoid any confusion, the Society of Hop Research in Germany more recently adopted the label of “Special Flavor Hops”  for varietals bred to exhibit attributes previously considered “un-hoppy.” Peter Darby, who oversees hop breeding in Great Britain, refers to them as “impact hops,” while others simply refer to them as “special.” That many drinkers want these new flavors is clear. Why some hops deliver them and others do not is not.

What Makes American Hops Different

Hops add many positive attributes to beer, most notably aroma and flavor, the latter a result of what’s experienced in the mouth, including bitterness. The chemistry involved in bitterness is relatively well understood. Not so with aroma. More than 20 years ago, two researchers in Oregon proposed establishing an Aroma Unit (AU) comparable to the International Bitterness Unit (IBU). They intended that brewers would use their Hop Aroma Component Profiles, which identified 22 specific compounds, along with the AU, much as they would use the alpha acid content of a particular variety to adjust hopping rates. Since the 1960s “scientists have tried to identify the compound responsible for hoppy character in beer without success. Hoppy aroma in beer is probably not attributable to a single component but rather to the synergistic effect of several compounds,” they wrote in 1992.

Hop scientists have learned much since then, but that statement remains frustratingly valid. J.L. Hanin first used steam distillation to isolate hop oils in 1819, and later in the century Alfred Chapman isolated the key compounds myrcene, humulene, linalool, lynalyl-isonate, geraniol and diterpene. After the introduction of gas chromatography in the 1950s, researchers soon identified more than 400 compounds. The contribution many of them make is not clear because they occur at low levels, individually below perception thresholds. Synergy may change that.

Twenty-first century discoveries have brought some elements into focus. In 2003 Toru Kishimoto at the Asahi Breweries  research laboratory in Japan, determined a compound called 4-mercapto-4-methylpentan-2-one (otherwise referred to as 4MMP) is a main contributor to muscat grape/black currant character apparently unique to hops of American heritage. It has a low odor threshold and occurs naturally in grapes, wine, green tea and grapefruit juice. Hops grown in the New World, including New Zealand and Australia as well as the United States, contain 4MMP and other compounds observed only at trace levels in hops grown in England and on the European continent.

Other researchers in Japan found several American varieties contained compounds that were transformed into limelike and other citrus flavors during fermentation, but that existed only at very low levels in European aroma hops.

Something obviously happened to European and American hop varieties during the course of at least 1 million years since they split from one another. The genus Humulus likely originated in Mongolia at least 6 million years ago. A European type diverged from that Asian group more than 1 million years ago; a North American group migrated from the Asian continent about 500,000 years later. Although three North American botanical varieties exist in the wild, scientists don’t agree on how much they differ from one another, but they clearly are genetically unlike the Europeans. Virtually all hops cultivated today either are direct descendants of European types selected for their brewing and agronomic qualities, also known as landrace hops, are European varieties crossbred to improve those brewing and agronomic qualities, or are a cross between European and American varieties.

That last category didn’t exist until 1917, when E.S. Salmon of Wye College in England took a hop collected in the Manitoban wild, and thus obviously native American, and pollinated it with English hops. Today, every hop in demand for exotic, fruity flavors is such a cross.

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Hop selection at Sierra Nevada Brewing Co.

Consider Citra, the poster child for special flavors. Descriptors of its aroma include grapefruit, lime, citrus, gooseberry, tropical fruits, lychee, melon and sauvignon blanc winelike. It is half Hallertau Mittelfrüh, a German landrace variety often referred to as “noble”; one-quarter U.S. Tettnanger, which is in fact the English landrace variety Fuggle; 19 percent Brewer’s Gold, one of the first hops to result from the crosses Salmon made in 1917; 3 percent East Kent Goldings, another English landrace variety; and 3 percent unknown, which might well be—like Brewer’s Gold—influenced by American wild hops.

Hop Oils and the Magic of Biotransformation

When brewers talk about hops, they actually mean the cones of a female hop plant. Hop oils constitute up to 4 percent of those cones, but more often much less. The four most prominent are myrcene, caryophyllene, humulene and farnesene. The first is a monoterpene, meaning it consists of 10 carbon units, while the latter are sesquiterpenes (15 carbon units). Myrcene has a green, herbaceous, resinous aroma associated with fresh hops and not always considered desirable.

It often constitutes 50 percent or more of the oils in American cultivars. Most of its aroma will disappear during boiling, but it can be prominent in dry-hopped beers because it has a low perception threshold on average. People’s perceptions of aromas, in fact, vary widely, presenting another challenge for all seeking to understand hop aroma. The prominent oils in hops are found in many other plants, and in the case of myrcene these include thyme, lemongrass, verbena, pistachio and fruits such as mango and grapefruit.

In their oxygenated form, sesquiterpenes are more likely to survive boiling, resulting in herbal and spicy aromas also described as “fine” or “noble.” Farnesene, for instance, makes a distinct floral contribution to beers hopped with Saaz. It often constitutes less than 1 percent of oils in bred hops but up to 20 percent in Saaz. One specific caryophyllene alcohol compound, an oxidation product, may add a very strong cedar wood note many describe, again, as “noble.” However, as much as half of the drinking population may be blind to this particular compound. Floral, mildly woody, spicy are qualities that not long ago almost exclusively defined pleasant hop aroma. They remain desirable today, but may be overwhelmed by the addition of hops—particularly bold hops—late in the boil or post-fermentation.

As a hop ripens, many other monoterpenes form along with myrcene, including linalool, geraniol, nerol, citronellol and limonene. Although their presence is often measured in tenths of a percent, they are essential to producing citrus, fruity, floral and woody aromas, whether through synergy or interaction with yeast.

Research in Japan points to the importance of the physical interaction and biotransformations that take place in the presence of yeast. In one study, scientists investigated how geraniol and citronellol, accompanied by an excess of linalool, contribute to citrus aromas and flavors. Focusing on the Citra hop, they sought to identify the key flavor compounds contributing to the aromas specific to the variety—including passion fruit, gooseberries and lychee.

They brewed one beer with only Citra hops and another with Hallertau Tradition and coriander seeds. Both Citra and coriander are rich in geraniol and linalool. The finished Citra beer contained not only those two oils but also citronellol, which had been converted from geraniol during fermentation. The same transformation from geraniol into citronellol occurred during fermentation of the beer made with coriander. Taste panels perceived the beers as relatively similar.

The same researchers followed with another study that compared the composition of monoterpene alcohols in various hops and examined the behavior of geraniol and citronellol under different hopping regimens. They concluded that blending geraniol-rich hops increased the amount of geraniol and citronellol in beer, and that this enhanced citrus character. They determined that hops more easy to find in the market, in this case Apollo and Bravo, could produce aromas similar to hops in short supply.

Still another study in Japan pointed to just how complex sorting out the variables can be. In that one, scientists examined the changes in hop-related compounds during the fermentation process, finding that keeping everything else the same and changing only the yeast strain resulted in noticeably different hop aromas.

Such results illustrate the need for still more research.

Oregon hop merchant Indie Hops, founded only in 2009, pledged more than $1 million for what can broadly be described as aroma research at Oregon State University. “Our ultimate goal is to determine what is it in hop oil that drives flavor,” said Thomas Shellhammer, who is in charge of the brewing science education and research programs at OSU. Shaun Townsend, an OSU faculty member who heads the breeding aspect of the partnership with the Indie Hops, would then use the information to develop cultivars with particular oil profiles.

Meanwhile, There’s Alchemy

Greater understanding of the hop aroma and flavor matrix doesn’t automatically make it easier to integrate bolder hop aromas and flavors into the larger beer flavor matrix. Patrick Ting, for 30 years a hop chemist at Miller (and MillerCoors) before recently retiring, pointed out it is a mistake for brewers to try to equate specific oils with specific odor compounds. “You can’t say we’ll add a little bit of this, a little bit of that,” he said.

Although hop aroma remains something of a black box, brewers find ways on what seems a daily basis to maximize these new flavors. Given a chance to brew with two of the new German “Special Flavor Hops” early in 2012, Bear Republic Brewing brewmaster Richard Norgrove started with a base best described as a wheat wine. He blended Mandarina Bavaria and Polaris in a ratio of 60 to 40 or 40 to 60, depending on the addition, making one at the beginning of a 90-minute boil, one with 60 minutes remaining, one with 40, and then dry hopping with the pair.

“I like to do a lot of blending, maybe change the way the oils come across,” he said. He talked in terms of abstract art versus portrait art, probably because he paints with watercolors himself. “With watercolors you dilute or strengthen the vibrancy of color by the way you use water.”

Dry hopping may eliminate one bit of the mystery for brewers, because post-fermentation hopping preserves much of the aroma in a freshly kilned hop. However, the translation is not direct, and Sierra Nevada brewmaster Steve Dresler said that can be good for his beer. The brewery has learned if yeast is not still active at the beginning of dry hopping, some odor compounds will not develop. “We don’t get the same floral estery notes in some other beers if we use the torpedo [dry hopping] process simply cold without yeast contact time,” he said.

Sierra Nevada literally invented its “torpedo,” a device packed with hops through which its brewers circulate beer after fermentation, to dry hop more efficiently. It uses Magnum (a high alpha hop rich in oil), Crystal and a restrained amount of Citra to dry hop Torpedo Extra IPA. “You can overbrew with Citra,” Dresler explained.

Once again, Citra is a poster child for the new. It can be divisive, and that’s likely part of its appeal. When Sierra Nevada began evaluating Citra about five years ago, men on the brewery’s tasting panel described tropical fruit flavors, while women called the same beer catty or said it reminded them of tomato plants. Women on average detect odors at much lower concentrations and are more likely to rate smells as more intense and unpleasant, but many men share an unfavorable perception of Citra.

Gene Probasco, who star ted the first American private breeding program at John I. Haas, oversaw the creation of Citra. “[The cross] was made for aroma,” he said, and at the time “mild” was a synonym for “good” when it came to aroma. That was 1990.

It was part of a project for a large brewery client, and nothing came of it. Two other large brewing companies essentially owned the rights, one after the other, to the hop during the next dozen years, but ultimately neither of them had a use for it. Only after the Hop Breeding Co. began sending samples to craft breweries was it recognized as special. That was 2008. Understanding why other equally special hops have the impact they do may take a little more time.

This story appears in the May 2013 issue of All About Beer MagazineClick here for a free trial of our next issue.


Stan Hieronymus
Stan Hieronymus, a contributor to All About Beer Magazine for 20 years, is the author of several books on beer and brewing. The most recent, For the Love of Hops (Brewers Publications), deals with all aspects of one of one of beer’s essential ingredients.