Researchers recently have begun to unravel the mystery of hops aroma, but many unknowns remain.
Published: March 16, 2016
First, the bad news. Although both Citra hops and coriander contain significant amounts of the essential oils geraniol and linalool, substituting coriander for Citra hops in an India pale ale recipe will not produce an award-winning IPA. There’s good news to come, but first, let’s dig into a few basics.
A hops cone contains 40 to 50 percent cellulose, 15 percent protein, and 10 percent water, but it is the alpha acids (from 2 percent to occasionally 20-plus percent) and essential oils (as little as a half a percent) that interest brewers. Isomerized, alpha acids provide bitterness; volatilized, the compounds in essential oils contribute to—and sometimes dominate—aroma and flavor.
The composition of oils varies based first on the hops variety, then on multiple other factors, but most of the compounds (50 to 80 percent) will be hydrocarbons (from 20 to 50 percent oxygenated hydrocarbons) and less than 1 percent sulfur compounds.
Those compounds classified as hydrocarbons are highly volatile, not very soluble, and are perceptible in finished beer only when added very late in the boil or post-fermentation. Myrcene is almost always the most prominent, constituting 50 percent or more of the oils in hops varieties such as Cascade and Simcoe. It has a green, herbaceous, and resinous aroma associated with fresh hops that will be scrubbed away during boiling and fermentation but that will provide distinctive piney character to a dry-hopped beer.
The oxygenated hydrocarbon compounds are more soluble and aromatic. Their aromas, or new ones resulting from the fermentation process, are more likely to show up in finished beer. They include linalool, geraniol, citronellol, and hundreds of other compounds. The most prominent of them will constitute less than 1 percent of the hops oils. Scientists first isolated some of these compounds almost 200 years ago, and Alfred Chapman put names to some of them near the end of the nineteenth century. But it was the introduction of gas chromatography in the 1950s that allowed researchers to identify almost 1,000 compounds.
Although sulfur compounds (thiols) are a small portion of the oils, they have very low perception threshold levels (meaning that it takes very small amounts to impact sensory perception) and can significantly influence aroma, either positively or negatively. Measuring thiols is more difficult than measuring hydrocarbons because thiols are present in such small amounts and very expensive and sophisticated analytical instrumentation is needed. Only recently have scientists linked individual thiols with passion fruit, tropical fruit, sauvignon blanc grapes, and other exotic aromas that characterize hops such as Citra and Mosaic. Thiols tend to be divisive because some drinkers are more likely to perceive ribes (currant, gooseberry) while others perceive tom-cat urine.
Individual compounds may be associated with specific aromas. For instance, citronellol is green, floral, and citrusy; geraniol is floral, often reminding one of roses or geraniums; linalool is floral and orange (see “Thirteen Hops-Odor Compounds to Know,” below, for other hops-odor compounds it is useful to know). However, when the compounds interact with the hundreds of other compounds in hops, still more odor compounds result.
“Our ultimate goal is to determine what it is in hops oil that drives flavor,” says Thomas Shellhammer, who is in charge of the brewing science education and research programs at Oregon State University. He and OSU PhD candidates Daniel Sharp and Daniel Vollmer made it clear at the 2015 Craft Brewers Conference how challenging that is.
A Japanese team brewed separate beers, one using coriander seeds and the other with Citra, to compare what happens to the essential oil geraniol in the presence of an excess of linalool during fermentation. In both beers, the geraniol was transformed into the compound beta-citronellol, which contributes to citrus and fruity flavor. Although the beer brewed with coriander had those fruity flavors, it did not have the same tropical character as the one made with Citra.
The researchers at Sapporo Breweries followed that experiment with one that examined the behavior of beta-citronellol and geraniol—again in the presence of linalool—when various different varieties were blended. They discovered, for instance, that when geraniol-rich Bravo hops are added along with Simcoe (rich in 4-mercapto-4-methyl-pentan-2-one), the beer is perceived as more flowery, citrus, and fruity.
That’s useful information, but unfortunately it does not assure that blending Chinook, or some other variety high in geraniol, with Nugget, which has more linalool than most other hops, will do the same. Our knowledge of how these low-level chemical interactions create compounds that then translate into sensory perceptions is still at a very formative stage.
Although researchers recently have begun to unravel the mystery of hops aroma, many unknowns remain. Panelists from OSU made that clear during the seminar “Understanding How to Control Flavor and Aroma Consistency in Dry Hopped Beer” at the 2015 CBC. “I imagine you guys probably have more questions about this than when I started. I do, too. That’s kind of the way science goes,” Daniel Sharp admitted candidly. “It’s job security on our part.”
“[Brewers] want a checklist that matches oils and flavors,” says Peter Darby of Wye Hops in England. “It’s not that simple.” For example, brewers have asked hops breeders for new hops varieties with high oil levels. But in a first round of research focused on Cascade, Vollmer found that the quality of the oil—as determined both by sensory panels and by examining the levels of specific compounds—influences perception more than quantity.
No single compound is responsible for a specific “hoppy” aroma. “You can’t say, ‘We’ll add a little bit of this, a little bit of that,’” says ex-Miller Brewing hops chemist Pat Ting, explaining that brewers make a mistake when they try to equate specific oils with specific odor compounds.
The concentration or amount of a given compound alone is not an indicator of its contribution to aroma, but it is relevant whether it occurs above or below its threshold of detection.
Depending on a compound’s concentration, the sensory perception of that compound can change. Less can be more, which is why thiols at low levels result in pleasant fruity aromas, but at higher levels are perceived as catty.
Synergy and masking play a major role in perception. Synergy occurs when two or more compounds interact to create something different. Masking occurs when one compound suppresses the perception of another.
Most research focuses on the analysis of individual components—because those still aren’t understood—rather than the interactive sensory effects. The interactions are the keys to unlocking the secrets of aroma.
Hops-derived compounds change constantly, beginning even before cones are picked and continuing post-fermentation. Much recent research has focused on biotransformations (the biochemical changes that occur in the presence of yeast). The matrix that includes different hops varieties and different yeast strains in different wort environments is confoundingly complex.
Studies at OSU and elsewhere ultimately will help commercial breweries make beers with more consistent hops flavor and aroma profiles. Vollmer’s ongoing research determined that several lots of Cascade grown on different farms in the Pacific Northwest might have the exact same overall level of hops oils but the composition of those oils is very different, and not surprisingly a sensory panel can confirm the difference. “[Brewers] need to be aware that this phenomenon can happen,” he says.
When Sharp began working on his masters degree in 2010, brewers often couldn’t even get information about the percentage of essential oils in the hops they bought. “Now you are starting to see linalool and geraniol [content] listed more often. It’s getting there,” he says. Perhaps information about the amounts of those compounds and still others will be available to even the smallest brewers, and of course homebrewers, by the time scientists determine which compounds are meaningful.
The research at Sapporo suggests how broad the possibilities are, although it doesn’t guarantee what might result from blending any particular geraniol-rich hops, or even two of them, with one containing an abundance of linalool. For one thing, the sulfur compound 4-mercapto-4-methylpentan-2-one (4MMP) also occurs in many of the same varieties, and it influences the creation of fruity-odor compounds.
But coriander contains no 4MMP, which is one more reason to stick to hops when brewing an IPA.
caryophyllene » woody
citronellol » citrusy, fruity
farnesene » floral
geraniol » floral, rose, geranium
humulene » woody, piney
limonene » citrusy, orange
linalool » floral, orange
myrcene » green, resinous, piney
nerol » rose, citrusy
pinene » spicy, piney
3-mercaptohexanol » guava, tropical
3 mercaptoheyl acetate » muscat, passion fruit
4-mercapto-4-methyl-pentan-2-one » black currant, tropical
Aurora, Bravo, Cascade, Centennial, Chinook, Citra, Mosaic, Motueka, Styrian Golding
Amarillo, Cascade, Centennial, Citra, Glacier, Millennium, Mount Hood, Nugget, Pacifica, Willamette
Hops that Contain 4-mercapto-4-methylpentan-2-one (4MMP)
Apollo, Cascade, Centennial, Citra, Chinook, Cluster, Equinox, Mosaic, Simcoe, Summit