This first appeared in the second issue of Hop Queries (June 2017). It is a reminder that all the components matter.
To repeat the quick summary of essential oils from last month: They constitute up to 4 percent of the hop cone. They include 50 to 80 percent hydrocarbons (such as myrcene), 20 to 50 percent oxygenated hydrocarbons, and less than 1 percent sulfur compounds. Hydrocarbons are highly volatile, not very soluble, and are perceptible in finished beer only when added very late in the boil or post-fermentation.
The oxygenated compounds (such as linalool and geraniol) are more soluble and aromatic. Their aromas, or new ones resulting from interaction with yeast (biotransformations), are more likely to show up in finished beer. Although sulfur compounds (thiols) are a small portion of the oils, they have very low levels of threshold and they play a prominent role in creating fruity, tropical aroma currently in vogue.
In 2010, researchers working for Japanese brewery Sapporo reported on a study about how geraniol metabolism might add to the citrus flavor of beer (P26, “For the Love of Hops”). They brewed two beers, using Citra hops in one and coriander seeds in the other because both are rich in geraniol and linalool. The finished Citra beer contained not only linalool and geraniol 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 concentration of geraniol and citronellol in both increased depending on the initial concentration of geraniol. The results suggested the importance of citronellol and an excess of linalool in the hop-derived citrus flavor of beer, but because there was little citronellol in raw hops the generation of citronellol depended on the geraniol metabolism by yeast.
In 2012, those researchers at Sapporo reported on a study in which they compared the composition of monoterpene alcohols in various hops, and examined the behavior of geraniol and citronellol under various hopping conditions as well as hop blend conditions influencing the citrus flavor of hopped beer. First up, they found many U.S. varieties contained relatively high amounts of geraniol, but that European hop varieties did not. (This also extends to many varieties grown in New Zealand and Australia, because American varieties were used to breed them.)
Among conclusions offered in their poster presentation at World Brewing Congress 2012 were ones that a) the contents of geraniol and citronellol in finished beer could increase by blend of geraniol-rich hop, and b) citrus character in beer could be enhanced by blend-hopping of geraniol-rich hops; and composition of monoterpene alcohols and flavor character in beer could be controlled by blending two hops having different flavor characteristics.
They also provided a graphic showing levels of various compounds in several hop varieties. I’ve shared this during presentations along with the suggestion that brewers who could not get their hands on varieties like Citra (or did not want to pay the price) might look for hops high in linalool (for instance, Nugget) or high in geraniol (for instance, Chinook) and use them in the whirlpool or in dry hopping.
Anecdotally, homebrewers have told me about using such varieties (most particularly Bravo) to make beers they described as smelling and tasting tropical. Or blending them with hops known for that character to further enhance it.
But perhaps it is not that simple. The team from Sapporo, headed by Kiyoshi Takoi, continues to follow up, reporting on their latest discoveries in BrewingScience. “Control of Hop Aroma Impression of Beer with Blend-Hopping using Geraniol-rich Hop and New Hypothesis of Synergy among Hop-derived Flavour Compounds” delivers exactly what the title suggests.
Pardon the detour, but we should back up a little. Geraniol is not the only compound more prominent in New World hops. There are many thiols (sulfur-containing compounds). In 2003, Toru Kishimoto, at the brewing and research laboratory for Asahi Brewing in Japan, suggested 4-mercapto-4-methylpentan-2-one (4MMP) could be a contributor to exotic aromas. “We had evaluated the hop aroma of many cultivars from the U.S., Europe, Australia, and New Zealand. At first mainly sensory evaluation,” he wrote in an email. “Then I noticed the U.S., New Zealand, and Australian hops had a common fruity character in the aroma none of the European hops had.”
The new hypothesis from Takoi and researchers at Sapporo puts all of this together. As this graphic illustrates “tropical” is a result of synergy. (Note: 4MMP is also known as 4MSP; blame scientific nomenclature.) Basically, “We assumed that the synergy among volatile thiols and monoterpene alcohols could contribute to such flavours in beer made with certain ‘Flavour Hop’ varieties, containing both groups of compounds.”
(BrewingScience November/December 2016, Vol. 69)
To cut to the chase, they write, “This result is also useful for determining blend-hopping recipes based on composition of flavor compounds of each hop variety.”