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The Complex Case of Thiols

Thiols, also known as mercaptans, are sulfur-containing organic compounds with a sulfur atom bound to a hydrogen atom. Thiols make up less than 1% of the essential oils in a hops cone but might hold a key to the fashionable in-demand tropical flavors.

Stan Hieronymus a month ago

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“Here I am in the 21st century
I have to say it ain’t as cool as I hoped it would be
No man on the moon, nobody on Mars
Where the hell is my flying car?”
—From “21st Century Blues” by Steve Earle

Waiting for technology that turns brewing research into information useful to brewers can feel a little like waiting for a flying car. At the International Brewers Symposium on Hop Flavor and Aroma in Beer last summer at Oregon State University, five of the dozen presentations emphasized the role thiols play in creating the bold, often tropical, flavors currently fashionable in beer. Scientists in France, Belgium, Japan, the United States, and Germany are all involved in related research.

The potential seems so obvious that Nyséos, a specialty laboratory in France that focuses primarily on wine, has created a Thiol Potency (TP) indicator. Laurent Dagan, director at Nyséos, explains that discoveries about the role thiols play in developing flavor and the TP indicator are his “call” to brewers. “Hey, we have new information. It is complex, but we are trying to find indicators useful for you. What do you think about that?” he asks. The key word is complex because thiols make up less than 1 percent of the essential oils in a hops cone, and there are uncounted hundreds of compounds that contribute to hops odor that our brains convert into hops aroma.

What Are Thiols?

Thiols, also known as mercaptans, are sulfur-containing organic compounds with a sulfur atom bound to a hydrogen atom. Scientists first identified them in hops in the early 2000s, focusing on three that winemakers have known about for decades. One, 4-mercapto-4-methylpentant-2-one (4MMP), smells and tastes of box tree, black currant, and ribes. It is also known as 4-methyl-4-sulfanylpentan-2-one (4MSP). Another, 3-mercaptohexan-1-ol (3MH), is often described as exotic, smelling of rhubarb and citrus. And the third, 3-mercaptohexyl acetate (3MHA), is reminiscent of passion fruit and guava.

These compounds are all prominent in Sauvignon blanc, Riesling, and other wines, although they do not occur in grapes. They are formed during fermentation from precursors present in grape must. However, free thiols have been identified in many hops varieties along with two different bound forms (precursors) in both hops and barley malt. “Little is known about how these analytes develop in malt and hops over time or what process changes directly result in the increase of these analytes,” says Scott Lafontaine, a graduate research assistant at Oregon State University. Included are not only the thiols found in wine, but several others.

Analyzing thiols is challenging because of their low concentration and high reactivity. The standards and instrumentation that are needed to measure them are very expensive, and few laboratories have the equipment needed. “Part of the reason [this is challenging] is that thiols have very low odor-detection thresholds, and therefore, very small concentrations of these analytes can potentially have a large impact on beer flavor,” Lafontaine says. “In terms of concentrations, we are looking at parts per trillion. To give you an idea of what we are trying to find, it is like looking for the top of a pin in an Olympic-size swimming pool.”

Whoa, There’s Never a Single Marker

Stop and take a breath. Thiols are not a magic bullet for creating tropical flavors. The hops aroma/flavor matrix remains a product of synergy. Remember the lesson learned with linalool, a monoterpene. In 1981, Val Peacock and associates at Oregon State had suggested the importance of linalool to hops aroma. They developed a model to predict the amount of floral-hops aroma likely in a beer based on the amount of linalool, geraniol, and geranyl esters in the essential oils. This led to research by other scientists. Peacock noticed that “because of the amount of attention given to linalool during recent decades, its perceived importance has been elevated far beyond its true relevance.” As a result, he concluded, “this distorts brewers’ understanding of the nature of hops aroma in beer. Hops have more to contribute to the aroma of beer than just linalool.”

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Thiols in Action

As reported in “Hops Oils & Aroma: Uncharted Waters” (Craft Beer & Brewing Magazine®, March/April 2016), a team at Sapporo Breweries in Japan conducted a study in 2012 in which they compared the compositions of monoterpene alcohols in various hops, examining the behavior of geraniol and citronellol under various hopping and hops-blend conditions. Among other things, their experiments demonstrated that the co-existence of linalool, geraniol, and citronellol could increase the perception of “citrus” in a model solution. Anecdotally, using this information, adventurous brewers combined hops high in linalool (such as Bravo or Nugget) with others rich in geraniol (for instance, Chinook) to brew beers they described as fruity, sometimes tropical. But it turns out linalool, geraniol, and citronellol may not have been totally responsible for those flavors.

Following additional research, the team from Sapporo has a new hypothesis. In their experiments, sensory panelists rated various aroma attributes of a solution dosed with 4MMP, scoring it two (out of four) for “tropical.” Similarly, a solution dosed with a mixture of linalool, geraniol, and citronellol scored less than two for tropical. However, when all four were blended, the tropical score jumped to a full four. (That may be what happens when brewers use Chinook because not only is it geraniol-rich, it also contains 4MMP.)

The Thiol Potency Indicator

The formula Nyséos proposes to measure thiol potency generates a TP indicator, but the lab also provides information about bound and free thiols. For instance, Calypso has a TP of 112, but 95 percent of that is bound 3MH potency and 5 percent bound 4MMP potency. To free bound thiols, Nyséos suggests using hops in the kettle. Citra, on the other hand, has a lower TP (57), but 33 percent is free 4MMP, 8 percent bound 4MMP, 1 percent free 3MH, and 58 percent bound 3MH, making it suitable for use in the kettle or dry hopping. Unfortunately, Citra doesn’t actually have a single TP. Its TP varies from one lot to the next.

For instance, Nyséos analyzed two different lots of Citra during its research. One contained two-thirds the amount of free 3MH as the other, but 50 percent more free 4MMP. During the aroma and flavor conference at Oregon State, Martin Steinhaus presented information from the German Research Center for Food Chemistry about 4MMP in forty-six varieties from five countries. Not surprisingly, the group concluded that variety was the largest factor in determining 4MMP concentrations. However, the research also found variations related to harvest year and provenance. The study did not take into account other factors—such as harvest maturity, post-harvest processing, and hops storage—that other researchers have reported may have a significant impact on the composition of essential oils, including thiols.

That’s why the possibility of a TP indicator number printed on a package of hops, along with a list of other components, is so appealing. It would measure the final product. For now, the cost of technology makes this look like a flying car, but Dagan is an optimist. “Thiol management is tricky in wine,” Dagan says. “But after twenty years, winemakers are able to produce great Savignon blanc wine with better management of thiols. So, I think that will be the same for beer in five years.”

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