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Craft brewers now have a new tool for sniffing out trace flavor compounds


In dry hopping, hops are added during or after the fermentation stage of the brewing process.
Enlarge / In dry hopping, hops are added during or after the fermentation stage of the brewing process.

Natasha Breen/Getty Images

Craft-beer aficionados relish the endless flavor variations that can be achieved by mixing and matching different varieties of hops, whether one favors refreshing citrus or fruity notes or something a bit more earthy or pine-scented. But some of the chemical compounds that contribute to those flavors are present in such trace amounts that it’s difficult for brewers to measure and track them during the brewing process. Now German scientists have devised an automated, efficient method for doing just that, according to a recent paper published in the Journal of Agricultural and Food Chemistry.

As we’ve previously reported, all beer contains hops, a key flavoring agent that also imparts useful antimicrobial properties. (Without them, beer spoils quickly.) Brewers mash and steep grain in hot water, which converts all that starch into sugars. This is traditionally the stage when hops are added to the liquid extract (wort) and boiled to give the beer that hint of bitterness. During the boiling process, a certain portion of the resins (alpha acids) in hops isomerize into iso-alpha acids. That chemical rearrangement of the molecules is what produces bitterness. Yeast is then added to trigger fermentation, turning the sugars into alcohol.

But a little hops goes a long way. Add too many hops, and the beer will be so bitter as to be undrinkable.

Dry hopping is a way to put more hops in beer without getting that excessive bitterness, and it’s especially popular with craft brewers. Dry hops are added during or after the fermentation stage, after the wort has cooled. In this case, there is no isomerization of the alpha acids, so you get all the aromatic hoppy flavor without too much bitterness. Brewers can use as much as 20 times the usual amount of hops if they’re dry-hopping. (Just beware of “hop creep,” which can cause such bottled beers to explode.)

With the increasing demand for fruit-forward hoppy beers, there is growing interest in gaining a better understanding of the various chemical compounds that can impact aroma and flavor. In order to create unique beers with a special character but without any additional ingredients, you need to know which odor-active compounds are present in the different hop varieties, how high their content is, how they affect beer aroma, and how they change during the brewing process and storage.

For instance, last summer we reported on how chemists at the University of Redlands in California have been tracking the chemical compounds that contribute to the flavor profiles of sour beers (made with wild yeasts), monitoring how their concentrations change over time during the aging process. Using NMR spectroscopy, the chemists studied the levels of acetic acid, lactic acid, and succinic acid, all of which are produced as the yeast ferments and contribute to the distinctive flavor profile of a sour beer.

The team also used liquid chromatography and time-of-flight mass spectrometry to identify and track changes in trace compounds that can also contribute to the overall flavor profile, such as phenolics or vanillin. That analysis revealed trace amounts of hordatines, which come from barley and are known to possess antimicrobial properties, as well as the amino acid tryptophan. The end goal is to gain better control over the process for craft brewers so they can craft better sour beer. 

Other compounds that contribute to flavor in hoppy beers include (3R)-linalool, which imparts citrus and floral notes, myrcene (which smells like geraniums), rose-scented geraniol, and 4-mercapto-4-methylpentan-2-one (4MMP for short)—it’s what gives certain craft beers that distinctive black-currant-berry aroma. And if you’re a fan of hoppy beers, just remember to heed the science that says to store your beer in a cool place and drink it within three months or so, lest it lose that rich aroma.

The latest study, led by Nils Rettberg of the Research Institute for Beer and Beverage Analysis in Berlin, Germany, focuses on aroma compounds called thiols (or mercaptans). These include the aforementioned 4MMP, as well as 3-mercapto-1-hexanol (3MH) and 3-mercaptohexylacetate (3MHA), which impart grapefruit and passion fruit/guava aromas, respectively. Even small amounts of thiols can have a big impact on beers, so brewers are keen to learn as much as possible about their chemistry as it pertains to “hop-forward” styles of beer. But because only very small levels of thiols are present in most hoppy beers, detecting them is a much bigger challenge.

Rettberg and his co-authors noted that thiols also contribute to the distinct aromas of wines like Sauvignon Blanc, and they were able to draw on a rich array of prior research in viticulture and oenology. Specifically, they adapted a method for measuring wine thiols involving coated polymers placed into the air above a wine sample, thereby converting any aerosolized wine thiols into compounds that can be more easily measured. The method was still insufficient to detect the trace amounts of thiols in beer, so Rettberg et al. combined it with gas chromatography-tandem mass spectrometry to reach the necessary sensitivity.

More beer!

To test their method, Rettberg and his colleagues collected a sampling of commercially available dry-hopped ales: six different beer styles and 13 beers brewed with German, Australian, and US hops, all of which they expected would have high levels of thiols. The cooled beer samples were placed into laboratory glass bottles and flushed with nitrogen to remove most of the excess carbon dioxide, thereby making it easier to accurately pipette small volumes. And then the researchers analyzed the samples with their two-tiered method. They used a non-dry hopped lager beer for calibration.

The result: thiol distribution in the beers was consistent with prior studies, demonstrating the efficacy of this new solvent-free, automated method to measure and track thiols for brewers of hoppy beers. But there was one surprise: in one beer (a US IPA) that had real grapefruit added, they only detected one of the three kinds of thiols they were expecting to find. This suggests that the aromas of this particular beer were due to other scent compounds present in the grapefruit, in addition to any thiols. The authors concluded that this indicates “a need to identify the key aroma compounds of beers with different fruit additions as these gain more and more consumer acceptance.”

DOI: Journal of Agricultural and Food Chemistry, 2020. 10.1021/acs.jafc.0c06305  (About DOIs).



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