Q & A with Drs. Cecile Chenot and Tom Shellhammer, Oregon State University

How do you see the role of thiols evolving in hop-forward beer styles, and what should brewers prioritize to extract consistent thiol expression at scale?

Thiols are an important contributor to hop aroma in some varieties and beer styles, but brewers should remember that they are only one part of the overall aroma picture. There is currently a great deal of attention on these compounds because they are extremely potent and can deliver interesting tropical and exotic aromas. At lower concentrations they can add richness and fullness to hop aroma, while at higher levels they can produce sweaty or burnt rubber notes.

That said, thiols work alongside many other important hop-derived compounds, including terpene alcohols, esters, and fermentation-derived aromatics. The best hop-forward beers are rarely defined by a single compound class. Brewers should think about balance and integration rather than maximizing any one component. For some brewers, the adage that if a little is good then more is better can be taken too far, in our opinion. Beers that are overly thiol-dominant can become one-dimensional and, for some consumers, unpleasant.

For consistent thiol expression at scale, brewers should pay close attention to hop selection, fermentation conditions, and yeast strain choice, since these factors strongly influence both the presence of thiols and their release from bound precursors during fermentation.

With more breweries adopting thiol‑enhancing yeast strains and processes, what are the most common misconceptions you see about biotransformation, and how can brewers avoid these pitfalls?

One common misconception is that biotransformation is a guaranteed outcome simply by using a thiol-enhancing yeast strain. In reality, yeast can only release thiols if the appropriate precursor compounds are present in the hops or wort, so hop selection, dosing rate, and timing remain critical.

Another misconception is that only ale strains are strong biotransformers and therefore the only suitable yeasts for hop-forward beers. In recent work conducted in my lab in collaboration with Lallemand Brewing, we observed that a lager strain (LalBrew Diamond™) released more thiols than several ale strains. This aligns with the experience of some brewers who produce highly aromatic hop-forward beers using lager yeast.

And yet another misconception is that biotransformation will always dramatically increase hop aroma intensity. In practice, the outcome depends on the interaction between hop variety, hop timing, yeast strain, and fermentation conditions. Brewers often focus on one factor in isolation when the outcome actually depends on the interaction of hop variety, hop timing, yeast strain, and fermentation conditions.

Brewers can avoid these pitfalls by thinking more holistically about recipe design and fermentation management. Understanding which hop varieties contain significant precursor pools and aligning those with yeast strains capable of releasing them will lead to much more predictable results.

What practical strategies would you recommend for brewers aiming to boost thiol expression without significantly increasing hop usage or production costs?

Brewers should think of hop variety, timing, and fermentation conditions as separate tools in the toolbox. From a hop selection perspective, it is important to consider both the amount of free thiols present in the hops and the pool of bound precursors that can be released during fermentation. All hops contain both forms, but varieties such as Citra®, Mosaic®, Strata®, and Nelson Sauvin™ are known for relatively high levels of free thiols and can therefore contribute strong thiol character directly.

Other varieties, such as Cascade and Centennial, may have lower levels of free thiols but contain substantial precursor pools that yeast can convert during fermentation. Because of this, timing of hop additions and fermentation conditions matter. Hop varieties rich in free thiols lend themselves to later dry-hopping approaches and do not necessarily require yeast interaction. In contrast, varieties with larger precursor pools may benefit from earlier additions, where extended contact with fermenting yeast increases the potential for thiol release.

Whirlpool hopping and/or early fermentation dry-hopping can increase interactions between yeast and hop compounds.

Fermentation conditions also play a role. Warmer fermentation temperatures and lower starting wort free amino nitrogen (FAN) have been associated with greater release of bound thiols.

Of course, yeast strain selection is also important, as strains vary in their ability to release thiols from precursors just as they vary in ester and higher alcohol production. As mentioned earlier, excluding lager strains from hop-forward beers may unnecessarily limit opportunities for thiol expression. Optimizing these various factors can help brewers enhance thiol expression without necessarily increasing hop rates.

What do you see as the next advancement or horizon for hops and hoppy beverage styles?

There is considerable excitement in the industry around advanced aroma extract products. In addition to traditional supercritical CO₂ extracts and their aroma fractions, new products are emerging where extraction conditions are tuned to selectively capture specific aroma compounds. Some of these extracts contain more hydrophilic components that may include thiols or terpene alcohol precursors.

Brewers may want to think about using these products not only as post-fermentation aroma additions, but also earlier in the brewing process where yeast can interact with them. Much like with traditional hopping, yeast activity during fermentation can modify these compounds and potentially shape the final aroma profile. This approach is already gaining traction in the non-alcoholic and low-alcohol beer space, where brewers are looking for ways to build hop character with fewer traditional hopping steps.

Another important frontier is improving our understanding of how different hop aroma compounds work together to shape the overall sensory profile of beer. While thiols have received a great deal of attention recently, the future likely lies in understanding the broader interactions among thiols, terpenes, esters, and other fermentation-derived compounds.

Advances in hop breeding and processing will likely follow this direction as well, focusing not only on alpha acids and total oil content but also on specific aroma compounds and precursor pools and how they behave during brewing.

Ultimately, the goal is to give brewers more precise tools, whether through hop varieties, hop products, or yeast strains, to build layered and expressive hop character rather than chasing any single compound class.

About the authors