Free the thiols

Mark Dredge shines a light on a key hop compound

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Sometimes the aroma of an IPA just pops out your glass with a freshness that’s somehow more intense, more impactful, more exciting, with top notes of gooseberry, grapefruit, guava or passion fruit, coming above a broader palate of citrus and stone fruit. Those tropical aromas are known as thiols, and they’re becoming some of the most sought-after flavours in beer. 

When we look at a hop, most of its flavour comes from oils in the flower, and those oils make up just 0.5% to 4% of the hop’s overall composition. Of that already-small percentage, thiols, which are sulphur-containing compounds, account for less than one per cent of it, but they can be one-thousand times more flavour-active than the hop oils, detectable at parts-per-trillion: even at very low concentrations, thiols can contribute hugely to the flavour of a beer.

The main positive thiols (there are also bad ones, as we’ll see) in beer include: 4MMP (smells like blackcurrant, passion fruit, and technically it’s called 4-mercapto-4-methylpentan-2-one but we’ll stick to the abbreviations), 3MH/3SH (grapefruit, passion fruit), 3S4MP (grapefruit, rhubarb), 3MHA/3SHA (passion fruit, guava, citrus), 3MO (stone fruit) and 3M4MP (rhubarb, grapefruit). Where other hop oils like myrcene, humulene, linalool and geraniol give herbal, resinous, spicy, woody, floral and light fruity aromas, it’s the extra pungent pop of tangy tropical fruit which makes the thiols stand out – just think of the distinctive aroma of New Zealand Sauvignon Blanc wine as that’s primarily from these same thiols. 

There are two particularly interesting things to know about thiols: they aren’t just in hops, and some barley and malt contains significant volumes of thiols, mainly the grapefruity 3MH (as it’s a specific chemical compound it smells the same whether it’s derived from malt or hop, and research is still ongoing to find the barley with the highest thiol potential); and sometimes thiols can be abundant in a beer but we simply can’t smell them.

Thiols can exist in either a free form, which are volatile and you can easily smell them in beer, or they can be in a bound form, which are non-flavour-active, so we can’t smell them at all. Hops like Citra, Mosaic, Simcoe and Nelson Sauvin are all naturally high in free thiols and whether the hops are added in the kettle, whirlpool or as a dry hop, you can smell their distinct tropical aromas, and that easy access to their strong fruitiness is what makes those varieties some of the most popular (and expensive) hops.


Hops like citra, mosaic, simcoe and nelson sauvin are all naturally high in free thiols

But most hops (and malts) contain many more bound thiols than free ones, sometimes a thousand-times more: there’s a lot of highly desirable aroma compounds stuck in the beer, unable to get out, and we’ve only recently realised it. Thankfully there are ways to free those thiols.

FREE THE THIOLS

The chemistry is complex so the simplified explanation is that there are flavourless precursors to thiols naturally found in malt and hops. During the mashing process, there’s some enzyme activity which converts those precursors into accessible thiols. Having discovered that this initial enzymatic conversion happens in the mash tun, more brewers are adding hops into the mash, or ‘mash hopping,’ saturating their wort with thiols. Or potential thiols, because they aren’t fully free at that stage and so don’t contribute aromas, meaning you won’t smell them in unfermented wort. To become flavour-active these thiols need to interact with another enzyme, beta-lyase, and that happens during fermentation. 

The thiol is bound to an amino-acid which beta-lyase metabolises, separating the sulphury thiol and allowing it to be released into its aromatic state, making it smellable alongside the other flavour-active hop compounds. Not many yeasts produce significant amounts of beta-lyase, but research has led yeast laboratories to develop new strains with high beta-lyase activity for this purpose. It’s also possible to add a powdered form of beta-lyase, and several have been developed for the wine industry, specifically for New Zealand Sauvignon Blanc, with brewers using these wine enzymes to exaggerate thiols in beer with great results.

Hops with the most bound thiol potential include Cascade, Saaz, Perle, Hallertau Mittelfrüh and – anecdotally as research hasn’t confirmed this – numerous English hop varieties. These aren’t hops traditionally seen as being intensely aromatic, certainly not with tropical aromas, but if used in the right way they can give those qualities to beer, revealing entirely new flavours than what we’re familiar with, and as an added benefit to brewers and drinkers, those varieties are typically cheaper and more available than Citra and Nelson Sauvin, and often fewer hops are needed to be added because of their high flavour intensity.


The industry is getting a better understanding of how to maximise the desirable aromas

Research so far suggests that the best results for thiol-rich beers have used hops like Cascade and Saaz in the mash, a specific beta-lyase yeast (or exogenous enzyme) and then also added aromatic varieties as a dry hop; there’s a great complementation between the tropical top notes and the fruity-floral hop oils which is able to increase the overall hop impact in a beer.

But there’s a warning here, because it’s not all gooseberries and guavas and there are more negative sulphur-containing thiols than positive ones. The smell of ‘skunked’ or light-struck beer is a thiol, as is an off-flavour called mercaptan, which smells like an old bin. If you’ve ever smelt garlic, onion or vegetal aromas, ‘catty’ or cat pee aromas, or a general ‘sweaty’ quality in beer, then they are all thiols. It’s not easy for a brewer to predict what they’ll get, and sometimes the flavour threshold of the thiol means that it tips over from grapefruit to garlic – anyone who’s ever smelt or tasted fruits like durian, jackfruit or papaya will have a heady idea of the sulphurous overlap between delicious and disgusting. As more investigations are done with hop and malt varieties, the industry is getting a better understanding of how to maximise the desirable aromas. 

All of this on-going research into thiols is giving results which challenge what we thought we knew about fruity aromas in beer, and goes against several pieces of commonly held knowledge about beer: we can get tropical aromas from malt and from traditional European hop varieties; we can add hops in the mash tun and they’ll smell like passion fruit and grapefruit; we can add fewer hops and get more aroma from them; and that these new kinds of thiol-focused IPAs ultimately succeed (or fail) in the fermenter and the interaction of yeast with volatile chemical compounds. 

Next time you have a beer which pops with top notes of tangy grapefruit, pungent passion fruit, tart rhubarb and funky guava, then that’s the thiols you’re tasting.

This article is a compilation of conversations with numerous people: Ben Palmer, researching a PhD on thiols at the VLB in Berlin; Philip Woodnutt, Technical Director WHC Lab, a fermentation and quality control company based in Ireland which released Hop Unlock in the summer of 2022, a yeast which specifically produces lots of beta-lyase; Andy Garbett, Marketing Manager at Brook House Hops, working with WHC Lab on the thiol potential in English hops; Patrick Jensen, Director of R&D, and Lindsay Koby, Field Research Agronomist, both at Yakima Chief Hops; Jamie Williams, Lead Brewer at Brick Brewery who made a Thiolised IPA using WHC Lab’s Hop Unlock; Brett Ellis, co-Founder of Wild Beer Co. who have brewed several beers focused on releasing thiols.

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