Designing a beer for biotransformation

Lately it seems like brewers are trying all sorts of wild techniques to unlock new flavours in their beers. In this article I will break down thiol biotransformation in beer and how you can combine your ingredient selection, yeast, and brewery methods to unlock new flavour combinations in your beers. 

Previously, my colleague Eugene Fletcher published an excellent overview of the yeast and hop interactions we call biotransformation. I encourage you to check out that article for more on the background science of biotransformation, which is also posted on the Escarpment Labs blog. 

While biotransformation has been a popular term in the brewing industry, we found that practical resources are lacking for brewers to make use of yeast-hop interactions to produce more flavourful beers. The aim of this article is to use the science to provide some practical advice for enhancing biotransformation in beers. 

There are two major types of hop biotransformation: thiols and terpenes. Thiols are sulfur-containing molecules that are extremely potent, often with tropical fruit aromas. They are often bound up in the hop or malt and flavour-neutral, but fermentation can release them. Terpenes (monoterpene alcohols and esters) come from hops and can be transformed from one form to another by fermentation. In this article I am going to focus on thiol release due to its recent popularity. 

Enhancing Thiols 

Thiols and their release is all the rage these days, with new yeasts coming onto the market that have been modified or bred to enhance thiol release. This opens up a new world of opportunities for flavour development, and even turns on their heads some of the approaches being used to make highly aromatic beers. 

These yeasts have been developed to have enhanced activity from the IRC7 enzyme, which is a beta lyase that helps the yeast release aromatic thiols from flavourless precursors. 

Let’s clarify why thiols are important. Basically, most of us really like these aromatics from hops. Thiol compounds can smell like Sauvignon Blanc grapes, gooseberry, guava, grapefruit, and passionfruit! It’s no surprise that the hops with the most free, aromatic thiols are also the most expensive: Citra, Sabro, Mosaic, Nelson Sauvin. 

But there’s huge opportunity to release bound thiols from lower-cost ingredients, in unexpected places. Some common and lower-cost hops such as Cascade, Saaz, and Perle contain large amounts of bound thiols that can be released by yeasts with strong thiol release capabilities. 

We even see brewers starting to experiment with mash hopping to enhance thiols. This sounds like a crazy idea, but there’s some solid science behind it. The bound thiols in hops take the form of either cysteine-bound or glutathione-bound. In the mash, barley enzymes can facilitate the conversion of glutathione-bound thiols into cysteine-bound thiols. Since yeast releases cysteine-bound thiols, this can result in stronger thiol release. It’s also important to mention that malt also contains thiol precursors, so with the right yeast, you can get some tropical, grapefruit aromatics with malt alone! 

The best yeast strains for thiol biotransformation are strains that are both capable of beer fermentation and have high IRC7 enzyme activity. Unfortunately, this combination is quite rare among the yeast world. Some traditional yeasts like Foggy London Ale and Ebbegarden Kveik are good at thiol biotransformation. Luckily there is room for improvement among traditional yeasts. There has been a lot of attention toward developing new yeasts that are better at thiol biotransformation, some of which are starting to hit the market. 

Methods used to develop new yeasts for enhanced thiol biotransformation includes genetically engineering existing strains to have higher IRC7 activity, or breeding strains with naturally high IRC7 activity to achieve “hybrid vigor”. Recently, our team published a technique we used to make beer yeasts easier to breed with CRISPR-Cas9 gene editing, combined with classic yeast breeding to create yeast strains with enhanced IRC7 activity. 

Enhancing Terpenes 

There are many different terpenes that can come from hops and impact the flavour of your beer. In particular, the aromas that we perceive as “dank”, “floral” and “citrusy” are often the result of terpenes. 

Yeast can convert some hop terpenes into others, which is how yeast biotransformation impacts terpene character. For example, yeast can convert rose-like geraniol to lemongrass-like beta-citronellol. 

This means a very “dank” hop like Strata can become much more citrusy thanks to terpene biotransformation. 


Designing a Recipe for Biotransformation 

Based on available science, consider any or all of the following to enhance hop biotransformation. 

Yeast selection for thiols: Select yeasts that have been bred or modified for enhanced thiol release, or select traditional yeasts such as Foggy London Ale or Ebbegarden Kveik. 

Hop selection for thiols: look to Cascade, Saaz, Calypso, and Perle for high concentrations of bound thiols. Citra, Sabro, Mosaic, Simcoe, and Nelson Sauvin are all examples of hops with high levels of free thiols. 

Releasing hop thiols: Mash hopping is a promising technique for creating more precursor for the yeast to release. In general, getting the hops in earlier in the process (mash hopping, whirlpool, mid to late ferment dry hopping) will help with thiol release. 

Yeast selection for terpenes: Select yeasts with strong terpene biotransformation activity. This includes traditional yeasts like Vermont and Cerberus, as well as strains that have been bred with those strains as parents (such as Hydra). 

Hop selection for terpenes: Look to Bravo, Centennial, Cascade, Chinook, Mosaic, and Strata for above-average amounts of terpenes. 

Releasing hop terpenes: extraction in the hot side (late kettle, whirlpool) can help with getting terpenes into solution, although some biotransformation of terpenes during dry hopping is also possible. 

For both: consider using local ingredients! New yeasts can help you create totally new flavour combinations and can help unlock flavour and aroma in local hops and malt. We have seen promising results using local Ontario grown Cascade and Centennial hops when paired with the right yeast and brewing process. 



This article was also published in Brewers Journal Canada

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