Biotransformation: Keep your hops alive with yeast

The first lesson I learnt when I started brewing was that a brewer must be intentional in their choice of hops and yeasts. Apart from fermenting malt sugars to alcohol during brewing, the real craft of brewing comes to play when hop compounds are transformed in a biochemical process to add radically new flavours and aromas to beer in what is known as biotransformation.

What is biotransformation and what is all the fuss about? Biotransformation is the process by which yeasts change less desirable hop compounds into more desirable bold flavours and aromas. Essentially, inert hop flavours are brought to life by yeast.

Brewers are right in being intrigued about biotransformation for several reasons. Firstly, it offers them an opportunity to be creative with their craft beers as they develop recipes to add new flavours and aromas.

Also, with biotransformation, brewers are able to save money on hops since the right choice of yeast will amplify the perception of some hops flavour compounds, thus reducing the amount of hops required for brewing.

In this blog post, I will demystify the magic behind biotransformation of two important classes of hop compounds - terpenoids and thiols.

Fancy hop compounds

To appreciate how biotransformation works, we first have to be familiar with the chemistry of hops. Hops, as we know it, are rich in essential oils. Indeed, of all plants, hops is the species with the most complex known essential oils. No wonder it has been used for brewing over several centuries (and millennia even) to confer desirable flavours and aromas to beer as well as add some bitterness! Hundreds of compounds have been identified in hop essential oils but the composition and amount of these compounds depend on the hop variety.

Terpenoids

The most popular class of compounds present in hop oils are the terpenoids. Terpenoids possess strong sensory qualities. An example of a common terpenoid in hops is geraniol which can be transformed into floral aroma compounds citronellol, nerol and linalool. Alpha-humulene, another terpenoid compound, gets converted into caryophyllene which add a note of black pepper to beer.

As I mentioned earlier, the composition and amount of terpenoids present in hop oils vary depending on the hop variety. So next the obvious question is which hops varieties are rich in terpenes? Scientists have only scratched the surface of hop chemistry but Cascade has been reported to have a considerable amount of geraniol. Other terpenoid-rich hops include Chinook, Mosaic, Centennial and Bravo.

Still on the topic, there has been a growing sensation regarding brewing with New Zealand hops, making them the most sought after in recent times. This is partly because they have an interesting flavour profile since these distinctly flavoured hops are bred to generate hop varieties with a more complex composition of flavour compounds. They are great for IPAs as they give citrusy and floral notes of grapefruit and lemon aromas. Research published this year (2021) by a group of scientists at the University of Otago in New Zealand showed that the most popular New Zealand hops including Rakau, Riwaka, Wai-iti and Waimea are rich in terpenoid compounds.

Thiols

Moving on from terpenoids, another interesting group of compounds found in hops that gets transformed into unique fruity flavours and aromas are thiols and other sulphur compounds. Thiols constitute less than 1% of hop oils but are still able to add a tropical fruity tang to beers. While free thiols can be found in some hops, majority of thiols are inactive because they are bound to protein-type molecules so the flavour perception of the compound is essentially “locked”. Therefore, biotransformation of these bound thiols to free (active) thiols is an important process which makes the brewer utilize to the maximum level every thiol compound per gram of hops thus saving money spent on hops.

Thiols are referred to by a simpler version of a more complex chemical name which I wouldn’t bore you with. The common ones include in 4MSP which add a note of black currant to beers and can particularly be found in hops such as Simcoe, Summit and Cascade. Hallertau Blanc, Tomahawk and the New Zealand hop variety Nelson Sauvin are all rich in 3MH which contributes an exotic fruit, rhubarb-like and grapefruit flavour.

The yeast cell factory

The main force behind biotransformation is yeast. Think of a yeast cell as a factory where raw materials in the form of hop compounds are converted into desirable flavour and aroma compounds. In these yeast cell factories are tiny factory workers called enzymes. It is these enzymes that do the conversion of the hop compounds. For example, geraniol is converted to citronellol by the yeast enzyme NADPH dehydrogenase. Citronellol has a sweet citrusy aroma. Therefore, hops such as Riwaka that are rich in terpenoids will release geraniol into the wort which can then be biotransformed by a yeast enzyme to produce the citrusy citronellol flavour.

Clearly, the choice of yeast is an important consideration in making a beer recipe that requires biotransformation of hop compounds. The differences observed in yeasts can be traced to the genetics of the yeast strain that makes some yeasts able to make active forms of the enzymes required to transform the hop compounds.

Our own research here at Escarpment Labs has shown that some wild phenolic yeasts are able to effectively biotransform terpenoid compounds. However, the phenolic character presented by these wild yeasts is not ideal for making clean IPAs since the phenolic off-flavour masks the citrusy aroma of the biotransformed hop compound. In our study, we also identified non-phenolic yeasts such as Cerberus, Vermont Ale and Hornindal Kveik which could equally do the job of biotransforming terpenoids.

In the case of thiols, majority of the compound remain bound and inactive. During yeast biotransformation of thiols, the enzyme called cysteine beta-lyase releases the inactive bound thiols present in hop oils to generate free thiols that add fruity notes to beer. Information on yeasts that can perform this type of conversion is limited. At Escarpment Labs, however, we are actively doing some research to develop beer yeasts that are able to release bound thiols in hops to enhance the flavour of IPAs. In the meantime, some wine yeasts can do this biotransformation and can be co-pitched with beer yeasts as long as they do not inhibit growth of the beer yeast.

 

Biotransformation of terpenoids and bound thiols.
The yeast enzyme NADPH dehydrogenase transforms hop terpenoids such as
geraniol into the citrusy citronellol flavour compound. Another yeast enzyme (cysteine beta-lyase)
frees bound thiols present in hops resulting in an enhanced thiol (mostly tropical fruity) flavour.

 

Escarpment Yeasts for Biotransformation

Through our research partnerships with universities and research institutes, we've been able to measure some of the biotransformation products and enzyme activities in our yeast strains! While we can't yet share specific numbers as the studies are unpublished, we have summarized yeasts from our lineup that excel in either terpene or thiol biotransformation below: 

Product

Phenolic?

Terpenes

Thiols

Foggy London Ale

No

Low

High

Ebbegarden Kveik Blend

No

Low

High

Cerberus

No

High

Medium

Hornindal Kveik Blend

No

High

Medium

Vermont Ale

No

High

Low

Uberweizen

Yes

Very High

Medium

Wild Thing

Yes

Very High

Medium

Classic Wit

Yes

Medium

High

Ardennes

Yes

Medium

High

Ontario Farmhouse Ale

Yes

High

High

 
Hop o’clock

Last but not least, the timing of the hopping regime can make a great impact on the release of hop compounds that are biotransformed by the yeast. The difference in flavour based on the timing of hop addition is literally like night and day. Typically, to maximize the amount of terpenoids extracted into wort, hops is added during the late kettle boil or early during dry hopping. On the other hand, dry hopping or addition of hops during late fermentation is preferred to extract thiols from hops and make them available for biotransformation.

Taken together, the choice of hops, yeast and the timing of hop addition should be carefully considered in order to obtain the desired flavour and aroma when crafting one’s ideal beer. Also, craft brewers are getting more creative with the addition of flavourings and spices such as coriander, ginger and citrus peel. Even though this blog post mostly focused on biotransformation of hop compounds, some of these spices are also rich in terpenoids that could also be converted into interesting flavour compounds.

Resources:

Foods 2021, 10(2), 414; https://doi.org/10.3390/foods10020414

FEMS Microbiology Reviews 2019, 43,193-222; https://doi.org/10.1093/femsre/fuy041