What is biotransformation?
Our definition of biotransformation is any chemical reaction facilitated by microbes (yeast, bacteria) that changes the flavour profile of existing ingredients in your beer. This includes not only hops but also malt and adjuncts!
In general, biotransformation can be divided into specific chemical reactions: - Terpene conversion (e.g. linalool to citronellol, creating a more citrusy character) - Thiol release (e.g. cysteine bound 3MH to free 3MH, creating a more tropical character) - Release of glycosidically bound aroma compounds
Our understanding and definition of biotransformation is constantly evolving, which makes this an exciting area of fermentation science!
Current open questions include:
- Does timing of dry-hopping matter? - How do we maximize terpene bioconversion in beer yeasts?
There are a lot of opinions out there as to what works best to achieve biotransformation! We want to make this easy for you, so here are our general suggestions:
- Build-in terpenes early on in your recipe, e.g. in the flameout, whirlpool, or early dry hop addition. For terpene biotransformation, we find that Hydra, Vermont, Laerdal, and Cerberus work the best. - Build-in releasable thiols early in your recipe, e.g. in mash hopping or whirlpool. - Dry hop with your "big money" hops later on in fermentation, either just before final gravity or after final gravity. Dry hopping with Citra, Mosaic, Galaxy (and so on) too early can result in fermentation flashing off the delicate, volatile tropical aromatics that you paid for. - Dry hopping rate depends a lot on yeast strain used as well as the quality and intensity of the hops used. Double dry hopping (e.g. once before terminal gravity and once after) can help get a strong hop aroma and flavour.
We have some suggested combinations based on the experience of our brewers and brewery partners:
- Hydra + Bravo/Strata Whirlpool + Simcoe/Citra Dry Hop (balanced fruitiness, lots of citrus, hint of dank) - Thiol Libre + Cascade/Calypso Mash Hop + Citra/Mosaic Dry Hop (citrus and white wine) - Laerdal + Sabro or any fruity NZ hop (coconut, lime)
How do yeast strains compare in biotransformation?
In a 2019 study, we measured beta citronellol content in beers fermented by a number of our strains. This is a good measure of terpene biotransformation. This is because there is not much citronellol present in hops, this aroma compound is usually the result of conversion from other terpenes by yeast.
We found that terpene biotransformation is low in Lager yeasts, and tends to be high in saison, wild, and Belgian strains as well as some German weizens. In this study, the highest "clean" (IPA friendly) strains were Cerberus, Hornindal Kveik, and Vermont Ale. Since Hydra is a hybrid of Cerberus and Vermont, it is also a great terpene biotransformation yeast.
By contrast, measuring thiols is quite challenging and expensive. We were able to measure thiols in the 2021 study that resulted in Thiol Libre.
In general, most beer yeasts are poor thiol releasers and poor glycoside releasers. Of the "natural" beer yeasts, we find that Ebbegarden Kveik as well as some Saison yeasts are OK at thiol release. This is why new yeasts like Thiol Libre have been created.
We can't ignore other brewing organisms when considering biotransformation! As an example, Brettanomyces might be the greatest natural biotransformation powerhouse of them all given how much it can change the flavour profile of a beer. We also suspect more research will emerge on biotransformation by lactic acid bacteria in the future since we know they can produce quite unique flavour profiles.
More Biotransformation Resources
Biotransformation: Keep Your Hops Alive on our Blog
Designing a Beer for Biotransformation on our Blog
Check out our Guide to Yeast Flavour and Biotransformation.