This week I take a look at thiol enhancing yeast and its application in brewing fruity IPAs.
Thoils and the Modern IPA
With IPAs making up over half of US Craft beer production, there has been a huge surge in interest in understanding hop aromatic compounds, including both hop oils and thiols. Thiols are a tiny fraction of hop oils but play an oversized role in generating aromatics in a finished beer, particularly the fruity flavors desired in many IPA styles like Hazy/New England IPAs. We’ve discussed thiols in a number of podcasts including Hop Thiols with Stan Hieronymus, and Thiols and Fruity IPAs with Morebeer.
Chemically a thiol is a sulfur version of alcohol. They make up a tiny fraction of hop aromatics, but can be detected by humans at levels as low as a few nano-grams per liter! There are a variety of hop thiols, but four main ones drive the majority of thiol aromas which can vary from citrus, white grape, gooseberry, rhubarb, black currant, chive, passion fruit, guava and even sweaty or cat-pee aromas.
Thiols compounds can also be free or bound. Free thiols are the aromatic versions, while bound thiols are odorless precursors that are bound to amino acids. Various hop varieties have different levels of free or bound thiols but the bound ones typically remain bound unless the brewer takes some steps to free them. Some pale malts also contain bound thiols. Some thiols can be freed during fermentation.
We use the term biotransformation to describe chemical processes that take place during fermentation that change one compounds to another. One well known biotransformation takes place when specific hop aromatic compounds like geraniol, linalool and citronellol are transformed into their aromatic form during fermentation. This is why many modern IPAs use seleted hops like Citra, Columbus and Cascade which are high in theses compounds.
Thiols can go through a slightly different biotransformation where a portion of the bound thiols are released to become aromatic free thiols. While some naturally occurring yeast strains can perform this transformation, yeast specifically engineered or selected (so called thiolized yeast) to enhance thiols perform best.
Interestingly, mash hopping in many cases can enhance this effect. So by selecting certain hop varieties, high in bound thiols, and using them in the mash along with thiolized yeast we can achieve a higher percentage of freed thiols. I wrote a recent article for BYO magazine that covers the chemistry and hop selection in more detail.
Thiol Enhancing Yeast
With the IPA revolution and push to get more aroma out of our hops, yeast labs have responded by creating so called “thiolized” or thiol enhancing yeast. The work to create these new strains is heavily tied to work started some 20 years ago to understand the yeast genome. Specifically scientists have identified an IRC7 gene in many yeast strains that is largely inactive. That IRC7 gene, once activated, produces beta-lyase which is the key enzyme that frees thiols during fermentation. We discussed this work in some detail in a recent podcast with Dr Chris White and Nick Impellitteri here as well as an earlier podcast with Adam Mills here.
There are two major approaches to activating the IRC7 gene. The first is by natural selection as white labs did with their Cosmic Punch (WLP077 strain). This strain is carefully selected to produce more free thiols. The second approach is to genetically modify the yeast strain to create a new strain with the IRC7 gene activated. This is the approach Omega took with their pioneering Cosmic Punch OLY-402 as well as later Start Party OLY-404, West Coast Ale OLY-004, Lunar Crush OLY-403, and Mexican Lager OLY-113. Other yeast labs have followed with Berkley’s tropics line, Escarpment labs Thiol Libre. Nick Impellitteri mentioned in the podcast above that The Yeast Bay is also working on modified yeast strains.
For those of you looking to learn more I encourage you to listen to the podcasts linked above as well as read my detailed BYO article here on mash hopping.
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