I was flipping through some old ACS “Molecule of the Week” posts today when I came across this post about 4-vinylguaicol (4VG), one of the molecules that yeast can produce to generate a clove-like flavor in beer. This is particularly common in various yeast strains, such as Torulaspora delbrueckii (although the ACS post uses outdated nomenclature – Saccharomyces delbrueckii). This flavor is particularly desirable in wheat beers and is detectable in beers made with classic hefe yeast strains such as WLP300/WY3068. While the role of 4VG in hefeweizen wasn’t exactly news to me – many people have discussed 4VG and the best practices surrounding it’s production – it got me thinking about the strains we typically use for hefeweizens and related beers.
I’ve always assumed that WLP300 (the Weihenstephan strain, also one of my favorite strains) was S. cerevisae – after all, S. cerevisae has the capability of converting ferulic acid into 4VG.¹ That said, I was unable to find any documentation indicating that this strain was actually S. cerevisae with the exception of this paper (though it appears to lack validation that WLP300 was indeed S. cerevisae). The authors did sequence 4 loci in all of the yeast strains studied, including WLP300, but it’s not unreasonable that they wouldn’t be able to determine the species based on only the limited sequence data from those four regions (which they were expecting to be highly conserved or highly variable between S. cerevisae strains).
On the other hand, Randy Mosher’s “Tasting Beer” claims that “Bavarian Weissbier, aka hefeweizen, uses a unique Torulaspora delbrueckii yeast that produces a clove aroma, along with banana and bubble-gum fruitiness.” Even though he doesn’t directly address this specific strain, it’s hard to imagine him not including WLP300/3068 under the umbrella of authentic Bavarian hefeweizen yeast.
I also found some individuals claiming that WLP300 is indeed T. delbrueckii, in the HBD archive, the Mr. Malty yeast database, and at Eureka Brewing (WY3068), but with no backing documentation.² In fact, many references to WLP300/WY3068 being T. delbrueckii all write “Weihenstephan 68 (S. delbrueckii single strain)” leading me to think that most individuals are finding this information in a common location – perhaps the Mr. Malty yeast database.
Either way, with the information available on the internet, it’s a dead end, with no really convincing or conclusive information. Both White Labs and Wyeast’s descriptions of the respective strains are curiously vague when describing the strains, not mentioning the what the species is. I would imagine if these strains were not S. cerevisae, that they’d make it a point to highlight that fact.³ As indicated by the Eureka Brewing post linked above, there’s no available rRNA data for this strain, making it a bit more difficult to determine the species without access to specialized equipment or services. I could get the rRNA at work but there would be no way to fund the sequencing without digging into my own pockets – not exactly realistic for this grad student! There are other potentially cheaper routes of species ID, but I’ll get into that in another post.
There is a teeny light at the end of the tunnel. I found a study comparing the osmotolerance of S. cerevisae and T. delbrueckii – it turns out that T. delbrueckii is quite a bit more osmotolerant than S. cerevisae – that is, it can survive in saltier, sugarier, etc conditions than it’s fungal relative. This can be tested (in an admittedly quick-and-dirty way) using the solid media osmotolerance testing methodology described by Hernandez-Lopez et. al, and while it couldn’t conclusively demonstrate which species WLP300 is, it might provide some useful preliminary information. So that’s where the next step lies – growing some WLP300 up and comparing it’s osmotolerance to a reference strain. Stay tuned!
Have any information regarding these strain’s species? I’d love to hear it!
¹As another caveat, not all T. delbrueckii strains robustly produce 4VG (they can be POF-, meaning that they lack the enzyme to produce 4VG). In other words, both S. cerevisae and T. delbrueckii can generate 4VG, but either species may also lack the capacity to do so. Therefore, ability or lack thereof to produce 4VG probably doesn’t help us much in determining what species a yeast strain is.
²I didn’t know about the Eureka Brewing yeast science blog until I googled around a bit on this subject. If you haven’t read the linked article or others on the blog, I’d highly recommend it! Very interesting information.
³And if this strain is indeed not S. cerevisae, is it really an ale yeast (as labeled by White Labs – “Hefeweizen Ale Yeast”)? It seems that most definitions of “ale yeast” specify that the strain is S. cerevisae. Sure, this is being a little pedantic, but I still think it’s an important question to ask!
Edit: I did a bit of reading and it actually might not be cost prohibitive to ID this strain via sequencing, so I may try that as well.