Although I spend an inordinate amount of time chasing down and thinking about new beer, in my day job, I’m employed as a microbiologist. So when I catch wind of something in my professional life that impacts my hobby, I tend to take notice. To that end, a recent publication in the Proceedings of the National Academy of the United States (PNAS, for the uninitiated) caught my attention. From the laboratory of Jose Sampaio at the Microbiological Resource Center in Caparica, Portugal, the study solves a long-standing mystery in the brewing world, namely the origin of the cold-tolerant portion of the lager yeast genome.
One of the first things you learn in the pursuit of good beer, is the difference between lagers and ales – to recap, ales are fermented at warm temperature and utilize a strain of yeast called Saccharomyces cerevisiae. This yeast is the same as that used in bread and wine making (although individual strains have important differences, they are still the same species). In contrast, lagers are fermented at cooler temperatures for a longer time, lending the finished product fewer esters and a generally “cleaner” product. The strain of yeast used for lagers is called, S. pastorianus (S. carlsbergensis in some older sources) long standing question has been, “What makes these strains of yeast different?”.
Several groups have taken a crack at this problem, and a few years ago, the genome of the lager strain Weihenstephan 34/70 was sequenced, revealing that the strain was an allotetraploid – basically a hybrid, consisting of some genetic material from the ale-type S. cerevisiae and some genetic material from another yeast type (S. bayanus). This genome analysis revealed that the S. bayanus type DNA was important for several of the known characteristics of lager brewing, particularly sugar uptake efficiency, high sulfite production (that sulfurous flavour that some young lagers have is important for flavour stability), and production of sugar polymers that are important for yeast flocculation.
Even with this knowledge, it was unclear where the S. bayanus type DNA in the lager yeast genome came from… extensive surveys had never found a yeast that was identical to that found in the S. pastorianus. The Portuguese group seems to have solved this problem, identifying a yeast strain that is 99.5% identical to the S. bayanus portion of the domesticated lager yeast genome – a strain they term S. eubayanus. Intriguingly, although the well-established origin of lager brewing dates to 15th century Bavaria, the yeast found by the Portuguese group was found on beech trees in Patagonia.
Huh? Patagonia? This result was a little surprising to both the researchers and to others who follow this sort of thing, but it may make sense from an ecological point of view. Trees serve as hosts to a number of bacterial pathogens, some of which produce galls on the tree – these galls divert sugars from the plant to the pathogen, thereby giving the pathogen an energy source.
Yeast of the Saccharomyces genus are well-adapted to growing on concentrated sugar sources, and so these galls are often also colonized by yeast that make use of the abundant sugars present. Hardwood trees at middle/northern latitudes are often colonized by yeast that are tolerant of both cold (cryotolerant) and that prefer warmer temperatures (thermotolerant), due to the seasonal nature of the climate. In contrast, the beech trees on the Patagonia plateau experience more or less constant low temperatures year round (average temperature 6-8°C) and so we might expect selection for more cryotolerant species in this environment.
The authors propose that sometime during the 15th century, yeast from these Patagonian strains somehow made their way to a brewery and formed a hybrid with the ale-type S. cerevisiae then in use. These hybrids were then under positive selection within the brewery environment and became enriched over time. The main problem with this model as I see it is that there was no trans-Atlantic trade between Patagonia and Europe until at least the early 16th century with most settlement coming much later. This discrepancy leaves a sizable 150-200 year gap in the record between the recorded beginnings of lager production in Bavaria and any sizable trade between Europe and Patagonia. Although it’s clear that the authors have found the true source of the non-cerevisiae portion of modern brewing strains (genetics doesn’t lie), I suspect that these strains evolved after the original low-temperature brewing strains that were in use from ~1400-1600. It is possible that other hybrids of cold-tolerant European yeast may have formed early but have since been lost due to competition from better adapted Patagonian hybrids. Further molecular archaeology may shed further light on these possibilities (anybody out there have any samples of lager from the 15th century?). No matter how you slice it – the discovery of the yeast responsible for lager production is something worthy of raising a toast. Prost!