Fermentation is one of the oldest metabolic pathways. It doesn’t require oxygen because it existed even before Earth’s atmosphere had any.
The first organisms to ferment were bacteria, and roughly one-fourth of living bacteria today are still able to ferment. Gaseous oxygen arrived only after the evolution of photosynthesis—using sunlight to make sugar from carbon dioxide and water, releasing oxygen as a by-product.
That sounds like great news, to those of us who need O2 to survive, but its arrival was a disaster for most organisms living at the time. Oxygen killed most of the bacterial species in the ocean and forced most of the rest into anoxic microhabitats—places where oxygen isn’t present.
Relatively soon after that, geologically speaking, some organisms learned how to use oxygen. These organisms developed aerobic respiration—breaking down organic molecules in the presence of oxygen to gain energy. When that process breaks down glucose, it releases many more ATP molecules—that’s adenosine triphosphate, the major energy provider in a cell—than fermentation does. And the only by-products are CO2, water, and heat.
Then something amazing happened: A free-living bacterium that could perform aerobic respiration became enveloped by a bacterium in the kingdom Archaea. Archaea are bacteria that differ in several ways from “ordinary” bacteria. The archaebacteria couldn’t aerobically respire on their own—but with the proteobacteria living inside it, it was protected from oxygen. It also gained the ability to benefit from the large amount of ATP the bacteria produced.
Today, we call the remnants of that proteobacteria mitochondria—often called “the powerhouse of the cell.” Mitochondria have their own genome, which is similar in sequence to modern free-living proteobacteria. Cells that have mitochondria are called eukaryotes, and their DNA is similar in sequence to free-living Archaea.
Okay, but What about Beer?
Somewhere down the line, one lineage of eukaryotes evolved to become fungi, and one of those fungal lineages evolved to become Saccharomyces cerevisiae—which we know as brewer’s yeast, baker’s yeast, or ale yeast.
S. cerevisiae retains the ability of its archaebacterial ancestor to ferment, but—thanks to the mitochondria’s proteobacterial ancestors—it can also aerobically respire. So, two organisms started living symbiotically, then gradually morphed into a single organism while still retaining vestiges of their free-living past. And that’s what eventually led to ale yeast.
Now, fast forward to Homo sapiens. Humans learned to use fermentation in many ways before they had any inkling of what was really going on. They used fermentation to make bread, various types of yogurt, cheese, vinegar, kimchi, fermented fish sauce, and more. Of course, humans also fermented beer, wine, cider, mead, and other alcoholic beverages. Most of these fermentations used S. cerevisiae, though other yeast or bacteria often got involved.
Early humans couldn’t have understood that these fermentations were being conducted by microscopic living organisms. One early hint came from the early microscopist, Anton Von Leeuwenhoek, who saw small “animalcules” under his scope and correctly identified many of them as living organisms. (He assumed they were tiny animals.) Unfortunately, when he looked at yeast, he thought it was just a starchy globule.
Later, Louis Pasteur proved that alcoholic fermentation was the transformation of sugar into alcohol (and CO2), facilitated by living yeast cells. Humans were starting to get a grasp what was going on beyond the practical.
Okay, but What about Lager?
Let’s rewind to the 1400s—four centuries before Pasteur—when Bavarian brewers were fermenting beer at colder temperatures than was typical for ale.
Nobody was isolating yeast strains back then, and the yeasts doing the fermenting in the breweries were mixed slurries. Evidence suggests that in Bavaria, these slurries included S. cerevisiae and other organisms—including a yeast strain that could function at lower temperatures. The stage was set for true lager fermentations to arise.
In 1845, J.C. Jacobsen—the founder of Carlsberg—visited Munich and collected yeast from the Spaten brewery, carrying it back to Copenhagen via stagecoach. Later, in 1883, Emil Christian Hansen—a scientist in the Carlsberg Research Laboratory—isolated a pure strain of lager yeast from that culture. He originally designated it Unterhefe No. 1.
Today, we now know it as Saccharomyces pastorianus—lager yeast.
But what are lager yeast, and where did they come from? Lager yeast aren’t found in nature; they’re found only in breweries. Scientists have known since the 1980s that S. pastorianus is a hybrid—the result of two different types of yeast mating.
Also, there are two types of lager yeast:
- Group I, the Saaz group, are triploids, meaning they have three sets of chromosomes. That (and an examination of the chromosomes) suggests that they inherited two sets of chromosomes from one yeast strain and one set of chromosomes from another.
- Group II, the Frohberg group, are tetraploid, containing four sets of chromosomes. That may have been the result of a hybridization of two diploid species, or it may have been a more complex series of hybrids.
It’s fairly common in the world of plants for new species to emerge through hybridization, but it’s significantly rarer in animals. Fungi, however, are less well studied, and scientists are unsure how often it happens in that group.
Not surprisingly, when DNA sequencing became possible, scientists found many sequences within S. pastorianus that have a high degree of similarity to S. cerevisiae—and that was expected because we use both strains to make beer. However, the other sequences in S. pastorianus sequences were a mystery. They clearly fell into the Saccharomyces genus, but they didn’t belong to any known species.
The Search for Clues
In 2011, researchers caught a break when they identified a species with a match for the other S. pastorianus sequences.
It was a yeast living in Patagonia, a region in South America that covers parts of Argentina and Chile. Scientists named it S. eubayanus because it shared some sequence similar with S. bayanus. But wait: How did a yeast from South America hybridize with ale yeast from Europe?
The answer: It didn’t. Further explorations found that S. eubayanus also lived in the Northern Hemisphere, including in Ireland—a bit closer to Germany and a brewing country with which yeast slurries were likely exchanged on occasion. These northern S. eubayanus strains also showed a higher degree of sequence similarity to the non-cerevisiae sequences in lager yeast.
So, the major mystery was solved: Lager yeast is a hybrid of two Saccharomyces yeasts; one is cerevisiae, and the other is the cold-tolerant eubayanus.
But what about the details? How and when might that have happened?
Since piecing together the big picture, scientists have continued to look at the genetics of lager yeast, and beer historians have examined brewing records of the time. A story has emerged.
Some scientists hypothesize that the hybridization occurred in Munich’s Hofbräuhaus between 1602 and 1615, a period when the brewery was producing both lager and wheat beer. Thus, both ale yeast and S. eubayanus would have existed within their slurry.
Strangely, however, sequencing data show that the ale strain that hybridized with S. eubayanus was an outside ale strain—not one from the house ale culture at Hofbräuhaus. Instead, it appears to have originated from the Schwarzach wheat beer brewery in the city of Einbeck.
To add another wrinkle: Even more recent studies have found that the S. cerevisiae–like sequences in the Saaz and Frohberg types of lager yeast are different. The inference is that they came from two different strains of ale yeast.
While researchers continue to uncover new evidence, the data currently support the idea that two different strains of S. eubayanus hybridized with two different strains of S. cerevisiae, forming the two groups of lager yeast. Some evidence points toward the first hybridization occurring in an unknown Bavarian brewery, then occurring again in the Hofbräuhaus.
Interestingly, S. eubayanus also has hybridized with S. uvarum and S. cerevisiae to form S. bayanus. That would seem to indicate that this species is prone to hybridizations with other Saccharomyces yeasts. And, in fact, scientists have recently made new S. eubayanus and S. cerevisiae hybrids in the lab.
The Cold Truth
S. pastorianus didn’t always exist, and it never existed in the wild. It arose independently in at least two breweries—very recently in human history—and it continues to reside only in breweries.
Lager yeast aren’t the pinnacle of fermentation or anything of the sort. However, they do represent a fascinating side-branch in the realm of human-run fermentations. And, in a way, they did take over the planet—because they now produce what are far and away, by volume, the planet’s most popular and widely consumed beers.
Correction: A previous version of this article stated that the “orginal Hofbräuhaus” in Munich had sent yeast to Carlsberg in Copenhagen in the “17th century,” neatly combining a typo with a factual error: It was the 19th century, not the 17th; and J.C. Jacobsen collected the yeast from Spaten, not the Hofbräuhaus. We regret the errors.
