Every time you eat a piece of bread or drink a glass of beer, you are participating in
what might be (depending on how strict you are about your definitions) one of the longest-running
microbiology experiments in human history.
While the earliest scientific studies of microbes usually go back only a few centuries
when it comes to yeast, the microbe
at the core of some of our favorite foods and drinks, well...those past few centuries
barely scratch the surface.
But before we get to that history, let’s start with some of the basic biology.
Yeast are a fungus, though unlike many in that kingdom, they don’t grow the branching
hyphae that characterize organisms like mushrooms.
Instead, yeast grow primarily through a process of asexual reproduction called “budding.”.
While there are many yeast species found among different fungal phyla, perhaps the most well-known
is Saccharomyces cerevisiae, which you might see labelled as brewer’s yeast or baker’s yeast.
Along with a few other yeasts, these various strains of S. cerevisiae are what we’ve
come to rely on for our beer, wine, and bread--and all through the magic of fermentation.
Yeast, like many organisms, rely on different metabolic pathways to break down sugar and
create energy.
These processes usually rely on oxygen.
But to deal with situations where oxygen may be less readily available or some of their
key metabolic processes are shut down, yeast turn to alcoholic fermentation.
Fermentation helps the yeast get the energy it needs, but the particular chemical path
they follow also produces ethanol and carbon dioxide.
The yeast doesn’t need these byproducts—in fact, too much ethanol could be toxic for them.
But what is waste to a yeast is treasure to us, and that shift in metabolic activity has
been making our bread rise and our brains woozy for millennia.
Molecular studies of pottery jars found in China found that they may have held some kind
of fermented beverage as far back as 7000 BCE.
And archaeological evidence has turned up for leavened bread from 2000 BCE in Egypt
and 1000 BCE in North Western China.
It’s easy to see why our use of yeast has persisted for millennia.
To quote one group of scientists, it helped us make food and drinks with a “enriched
sensorial palate” and also a “euphoriant effect.”
But the long-standing ubiquity of yeast also emphasizes how mysterious their origins are.
We don’t actually know how yeast became involved with our cooking.
We just know that it was ancient, likely starting with wine, and would come to span continents,
potentially spreading through trade.
The results were food and drinks produced through a series of observations about how
factors like temperature, time, and air might affect the final product.
To quote Samuel Johnson’s 18th century dictionary, barm--a type of yeast--is “the ferment put
into drink to make it work, and into bread to lighten and swell it.”
At the time they didn’t know what was actually in yeast or how it worked to shape those results.
We just knew it was a thing that provided…something…somehow….
Moreover, the earliest glimpses of yeast under the microscope seemed to support the idea
that yeast was just some sort of chemical, a fermenting agent, not a biological entity
of its own.
When Antoni van Leeuwenhoek observed yeast in the 17th century, he thought they were
just bits of globular particles.
Against the new and incredible bits of life he was seeing for the first time through the
microscope, yeast seemed useful but ultimately unliving.
But over the next few centuries, as more scientists worked to try and understand how fermentation
actually worked, they identified not just the chemical reactions that shaped the process,
but that yeast was an actual living organism essential to fermentation.
Eventually, Louis Pasteur would put all the pieces together, publishing a paper titled
“The Memoir of the Fermentation of Alcohol”, or something like that, but in French.
It was published in 1857 and it established the interplay between chemistry and biology
that allowed yeast to turn to alcoholic fermentation to survive.
He would also later uncover another form of fermentation carried out by bacteria, which
is called lactic acid fermentation.
Pasteur’s work didn’t just change our understanding of how bread was made, it would
go on to inform our understanding of germ theory, and how the invisible world of microbes
connects to the grim realities of disease.
From this early history, yeast have gone on to become perhaps one of the most studied
eukaryotic organisms in modern biological research.
Saccharomyces cerevisiae in particular has had an illustrious research career.
In 1996, its genome was fully sequenced, making it the first eukaryotic organism to be genetically
mapped out.
Part of what makes yeast such great research organisms is that they are easy to grow and
manipulate--both through environmental and genetic means--which has allowed us to use
them to study everything from DNA replication to prion diseases.
In the same way that cultures have refined the ways we use yeast in our food and drinks,
scientists have refined the way we use yeast in our lab experiments.
They’ve even developed genetic engineering techniques that repurpose yeast to brew the
ingredients for everything from perfumes to antimalarial drugs.
When you consider just how far we’ve come with yeast, you might begin to wonder: did
we domesticate them?
After all, our early days of yeast use likely coincided with our early days of plant and
animal domestication.
So in the same way that we’ve come to rely on domesticated animals for food and labor,
maybe yeast are our microbial workhorses.
The answer to that question is one scientists are still studying, in part because the history
of yeast is so large and mysterious, and in part because domestication has a very specific
definition in these conversations.
One paper we found describes it as “human selection and breeding of wild species to
obtain cultivated variants that thrive in man-made environments, but behave sub-optimally
in nature,” now that’s quite a number of nuanced requirements to check off.
But with this definition in mind, beer yeast seems to show the most evidence for having
been domesticated, which is likely connected to specific aspects of the brewing process
that have limited the exposure of the domesticated yeast to wild yeast.
But all yeasts considered, the genetic analysis of beer yeast strains show that this domestication
is actually relatively recent—as in, it only happened a few centuries ago.
While it did happen before we even knew that yeast was a living thing that we were domesticating,
it still came long after we started using yeast to make beer, leaving much of the organism’s
history still a mystery.
It is often tempting to think that we have a microbial world and a human world that intersect
only in chemical reactions and biological connections.
But yeast reminds us that microbes are more than just a scientific reality, they are a
cultural one--even when we didn’t know they were around.
Yeast have created our foods, shaped our traditions, and both bonded and divided us.
And how we use and change yeast will only continue to shape our lives.
Their future, tied so much to our own, is every bit as grand and mysterious as their
past has been.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
We’ve been making videos on this channel for just over a year now, and we want to thank
all of you for supporting us during our first year.
To celebrate that, we’re adding a new tier to our Patreon as well as a Journey to the
Microcosmos Discord in the hopes of opening up our community to more people.
We often see people asking microscopy questions in the video comments, which we don’t always
have time or information to answer, but we love seeing other commenters jump in and answer
those questions.
But, we also know that YouTube comments aren’t always the easiest or best place
to have those conversations.
So, in our Discord, we’re going to have spaces for you to share any of those questions
you might have, along with your own microscopy, or just some of those good good tardigrade memes.
Additionally, Microcosmos team members like me and our Master of Microscopes James will
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questions you might have for us about the channel.
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