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 be there from time to time to discuss the
most recent episode or answer any lingering questions you might have for us about the
channel. Our new $2 Patreon tier will give you access
to the Discord, our Patreon feed, and a monthly hour-long video featuring just uncut footage
and background music. And we also still have the $8 tier where in
addition to the Discord and uncut video, you will receive weekly digital wallpapers and
your name will be featured in the credits. Just like all of these people that you’re
seeing on the screen right now. To learn how to become a Patron you can go
to patreon.com/journeytomicro Regardless of whether you join our Patreon
community, we are so grateful to all of you for watching and appreciating our channel. To see more from our aforementioned Master
of Microscopes, James, check out Jam & Germs on Instagram. And to see more from us, well, there’s probably
a subscribe button somewhere nearby.
