As you wander through the aisles of the grocery store, you might find your attention caught on any number of things.

Frozen pizza. Cupcakes.

Wine. And as delicious as all of those things are, we doubt that any of them undergoes as spectacular of a transformation as a packet of instant yeast does when you shoot lasers at it. Because that is not how yeast always looks.

In the packet, it’s dry and dull. And when you add water and look under a microscope— without any lasers added— yeast are more like stagnant, sturdy, living bubbles. So what makes yeast shine so bright when hit with a laser?

Well, we will get there. But first, let’s talk about the grocery store, and why you can buy a packet full of microbes while you are there. Because that is where James— our master of microscopes, our purveyor of ponds— got these yeast.

He bought them, in the form of instant yeast that was sitting on the shelf of a supermarket. And that’s because for most of us, yeast is an ingredient. It’s something a recipe calls for, like milk or sugar.

But yeast is also something much more than an ingredient. It’s more like our ally, a friendly baker on the inside who does so much of the work for us, as long as we give it a few things in exchange. When you buy a packet of instant yeast, what you are really buying is a lot of Saccharomyces cerevisiae.

That’s the name of the species, and it roughly translates to “sweet fungus of beer”. But it’s known by many other names, like Brewer’s yeast, Baker’s yeast, and budding yeast. But those first two names are a tribute to the role yeast play in helping to make our beer, wine, and— what we’re going to focus on today— bread.

If you’ve used instant yeast, you probably conjure up the image of the tiny, brown-ish white granules that fill the packet and froth up when you add water. Those granules are full of live yeast that had been dried out for later use, and their small size is important. It gives each of those minuscule yeast clumps more surface area relative to their volume, helping them dissolve faster when they are mixed with liquid and hydrated into something that looks more like these cells.

James is a master of microscopes, not a baker, so he wasn’t planning to use these yeasts to make bread. But yeast, of course, helps us make bread using the same basic processes that helps them survive. So the steps James took to bring these cells to life can help us see some of the ingredients and processes that make our favorite loafs possible, starting out with sugar.

We often describe the size of microbes as fractions of fractions of millimeters, but it can be tough to wrap our minds around what that really means. Looking at these yeast cells dwarfed by a few undissolved grains of sugar helps give a better sense of scale. Just think of how small a grain of sugar is in your hand, and imagine just how much tinier a little yeast must be.

Again, it’s easy to think of sugar and yeast as two ingredients in bread, one added after to make the right bit of chemistry happen. And while that’s true, there’s also a little more going on. Sugar is like a bribe that helps kickstart our yeast friends into action, giving them the sustenance they need to get moving.

As the yeast feeds, they begin to divide, forming smaller daughter cells that bud off from their mothers. You can see some of those buds here, forming ovals that branch off the circular mother cell. This is where the name “budding yeast” comes from, and it’s yet another way that yeast is different from our other ingredients.

Sugar doesn’t make more sugar, but for yeast replication is one of their objectives, driving the growth and survival of their population. But as the yeast accomplishes their goals, they happen to get some of our goals done as well. As the yeast digests sugar, they produce byproducts: various organic acids, ethanol, and carbon dioxide— which you can see here in bubbles expanding and pushing against the field of yeast.

We are watching these bubbles grow in water, and over the next few minutes, their edges will push further and further out until these two bubbles seem to take over the screen. But in bread dough, those carbon dioxide bubbles cause the entire structure around the yeast to expand and to rise. And when the dough is moved to the oven, the carbon dioxide will shape the bread that forms.

The ethanol byproduct isn’t visible like those carbon dioxide bubbles are, but in bread dough, the evaporation of ethanol also helps the bread rise in the oven. And it will also shape the flavor of the bread that comes out. While it is making bread for us, the world inside the yeast is busy as well.

Buried in dough, it’s impossible to see what that means. But microscopes are powerful tools when it comes to digging up seemingly invisible forms of beauty, especially when they have lasers in them. With a microscope that sends a laser of UV light straight at the yeast, molecules inside the organism begin to fluoresce, a display called autofluorescence that lights up the whole screen with rings of red and green and yellow that press up against each other and overlap.

To scientists who know how to crack the code, those colors create a map of the molecules inside the cell. Those colors are actually the amino acids and vitamins that make the yeast work, rising and falling throughout the yeast’s life cycle to provide the organism with what it needs to survive. It’s as if someone turned on the lights at the factory, illuminating all the processes that make the final product possible.

And if we could dedicate a whole episode to the stunning auto-fluorescing patterns of yeast, we would. But there is one big challenge. You may have noticed that the videos we’ve been showing of this are pretty short.

Most only last a few seconds before we switch to another video. And that is because those bright lights quickly fade after being struck with the laser. Let’s sit here and watch this group of yeast, initially bright red and green.

After only a few seconds, the red is already gone. And as more seconds pass, the green becomes more and more faint, like someone has been turning the brightness on your screen down at an agonizingly slow pace. After a full minute, the screen has gone completely black, with no hint at the vivid scene painted across it so recently— almost as if it had never been there to begin with.

But even as the lights fade, the factory remains. Every day, perhaps in your kitchen or in the bakery around the corner from you, there is a group of invisible budding bakers occupied with turning sugar into carbon dioxide and ethanol. However transient their lives or their work, the legacy they leave…well, there’s no arguing with that.

Thank you for coming on this journey with us as we explore the unseen world that surrounds us. The people you're seeing on the screen right now, those are our Patreon patrons. They're the ones who make it possible for us to get the yeast packets which aren't that expensive, but also so that we have access to the lasers, which are actually not that cheap.

If you would like to become a patron on Patreon, you can go to Patreon.com/JourneytoMicro. There you can get a bunch of cool perks and also just know that you're making it possible for this to exist. If you'd like to see more from our Master of Microscopes, James Weiss, you can check out Jam & Germs on Instagram.

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