Yeast and Fermentation: A Story of Fungus and Farts

Good evening, friends and neighbors!

It’s been a while since I’ve touched on baking science, and I promised a while back that I’d discuss the fermentation stage of baking in greater detail.

I’m a man of my word, so here’s a crash course on yeasty beasties and how to make them work for you!

Once again, like the Ten Steps of Baking, I’m taking “Advanced Bread and Pastry” by Michel Suas as my text for today. I strongly recommend it for a more detailed look at all this business.

Credit given where it’s due, let’s explore these freaky little fungi, their delicious excreta, and why mass extinction is the tastiest thing in the bakery.

What’s Yeast?

Yeast are single-celled microscopic fungi that, simply put, eat sugars and starches. They then excrete carbon dioxide (CO2), alcohol, heat energy, and flavorful compounds. That process is called fermentation.

Before the identification and cultivation of specific yeast strains, bakers relied on wild yeast finding its way into the dough (a process today called “spontaneous fermentation.”) Because different yeast strains have different lifespans, acidity tolerances, and CO2 production rates, this resulted in unreliable bread- sometimes batches wouldn’t rise well, or had unusual flavors, or even overproofed and ended up bland and airy.

Today, we know that there are over 1500 different kinds of yeast, and which strains serve best in which situations. Given how long humans have been making and eating leavened bread, yeast are likely the first creatures we domesticated. Sorry, Fido- but at least a bagel can’t play fetch!

Microscopic picture of yeast cells
Microscopic picture of yeast cells from Wikipedia

Why Is Yeast Important?

Because we have a mighty need to eat yeast poop and harness their farts.

Now that your inner second-grader is giggling maniacally, let’s talk about what’s really going on in there.

Yeast eats sugars and starches, and in addition to creating carbon dioxide to make your dough rise, it also creates alcohol. Obviously this important if you are a homebrewer, but it’s equally handy for bakers! Alcohol evaporates at a relatively low temperature (which is what distillers count on for making liquor.) This means that whatever alcohol your yeast made and left in the dough will evaporate very quickly once the bread hits the oven, assisting in the sudden rise of the “oven spring.”

While it SOUNDS like the main point of yeast is the creation of those leavening gasses that make your dough rise, the process of fermentation itself also changes the flavor, nature, and aroma of the dough- all of which contribute to the final bread. Chemical leaveners (like baking powder and baking soda) don’t ferment anything… so if you want those freaky good flavors, you need to go the natural route.

For example, during fermentation, the dough becomes more acidic- a process that can be artificially lengthened by slowing down the proofing stages in a fridge. This acidity creates the tangy flavor of baguette- and of course the unique punch of sourdough through the presence of lactobacillus bacteria.

Breaking It Down- How Sugar Becomes Gas and Booze

We now know WHAT yeast is. We know WHAT these little beasties do, and why we’ve kept them around for millennia. But how do they do it? HOW do these tiny little fungi gobble up sugar and produce such useful farts?

Since I’m guessing most of us still wake up in cold sweats after dreams of high school chemistry class, I’ll breeze through it real quick:

Photo by Suzy Hazelwood on

When I talk about “sugar” here, don’t think I’m just talking about the granulated white stuff you put in your tea. The sugars present in flour are carbohydrates, and they vary in complexity- which in turn changes what’s required to turn them into carbon dioxide and alcohol, and how quickly.

To break down ANY of them, however, requires compounds called enzymes, which may (or may not) be already present in the flour or yeast. Think of it like this- enzymes are like the cutlery yeast uses to eat, but eating a cheesesteak is different from eating a whole cow. Some extra equipment and breakdown is needed!

The simplest sugars are glucose and fructose– yeast handles these easily and goes to work on them with an enzyme called zymase within 30 minutes. They’re the “fast food” of the yeast world- and their simplicity also makes them an easy energy source for us humans too. Wanna know why you get a “sugar high?” There it is.

Saccharose and Maltose are a little more complex. They need to first be broken down by enzymes saccharase and maltase. That turns them into the simpler glucose and fructose, which then gets finished off by the zymase. Back to our beef analogy, these sugars are like whole cuts of meat. You aren’t just going to attack that with a knife and fork- you break it down into manageable, cookable steaks first with additional tools, like a meat cleaver and grill.

Trickiest of all are starches. They make up of the majority of flour, and yeast on its own has difficulty breaking them down. The process relies on the presence of two enzymes in particular, alpha and beta amylaze, and there may or may not be enough alpha amylaze if the wheat hasn’t sprouted enough before it’s milled. This is why millers will often enrich their flour with flour from malted (or sprouted) grain.

Imagine trying to eat a whole cow. One shot. Not happening, right? And you probably don’t have a full butcher shop in your house. Think of enriching flour and these enzymes as farming out the job of turning all that beef into burgers, so that you can finally have a cookout- and maybe some quiet words for the dude who showed up with a whole dead cow.

“Dammit, Steve…”

That’s how yeast does its job… but like any picky eater, it needs to be in top condition, and the finest facilities so it can go to work. That’s your job to provide.

What Could Go Wrong?

Here’s the deal. You’ve got this horde of tiny little mushrooms. You need them to eat to their little hearts content, and give you all of their amazingly useful byproduct. You don’t just want them to be happy… you want them to have a party. You want them to party harder than Andrew W.K. with a crate of Red Bull.

But you ALSO don’t want them to go too crazy. You’re looking for a rager in that dough, NOT Baccanalia. We’ve given them enough food- they DON’T need to rip a bull apart by themselves.

So what exactly CAN crash this yeasty party (or send it out of control?)

  1. Temperature
    Most commercial yeasts do their thing at around 76 degrees Farenheit (or 24 Celsius.) Any hotter, and they start pumping out gas like crazy- too fast for there to be a lot of aroma development. Conversely, you can chill down your dough to make fermentation go slower. This is called “retarding” the dough- the yeast is a bit too cold to produce gas quickly, but it still has a lot of sugar to go through, so it just makes more alcohol and aromatic acids. This contributes to a more developed, nuanced flavor in your bread. It also lets you decide when you are ready to bake a bit more, rather than needing to time it against the yeast.
  2. How Much Yeast?
    This ought to be a no-brainer… more people at the party? More gas gets created! Depending on what kind of bread you are doing (as well as what other stuff is going in the dough) your yeasty guest list maybe a little longer or shorter.
  3. Salt and Sugar

    There can be too much of a good thing- even to yeast. Adding a bit too much sugar can make the yeast go nuts- but even more can actually make them slow down. Think of it like this- if you are at a party, and there’s only so much pizza available, you’re gonna try to get a slice as quickly as possible. But if the pizza just… keeps… coming… you might let some of that sit on the table after a while. Yeast is the same way- when theres too much sugar around, fermentation will slow down, and all that excess sugar may make your bread weirdly sweet.

    Salt, on the other hand, is the opposite. Salt stops fermentation and kills yeast. If you forget the salt in your bread, not only will you probably wind up with your yeast having too much of a good time, your bread will also probably taste off.
  4. Acidity
    An animated gif of a kaliedoscope
    Commercial yeasts like to have fun, but they don’t like it when things get too weird. Bacteria strains in the dough (such as our buddy Lactobacillus) crank out acid that adds funk and tang to the bread. After a certain point, there’s too much acid for the yeast to keep eating sugar. Wild yeasts tend to be a bit heartier though, and can take a bit more acid than their commerical bretheren- just like that weird dude in college that lived in a cabin off campus somewhere.

Once our yeast are happy and having the time of the lives, we let them do their thing- but eventually, the party is over… and everything must burn.

Where Does Yeast Go After It Dies?

During baking, the yeast feels things start to get hot. They freak out, gobbling as much sugar and farting as much carbon dioxide as possible… but all for naught. At 140 degrees, yeast cells start to die off, exploding like tiny gas-filled bubbles. Their death-pops are their final contribution to the oven spring.

At 200 F (around 100 C,) when most breads are finished, all that is left of your yeasty paradise is ruin. Tasty, toasty ruins, as even the expired yeast cells contribute to flavor.

That’s the end of this little crash course on the life, death, and useful farts of our yeasty friends!

Anything I missed? Got questions you’d like answered?
Drop them in the comments and I’ll do my best!

Stay Classy,

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