How Does Yeast Actually Work?

Yeast is perhaps one of the first things you would put in your shopping cart when planning to make a fluffy loaf of homemade bread. But its uses don't stop there. If you've already familiarized yourself with a beginner's guide to baking with yeast, you probably know that it's also the secret to a chewy pizza dough or those soft, melt-in-your-mouth cinnamon rolls you just can't stop thinking about lately. And if that's not enough, yeast is the reason we get to appreciate beer and wine too. But what is it about yeast that makes it so important?

Understanding the difference between active dry and instant yeast or whether fresh yeast makes a difference in baking is essential for choosing the right yeast to use. But the most important fact to know about yeast is that it is, in fact, a living organism, just like you and me.

And like all living things, yeast also needs energy to survive. So when it comes into contact with the sugars in your dough or beer, it starts feeding on them, breaking them down through a very unique process called fermentation. During that metabolic activity, yeast releases two important byproducts. One is ethanol, a type of alcohol that gives bread that characteristic fermented smell and makes beer and wine intoxicating. The other is carbon dioxide, which causes the dough to rise, gives beer its bubbles, and shapes wine's character. But that's only the first step.

There's a lot more going on beneath the surface of this extraordinary anaerobic chemical process. Flour contains up to 8% simple sugars, such as glucose, fructose, and sucrose. Once the yeast consumes these, it switches to maltose — a sugar released when flour is mixed with water and enzymes break down the available starch. 

During this process, the yeast refines the dough's gluten structure, allowing it to expand while evenly trapping the rising gases. But at the same time, it also releases a variety of side products, all of which boost the richness and complexity of the dough, giving it distinct fruity and buttery notes. In addition to ethanol and carbon dioxide, carbonic acids, esters, aldehydes, and secondary alcohols are just some of the 500 volatile compounds that have something to do with it. However, to keep fermentation running smoothly, the annoyingly picky yeast requires just the right temperature, which ideally varies between 80 degrees Fahrenheit and 90 degrees Fahrenheit, only slightly lower than the temperature required to dissolve compressed yeast and revive dry yeast. 

The sugars and starches involved in fermentation also play a key role during baking because they trigger the Maillard reaction and caramelization, which are different processes but two sides of the same coin. The first one gives bread a delightful golden crust, and the second adds a touch of sweetness to it. 

One might think people always knew about culinary yeast — scientifically known as Saccharomyces cerevisiae. However, it took a while before it gained the recognition it has today. A 2020 study published in the journal Frontiers in Genetics shows that ancient Egyptians were the first to discover the benefits of yeast, with evidence of leavened bread found in a tomb dating back to nearly 5,000 years ago. 

But it wasn't until the 17th century that Dutch microbiologist Antonie van Leeuwenhoek first detected yeast while performing some experiments in his lab, though he didn't realize then that it was alive. That was proposed independently in the 1830s by two other scientists, Theodor Schwann of Germany and Charles Cagniard de Latour of France. However, it was another French chemist, Louis Pasteur, who realized the significance of yeast, publishing papers about its role in fermentation between the 1850s and 1880s. His work laid the foundation for Danish fermentation physiologist and mycologist Emil Christian Hansen to isolate and purify individual yeast strains, thus paving the way for commercial production. 

Once industrial yeast became widely available, people quickly realized that it not only made bread incredibly light and airy but also improved its aromas and flavor. That's because industrial yeast's strains are carefully isolated and selectively bred, so it outperforms wild strains in both productivity and fermentation efficiency. The scientific breakthroughs responsible for the commercial yeast we buy today transformed bread-making, ensuring consistent results with superior texture, rise, and flavor.