Yeast: best friend of the baker and brewer

Computer illustration of budding yeast cells. Photo: Getty Images
Computer illustration of budding yeast cells. Photo: Getty Images
Jeffrey Miller reveals the science behind yeast — something many people used for the first time during lockdown.

Due to the Covid-19 lockdown there is more interest in home-baked bread than ever before. And that means a lot of people are making friends with yeast for the first time.

Yeasts are single-celled organisms in the fungus family. There are more than 1500 species on Earth. While each individual yeast is only one cell, they are surprisingly complex and contain a nucleus, DNA and many other cellular parts found in more complicated organisms.

Yeasts break down complex molecules into simpler molecules to produce the energy they live on. They can be found on most plants, floating around in the air and in soils across the globe. There are 250 or so of these yeast species that can convert sugar into carbon dioxide and alcohol — valuable skills that humans have used for millennia. Twenty-four of these make foods that actually taste good.

Among these 24 species is one called Saccharomyces cerevisiae, which means ‘‘sugar-eating fungus’’. This is bread yeast, the yeast we know and love most dearly for the food and drinks it helps us make.

The process starts out the same whether you are making bread or beer. Enzymes in the yeast convert sugar into alcohol and carbon dioxide. With bread, a baker wants to capture the carbon dioxide to leaven the bread and make it rise. With beer, a brewer wants to capture the alcohol.

Bread has been ‘‘the staff of life’’ for thousands of years. The first loaf of bread was probably a happy accident that occurred when some yeast living on grains began to ferment while some dough for flatbreads — think crackers — was being made.

The first purposely made leavened bread was likely made by Egyptians about 3000 years ago. Leavened bread is now a staple in almost every culture on Earth.

Bread is inexpensive, nutritious, delicious, portable and easy to share. Anywhere wheat, rye or barley could be grown in sufficient quantities, bread became the basic food in most people’s diet.

When you mix yeast with a bit of water and flour, the yeast begins to eat the long chains of carbohydrates found in the flour called starches. This does two important things for baking: it changes the chemical structure of the carbohydrates, and it makes bread rise.

When yeast breaks down starch, it produces carbon dioxide gas and ethyl alcohol. This CO2 is trapped in the dough by stringy protein strands called gluten and causes the dough to rise. After baking, those little air pockets are locked into place and result in airy, fluffy bread.

But soft bread is not the only result. When yeast breaks down the starches in flour, it turns them into flavourful sugars. The longer you let the dough rise, the stronger these good flavours will be. Some of the most popular bread recipes use this to their advantage.

More than just bread and booze

Because of their similarity to complicated organisms, large size and ease of use, yeasts have been central to scientific progress for hundreds of years.

Study of yeasts played a huge role in kick-starting the field of microbiology in the early 1800s. More than 150 years later, one species of yeast was the first organism with a nucleus to have its entire genome sequenced.

Today, scientists use yeast in drug discovery and as tools to study cell growth in mammals and are exploring ways to use yeast to make biofuel from waste products such as cornstalks.

Yeast is a remarkable little creature. It has provided delicious food and beverages for millennia, and to this day is a huge part of human life around the world.

So the next time you have a glass of beer, toast our little friends that make these foods part of our enjoyment of life. — The Conversation

Jeffrey Miller is an associate professor of hospitality management at Colorado State University.

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