Above: Freshly baked bread loaves being removed from an oven (FotoDawg)
Did You Know? The scientific name for the most common species of baker’s, brewer’s and wine yeast is Saccharomyces cerevisiae. In Latin, Saccharomyces means “sugar fungus” and cerevisiae means “brewery”.There’s nothing quite like the mouth-watering smell of baking bread. A mixture of flour and water is a gluey mess, but add some yeast (and an oven) and you have the makings of a loaf of fluffy goodness. For thousands of years, people used yeast to make bread, wine and beer without really knowing what it was or how it worked (hint: it’s alive!). Yeast continues to be an indispensable part of food biotechnology, so let’s explore the science behind this hard-working ingredient.
A broad definition of biotechnology is the use of living things, or their products, to improve our lives through agricultural, medical or industrial applications. For example, many food crops have been engineered to withstand herbicide treatment or fight off insect pests, which improves crop yields. A good example of medical biotechnology is the use of bacteria or algae as “living factories” to make the insulin that keeps some diabetics healthy. Baker’s yeast and brewer’s yeast are indispensable for the food and beverage industry—they put the “bio” in food biotechnology!
Yeasts are single-celled, eukaryotic microorganisms that belong to the fungal kingdom, along with mushrooms and moulds. "Eukaryotic" means that a cell contains complex internal structures, such as a nucleus, that are bound by a membrane (like the cells in your body). When oxygen is abundant, yeast cells can break down sugar molecules to make lots of energy, which allows them to grow. However, when yeast cells are deprived of oxygen (anaerobic conditions), they can’t use the same chemical pathways to create energy. But they can still make enough energy to stay alive by breaking down sugar into carbon dioxide gas (CO2) and ethanol. This chemical pathway is called fermentation, and the CO2 and ethanol that result are the keys to baking bread, brewing beer and making wine.
Did You Know? The state of Oregon is in the process of passing legislation to make Saccharomyces cerevisiae the official state microbe. Yeast is indispensable for this state’s craft brewery industry.Grains like wheat and rye are used to make bread flour. Most of the sugar found in bread flour is actually in the form of starch, or long chains of sugar molecules bonded together. Baker’s yeast contains an enzyme called amylase that breaks down starch chains into individual sugar molecules, but extra amylase is often added in commercial baking to speed up the process. Next, the baker’s yeast ferments the sugar. The CO2 that is released gets trapped inside the stretchy dough, causing bread dough to rise and creating the light, fluffy texture of the finished loaf. Ethanol, the alcohol produced during yeast fermentation, is evaporated during baking, which is why there’s no legal eating age for bread!
Grains like barley and wheat are also used to brew beer. Just like in bread making, the grain starch is broken down into individual sugar molecules. Amylase, which is found in whole grains, as well as other added enzymes, help dismantle the starch chains. Brewer’s yeast ferments the sugar molecules, but unlike in bread making, the resulting ethanol is kept in the finished product. Most of the CO2 is released during the brewing process, so some is usually added back into the beer at the end to achieve the desired level of carbonation.
Grapes, not grains, are the starting point for wine production. But yeast does the same job of fermenting grape sugars in order to create an alcoholic beverage.
Did You Know? Selective breeding is not the only way to create different strains of yeast. Modern laboratory techniques can cause random or targeted mutations (changes) in yeast genes to create a useful strain.
Baker’s yeast, wine yeast, and most brewer’s yeast all belong to the same species, but there are hundreds of different strains that have been selected over thousands of years. Humans have “domesticated” wild yeast through selective breeding, a process that creates distinct strains by separating particular yeasts that have given desirable characteristics to bread, wine or beer, or that have made the industrial production of these products easier. Different yeast strains actually have differences in their genes that dictate their specialized use in food biotechnology. It’s just like the selective breeding of dogs. All dogs belong to the same species, but small gene differences make a Great Dane look a lot different from a Chihuahua!
Next time you enjoy a delicious slice of fresh bread, please spare a thought for the microorganisms that have fermented their way to a starring role in our bakeries. You may never look at your lunch the same way again.
A brief history of fermentation (Laura Brown)
What are yeast? (Saccharomyces Genome Database Wiki) Explore Yeast: Yeast From A to Z (Lesaffre) Science of cooking: Bread Science 101 (Exploratorium San Francisco) The Science of Bread (Kitchen Geekery) Finally, Microbes Get the Recognition They Deserve (Saccharomyces Genome Database)
Alba-Lois L, Segal-Kischinevzky C. 2010. Yeast fermentation and the making of beer and wine. Nature Education. 3:17.
http://www.nature.com/scitable/topicpage/yeast-fermentation-and-the-making-of-beer-14372813 Chambers PJ, Pretorius IS. 2010. Fermenting knowledge: the history of winemaking, science and yeast research. EMBO Reports. 11:914-920.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2999870/?report=reader Mortimer, RK. 2000. Evolution and Variation of the Yeast (Saccharomyces) Genome. Genome Research. 10:403-409.
http://genome.cshlp.org/content/10/4/403.full.pdf+html Nevoigt, E. 2008. Progress in Metabolic Engineering of Saccharomyces cerevisiae. Microbiology and Molecular Biology Reviews. 72: 379-412.