Baking bread, brewing beer, producing biofuel, getting a gross skin infection. What do all four activities have in common? Yeast. And it’s everywhere – there are thousands of species of these single-celled fungi living in all corners of the globe, including all over your body.
Despite being kind of gross, yeast are culinary powerhouses. In fact they were probably one of the first creatures that humans domesticated. Even today we use them as little fermentation factories – feed them sugar, deprive them of air and they turn out alcohol and carbon dioxide at a mean rate. When yeast is added to bread dough, the carbon dioxide gas it produces causes the bread to rise. As the dough bakes, it eventually gets hot enough to kill the yeast cells. Yeast also provides the alcohol content in beer, wine, and liquor. During production of beer or wine, yeast converts sugar to alcohol and carbon dioxide, which are actually just byproducts of the biochemical process yeast uses to extract energy from sugar when it doesn’t have access to oxygen.
Use of yeast in baking and fermentation dates back thousands of years. Its usefulness as a leavening agent in bread was probably discovered when someone baked some old dough that had been sitting around long enough to start growing yeast naturally. Ever since that unintentional breakthrough humans have preserved and cultivated yeast strains for use in baking and production of beer and wine without really understanding what they were doing. Yeast weren’t observed until microscopes became available in the 17th century and it wasn’t until the mid-19th century that Louis Pasteur proved that living yeast were needed for fermentation.
Even today we continue to use yeast to produce alcoholic drinks. Unfortunately (for the yeast), alcohol is toxic to yeast. So after the alcohol produced by the yeast reaches a critical concentration, the yeast cells die. This means that there is a limit to the alcohol content of beer and wine produced by normal fermentation. Depending on the species of yeast, most yeast cells will die at alcohol concentrations somewhere between 10 and 20%.
But recently, in a quest to produce beers with outrageously high alcohol contents, several breweries have developed special strains of super-yeast that can withstand unusually high alcohol concentrations. This year, a Scottish microbrewery, Brewdog, introduced a contender for highest alcohol beer ever with their brew, “The End of History” which weighs in at a formidable 55% alcohol content. This super beer was served up in bottles encased in stuffed roadkill rodents (seriously. Check out the picture). But don’t get too excited, only 12 bottles were produced and they’ve all been sold at 500 pounds apiece. Not to be outdone, the Dutch microbrewery, Koelschip, recently dethroned “The End of History” with their own 60% alcohol beer, “Start the Future.”
Yeast have provided drinking alcohol for centuries, but their powers of fermentation may also place them squarely in the center of a future biofuel revolution. The alcohol we drink, ethanol, is the same as the alcohol that makes up most biofuel, which many scientists and policy makers hope will someday replace gasoline as a renewable fuel for internal combustion engines. The major obstacle to this goal is the expense of producing ethanol using traditional fermentation methods. The high cost of ethanol means that it struggles to compete with gasoline. Also the sugars that are fermented to produce biofuel often come from food sources such as corn and sugar cane, and mixing up food and fuel sources is probably not a good idea.
An abundant and renewable non-food source of fermentable sugar for biofuel production is lignocellulose, the stringy fibers found in grass, leaves, plant stems, and even wood. Unfortunately yeast normally can’t ferment lignocellulose because fermentable sugar is bound up in the fibrous material. Several approaches have been tested in an effort to circumnavigate this problem. One approach involves chemically treating lignocellulose to release the trapped sugar before fermentation. Another approach uses genetically engineered yeast or other microbes that are capable of consuming lignocellulose and breaking it down into sugar. At this point both approaches have drawbacks and advantages and it remains to be seen whether either approach can be turned into an economically viable process for converting lignocellulose to ethanol biofuel.
Humans have had a close relationship with yeast for thousands of years. Even though this relationship has often been unintentional and sometimes unwanted, we have to give yeast credit for introducing us to alcohol. And leavened bread. And, who knows, maybe yeast will help us solve the energy crisis sometime in the future. Or maybe it’ll just leave us with a nasty skin rash – or worse. Either way it’s a scrappy little microbe that’s not going away anytime soon.