Yeast: Love and Fear, Death and Beer

Single-celled fungi all around us do so much good and so much bad.
Jamal Saidi/Flickr

You might say Saccharomyces cerevisiae yeast are the house-cats of the microbial world. Although they've been domesticated at least since the Pharaohs ruled Egypt -- the earliest records show that people by then already were using the "sugar-loving" yeast to leaven bread and brew beer -- this fungus's less civilized cousins can really mess you up.

Yeast are found everywhere in the environment. The one-celled organisms -- called "yeast" from the Old English gist/gyst and Indo-European root word yes, meaning boil, foam or bubble -- don't need sunlight, do need oxygen, and are naturally drawn to sugar-rich hosts for the carbon they thrive on.

"There are people who will contend that civilization began so that humans could make beer."

Fruit and berry skins are among their favorite habitats, but the spaces between your toes, your skin, gut, genitals -- and even crude oil, which is 84 percent carbon -- suit them just fine, too. Basically if there is carbon to be had, one yeast or another will have adapted to having it.

Although some species of fungi have a single-cell yeast "phase," they aren't what we ordinarily think of as "yeast." Of the 1,500 identified species of yeast that live around, in, and on us, three in particular stand out. Foremost is S. cerevisiae. Besides its ancient and ever greater role in food, beverages, and nutritional supplements, modern biotechnology has harnessed the metabolic process of S. cerevisiae -- we know it as fermentation -to manufacture lifesaving medicines, fuel our vehicles, and even clean up oil spills.

Harmless to healthy people and present in/on all of us, Candida albicans becomes a one-celled monster when it finds a weak immune system. It's notorious for afflicting three-quarters of all women at some time in their lives with the itching, irritation, burning sensation, and soreness associated with so-generically-called yeast infections. Candidiasis, the technical name for those infections, also shows up in the form of diaper rash on a baby, jock itch, or white milky-looking thrush on the tongue.

The far more sinister Cryptococcus neoformans this year will kill hundreds of thousands of people, as it does each year. Preying on those with suppressed immunity, it's found in soil all over the world -- especially where lots of birds, particularly pigeons, leave their droppings. We all inhale C. neoformans' microscopic, airborne fungal spores, mostly with no problems. But people whose immune systems are compromised -- because they have untreated HIV infection, take immunosuppressive drugs, receive an organ transplant, or are pregnant, for example -- are at risk for developing the pneumonia-type illness cryptococcosis or, if the infection spreads to the brain, the life-threatening cryptococcal meningitis.


Thinking past these unpleasantries, consider that 50 billion pints and 67 billion cans of beer are consumed in the United States each year, according to the Beer Institute. "He was a wise man who invented beer," said Plato.

In fact, said Jim Koch, founder and owner of the Boston Beer Company, brewers of Sam Adams, "There's some debate about which came first, beer or bread. It appears most likely it was beer."

Koch called yeast -- specifically the S. cerivisiae used in both beer and bread -- a "miracle organism." He explained, "There are people who will contend that civilization began so that humans could make beer. They figured out how to grow grain, but needed to figure out how to turn the grain into a source of nutrition and safe hydration."

The alcohol produced by yeast's fermentation -- breaking down carbon into carbon dioxide and ethanol (alcohol) -- is among the most powerful sterilizing agents available. This is why, Koch said, even the Pilgrims didn't pack the Mayflower with barrels of water that could carry all kinds of harmful organisms -- typhus, cholera, and hepatitis among them.

"Instead, they provisioned it with beer," he said, "almost a gallon a day for every man, woman, and child."

8190849377_d00487ae2d.jpgBoston Public Library/Flickr

It wasn't that the pleasure-phobic Puritans were closet lushes. "The reason was beer was a safe form of hydration," said Koch. The alcohol produced by yeast's fermentation kills every potentially harmful organism in the beer. "When God made the universe," said Koch, "he or she made nothing harmful to human beings that can grow in beer."

Gods have been invoked, thanked, and cursed since the Greeks credited Dionysus with creating wine and winemaking. But it was the Frenchman Louis Pasteur who revealed new knowledge that even the gods had never imparted. In 1857 Pasteur created the field of microbiology when he proved that alcoholic fermentation is conducted by living yeast rather than by either a chemical reaction or by magic. Viniculturists ever since have sought to capitalize on the new understanding about yeast that microbiology opened up for them.

One winemaker, noted for using "wild" yeast to make what many consider his magical zinfandels, is Joel Peterson. The owner of Sonoma, California-based Ravenswood Winery told me he prefers the indigenous yeast found on the grapes he uses because they have been used for millennia in the kind of traditional winemaking he practices. "I get more interesting characters out of the wild yeast fermentation than I do out of a monoclonal yeast fermentation," he said.

Because it takes longer -- and therefore costs more -- to let the wild yeast ferment at their natural pace, Peterson said most big commercial wineries "give an overwhelming dose of Saccharomyces" so it dominates the others, ferments faster, and is more likely to yield a predictably consistent flavor.

Presented by

The author of Victory Deferred, John-Manuel Andriote has specialized in HIV/AIDS reporting since 1986. His research materials, correspondence, and recorded interviews are part of a special collection curated by the Smithsonian.

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