A Closer Look at the Science of Baker’s Yeast

Yeast is one of the main reasons bread is so special and complex. Like other microbes, yeasts are tiny: there are roughly 15 billion individual yeast organisms in every gram of fresh, compressed baker’s yeast. These single-celled fungi reproduce either by budding or splitting in half, a process biologists refer to as binary fission. The image above, made on the electron microscope at our lab, shows individual Saccharomyces cerevisiae yeast cells at about 17,000 times actual size. After budding, yeasts have “outies”— circular scars where their offspring detached.

Yeast is ubiquitous, which is why a mixture of flour and water will more often than not start bubbling with the fermentative action of wild yeasts (and bacteria) within a day or two. The fine white powder that often covers grapes is a yeast—often S. cerevisiae, the same species (but a different strain) that is usually used in brewing and baking.

It is no coincidence that S. cerevisiae grows naturally on grapes and other sweet fruits. Its name, New Latin for “sugar fungus of beer,” hints at why: this species of fungus has evolved the biochemical machinery to convert sugars (like humans, yeasts love to eat sugar) of various kinds into ethanol and carbon dioxide when it finds itself in a high-sugar or low-oxygen environment.

This yeast gains two evolutionary advantages from that ability. First, fermentation allows the microbe to survive in anaerobic (oxygen-free) and sugar-rich conditions as well as in its more usual state in the wild, in which oxygen is plentiful but sugars are not. Second, by spewing out alcohol and acids that are toxic to most bacteria (and many other fungi), S. cerevisiae fends off other microbes as it ferments. In essence, yeast cells pee on their food to make it unpalatable to outsiders. With little competition, it feeds unhindered and has enough fuel for reproduction. What the microbes have evolved to produce is straightforward: more yeasts.

But one man’s trash is another man’s treasure. For thousands of years, bakers and brewers have co-opted yeast’s unusual ability to repel other microbes as an invaluable tool in their trades, and in so doing have created hundreds of different strains of S. cerevisiae. Brewers, vintners, and distillers have diligently bred strains of S. cerevisiae to excrete more ethanol when fed particular blends of nutrients. This produces more of the interesting flavor compounds and fewer of the off-putting ones, and keeps the yeast fermenting for longer as the liquid becomes increasingly alcoholic.

Bread makers, meanwhile, have used artificial selection to evolve and isolate numerous strains that have characteristics tailored for particular baking situations. Many strains of baker’s yeast give off more carbon dioxide to make doughs rise higher and faster, while keeping the alcohol output low enough that it evaporates away during baking. Some strains are cultured in extra-sugary liquid so that they thrive in sweeter doughs by producing more of the enzyme invertase, which breaks sucrose down into fructose and glucose, yeasts’ preferred food. Other strains are selected for their ability to grow in acidic (low pH) conditions, such as those that occur in sourdoughs, or to tolerate more salt. Although the major yeast producers don’t usually market these variants of baker’s yeast to retail outlets as different products, they often make them available to industrial bakeries.