As we survey the life on Owl Acres, we have been thinking about the big things. We owe a lot, though, to those tiny one-celled organisms that are everywhere. Yeast for instance. Yeast cells are so small that you need a microscope to see them. And yet, each individual cell is a living, active member of the kingdom of fungi. Before the invention of the microscope in the 1600s, people thought there was somehow magic from the gods that made bread rise and beer ferment. Indeed, for thousands of years, yeast was considered a divine spirit or energy, or a god in its own right. The ancient Sumerians worshiped Ninkasi, the goddess of fermentation, and the Greeks had their Dionysus, the god of wine, wine-making and drunkenness. It wasn’t until 1857 that the French scientist Louis Pasteur discovered that yeast were in fact living organisms.
A particular species of yeast, (Saccharomyces cerevisiae) has been responsible for a great deal of holiday cheer at my house this season. Dormant and waiting for moisture and sugar to eat, these single-celled organisms sat in jars and packets just waiting their turns to shine. When conditions were right, in a warm pan of bubble loaf, they banded together in colonies to make the bread dough rise. They’ve been busy for several months finalizing last summer’s rhubarb wine in time for Christmas dinner.
We use this sugar-loving strain of yeast intentionally, but their wild cousins are absolutely everywhere having their own impacts. They live on the skins of grapes and apples, on plants and in the soil. We host them on our skin and in our hair, in our guts and on the dog. We have them in our lungs, in our mouths, and other orifices. Some species are helpful, and some are not our friends. The species Candida albicans can cause yeast infections like thrush, diaper rash and vaginal yeast infections. Generally, though, our bodies are equipped to keep our yeasts under control.
Left to their own devices in the wild, yeast are responsible for helping to clean up what plants leave behind in the form of uneaten fruits and berries. The yeast colonize the leftovers, digest them and recycle the nutrients. They’ll do the same to fruit left out on my kitchen counter if I’m not careful. Imagine for a moment what the world would look like if nothing ever decayed and got recycled.
Humans have understood the value of yeast in making beer and bread for millennia, even though they didn’t know what it was exactly. Today, specific strains of yeast are carefully guarded and nurtured to provide the right essence to whisky, wine and beer. The reason is that each strain of yeast imparts a specific and characteristic smell and taste to its alcohol product. By selecting a specific strain of yeast and keeping it as pure as possible, a whisky distiller tries, usually with great success, to produce a standardized whisky product that is at the same time different from a competitor’s brand. To the discriminating palate, Maker’s Mark tastes and smells different than say Four Roses, and the yeast plays a major role in that differentiation. It’s also the yeast that makes fresh-baked bread smell and taste so good. Today, over 1,500 species of yeast are known, with many more to be identified. The species we use the most (Saccharomyces cerevisiae) comprises lots of variants or strains which have been carefully managed to give the wine, or beer, bread or whisky the desired character.
A yeast cell is very tiny, 5-10 micrometers in diameter–about the size of a red blood cell. Unlike many other single-celled organisms, yeast cells contain a nucleus and other structures similar to the structures in the cells of multicelled organisms like other fungi and humans. In spite of their tiny size, they have a big impact on our world.
The yeast we use for bread, beer and wine is particularly fond of sugar. It probably evolved to consume ripe fruit that fell from trees and leftover nectar from flowers. The yeast, which live and reproduce in colonies, convert sugar to carbon dioxide and alcohol. The carbon dioxide creates bubbles in bread and beer. The alcohol finishes the libations. Under good conditions, the yeast reproduce by budding. That is, they push out a daughter cell containing an exact replica of the DNA, and then disconnect from it. When stressed, they will create spores combining DNA from two yeast cells to carry out sexual reproduction. The spores will wait for better days before they activate, and the combined DNA presumably gives them a better chance of survival.
Like many other fungi, yeast are saprophytes. That is, they secrete enzymes that break down their intended food and then absorb the “digested” material. This can happen either in an oxygen-free environment such as in fermenting beer or wine, or with oxygen present, such as on decaying fruit. In either case, the yeast digests the sugars and produces carbon dioxide and alcohol. When the alcohol content gets too high, it will kill the yeast, stopping fermentation. In the case of bread, the alcohol produced by the yeast evaporates in the oven. The carbon dioxide creates the little bubbles that make the bread rise.
Of course, there are other uses of yeast besides bread and booze. If your heritage is British or Australian, you would be familiar with another use of brewer’s yeast—vegemite. This thick, black and salty stuff may be spread on your morning toast. It’s made with leftover brewer’s yeast combined with salt, malt extract and various vitamins and minerals. It’s an acquired taste to be sure. Yeast cells have also proven to be excellent partners in the scientists’ laboratory.
Out in the garage, the car’s gas tank is filled with a combination of gasoline and ethanol. The ethanol was produced by the same sugar-loving yeasts we use for bread and beer–and corn liquor come to think of it. The process to make the ethanol is not unlike that used to make moonshine. According to the Iowa Corn Growers Association, 57% of Iowa’s corn crop, 1.5 billion bushels, was used last year to produce 27% of the biofuels in the U.S. (42% of the crop went into livestock feed.)
So let’s raise a glass to those tiny confederates who brought us our holiday cheer.
Antique magazine illustration by George D. Buckley