Yeast are simple, unicellular fungi. The most common forms of yeast — baker’s and brewer’s yeast — are strains of the species Saccaromyces cerevisiae. Yeast is often taken as a vitamin supplement because it is 50 percent protein and is a rich source of B vitamins, niacin, and folic acid.

Yeast microbes are probably one of the earliest domesticated organisms. People have used yeast for fermentation and baking throughout history. Archaeologists digging in Egyptian ruins found early grinding stones and baking chambers for yeasted bread, as well as drawings of 4,000-year-old bakeries and breweries. Only in the last 150 years, since the experiments of Louis Pasteur, have scientist begun to explore how yeast works. Pasteur first proposed the production of carbon dioxide from yeast as responsible for raising a loaf of bread in 1859.

Yeast Facts:

As little as two pounds of yeast starter can raise 500 pounds of bread dough.

Wild yeast spores are constantly floating in the air and landing on uncovered foods and liquids. These wild varieties contributed some of the earliest kinds of sourdough bread mixes which did not depend on adding starter cultures.

Yeast is also a popular organism for studying genetics. Baker’s yeast is one of only a half-dozen microbes on Earth whose unique gene script has recently been comprehensively deciphered. Notable in the yeast gene is a host of signals that trigger the microbe to protect itself against extremes in cold and heat, called thermal shock proteins. Hopes now run high in the biological community that over the next several years, more than 50 to 100 additional microbes will also provide comprehensive genetic scripts for their lifecycles, including how these organisms might survive under relentless swings in near boiling water, deep ice, or even in the core of an active volcano vent and nuclear reactors.

The yeasts, like most fungi, respire oxygen (aerobic respiration), but in the absence of air they derive energy by fermenting sugars and carbohydrates to produce ethanol and carbon dioxide. When yeast are supplied with both sugar and oxygen, the colonies grow up to 20 times faster through cell division than without oxygen.

The “Stuff of life” as reconstitutable for simple experiments in extreme environments and astrobiology, including freeze-dried nutrient mix (far left, dried skim milk powder) and fast-growing Baker’s yeast (middle, powdered Saccharomyces cerevisiae), and as shown with the key element for life, water, added (right shown in vials from left to right in last panel, with left, metabolic dye to measure growth (chemical called resazurin), middle, reconstituted skim milk with color dye added, and right, yeast.

In 1815, Guy-Lussac understood how yeasts convert the simplest sugar, glucose (C6H12O6) to ethanol:

C6H12O6 (glucose)--->2CO2 (carbon dioxide) + 2C2H5OH (ethanol)

The great medical microbiologist, Louis Pasteur, played a central role in proving this conversion to ethanol required living organisms, rather than a chemical catalyst. Pasteur showed that by bubbling oxygen into the yeast broth, the cells could be made to stop growing, but ferment vigorously–an observation later called the Pasteur Effect.

Many higher animals share this property of oxygen balance with yeasts. When given nutrient (sugar) and oxygen, they will burn fuel quickly like a stoked fire, but when deprived of oxygen, they will reproduce by cell multiplication and division (rather than metabolize). This kind of behavior–burn fuel or divide–is common to many biochemistries and these kinds of organisms are classified as facultative anaerobes; they essentially scrounge a meager living out of whatever particular circumstances are handed to them.

Unlike many kinds of fermenting bacteria (such as yogurt making or lactic acid microbes), yeasts don’t require anything but sugar and water to maintain fermentation and growth. For example, their nutrient broth can be free from other complex molecules such as amino acids, minerals or vitamins, since the yeasts’ history of austere conditions in nature has brought them to a unique state of self-sufficiency, even by microbial standards. The ingeniousness of adaptation makes yeasts one of the most studied and robust microbes.

Suggested Reading 

The Early Days of Yeast Genetics. (1993) edited by Michael N. Hall and Patrick Linder. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.

The Molecular and Cellular Biology of the Yeast Saccharomyces cerevisiae: Gene Expression. (1992) edited by Elizabeth W. Jones, John R. Pringle, and James R. Broach. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.

Mortimer, R.K. and Schold, D. 1985. Genetic map of Saccharomyces cerevisiae, Edition 9. Microbiological Reviews 49: 181-212.

Mar. 16, 1999: NASA/Marshall’s “Life on the Edge” program is barely a month old but it’s already producing results in some grade school classrooms.

“It was wonderful,” says Mrs. Nancy Walters, whose 3rd grade class recently tried some simple microbiology experiments with yeast. “The kids felt like they were doing real science, and they couldn’t stop talking about it for days.”

Life on the Edge is an educational program that aims to expose grade school students to some of the basic principles of astrobiology and to explore the possibilities for life elsewhere in the Solar System. The program began just over a month ago when 50 lb of yeast and other microbes were delivered to a summit in California’s White Mountains. Conditions there present severe challenges for most forms of life, so it is a good place to test the response of microbes to extreme environments. Some of the microorganisms will remain there for months, and some for longer than a year before they are retreived and distributed to classrooms for experimentation.

“Eventually we’ll be sending thousands of yeast packets to schools around the country,” says Dr. John Horack, director of science communications at the NASA/Marshall Space Sciences Lab. “But even before the microbes are ready to go we have to develop some simple lab protocols that kids can use to measure how their samples were affected by exposure. That’s why we’re going into classrooms now to test some of our ideas.”

One of these ideas, called “Planets in a Bottle,” was field-tested in a 2nd/3rd grade class room in February.

“‘Planets in a Bottle’ is a simple way to test the viability of yeast samples, and a great way to teach young students about conditions on other planets,” explained Dr. Tony Phillips, who is evaluating the concept in classrooms. “The basic ingredients for a planet in a bottle are 1 cup of warm water, 3 sugar cubes, a 1/4 oz. packet of yeast, a half liter plastic water bottle, and a nine inch party balloon. Simply mix the sugar, water, and yeast in the bottle, and cap the bottle with the balloon. A healthy sample of yeast will inflate the balloon to 12 inch circumference in less than an hour.”

What happens is this: In the nutrient broth — warm water containing both dissolved oxygen and sugar — yeast metabolizes the sugar and produces carbon dioxide. The rate of carbon dioxide production at any given instant is proportional to the number of healthy microbes in the bottle. Because the yeast are constantly reproducing through cell division the number of microbes increases exponentially. Likewise, carbon dioxide production increases. The balloon inflates slowly at first, then rapidly accelerates.

In practice the balloon inflates to maximum volume in about 45 minutes. That’s when the yeast have consumed all the available nutrient. At room temperature the cells remain viable for several hours afterward and then begin to die. The maximum volume of CO₂ and the time required to produce the gas can be used to estimate the number of healthy microbes in the original sample.

Click here for a sample “Planets in a Bottle” lesson plan

“Two weeks ago we visited Mrs. Walter’s 3rd grade classroom in Bishop, CA” continued Dr. Phillips. “The class was divided into seven groups, each with the basic ingredients for a Planet in a Bottle. Rather than have every group do the same experiment, we added variations so that each bottle would represent a different planet. For example, the Moon has no atmosphere to protect its surface from solar UV radiation. So, one group exposed their yeast to a UV lamp before adding the microbes to the nutrient mix, creating a “Moon in a Bottle.” Another group used scalding hot orange juice as a nutrient mix for ‘Venus in a Bottle.’ Citric acid in the orange juice served as a substitute for sulfuric acid in Venus’s hot atmosphere.”

Above: Young scientists monitor yeast growth in a bottle labelled “Pluto”. In this case the yeast were frozen for weeks before being added to the nutrient mix.

“Clearly we can’t reproduce true planetary conditions in a simple water bottle, nor did we pretend to, but these excercises have powerful teaching value. Every kid in Mrs. Walter’s class now knows that Venus has acid in its atmosphere thanks to the orange juice experiment, and they also learned that weak acids are not deadly to yeast,” Phillips said.

“My students were really excited when their balloons began to inflate,” recalled Mrs. Walters, “but the best part came at the end when we measured the sizes of the balloons and held a classroom debate about the results. We argued about which planet was most congenial to yeast and what the limitations of our results were. It felt like real science.”

Left: Students in Mrs. Walter’s 3rd grade class debate the question: “Which planet is really best for yeast?”

NASA scientists have a crowded schedule of classroom visits planned in the months to come, even though the Life on the Edge yeast container won’t be retreived for some time. The goal is to develop safe and effective classroom protocols before the yeast packets are distributed nationally.

“We don’t want to spoon feed students with overly-detailed protocols,” says John Horack,” That’s not science. But, we do want to give them a good starting point for their own creative experiments with extremophiles. The only way to do that is by spending lots of time in the classroom now, while the microbes are still in the White Mountains.”

To view a prototype lesson plan for “Planet in a Bottle” yeast experiments click here. Readers are invited to try the experiments (they are lots of fun) and we welcome comments from educators and others to improve our procedures. Please send comments and suggestions to [email protected].