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Carolina Timeless Tips

Yeasts in the Classroom

Published: Oct. 1985 | Updated: May 2026

Yeasts have become increasingly important to basic biological research. Because of their rapid growth rate and ease of manipulation, certain strains of yeasts are convenient organisms for studies in physiology, genetics, cytology, and biochemistry. Yeasts have been used by biotechnology corporations to produce human growth hormones, interferons for treating cancers, and enzymes for reducing inflammation. Many believe that yeasts are superior to bacteria for use in genetic engineering because they secrete their proteins.

Yeasts are found in nature on the nectar of flowers and the surfaces of fruits as well as in milk, soil, salt and fresh waters, and animal excretions. Yeasts normally live harmlessly in the mucous membranes, skin, and feces of the human body. Under certain conditions, however, some of these yeasts may become pathogenic. For years, there has been an increase in yeast infections resulting from clinical changes to the host’s defenses due to steroid and immunosuppressive drugs and by indiscriminate antibiotic use.

Here with a Loaf of Bread beneath the Bough,
A Flask of Wine, A Book of Verse—and Thou
Beside me Singing in the Wilderness—
And Wilderness is Paradise enow.
—Omar Khayyam

A warm, crunchy loaf of fresh-baked sourdough. A relaxing glass of wine to close out a busy day. Neither would exist without yeasts.

Their importance can hardly be overstated. The living organisms are used in baking as leavening agents for many types of baked goods (baker’s yeast) and brewing for fermentation (brewer’s yeast) and in the production of food supplements, dry ice, cheeses, amino fats, vaccines, enzymes, fuels, and vitamins. Commercial yeast products can be found as active dry yeast, instant yeast, rapid rise yeast, nutritional yeast, and compressed yeast. Yet some species of yeast, especially wild yeasts, can also be destructive plant and animal pathogens and cause food spoilage, decreasing shelf life.

Structure and Reproduction

Most yeasts are single-celled fungus, belonging to class Ascomycetes, but some are also found in the Deuteromycetes, the Basidiomycetes, and the Zygomycetes. Most yeasts are unicellular throughout their life cycles, but some produce several buds in succession that remain joined for a while as a pseudomycelium. Some of the mycelial yeasts are dimorphic, having the ability to grow as either yeast cells or mycelia depending on environmental conditions.

A typical yeast cell is oval or spherical with a thin cell wall surrounding the cell membrane. The most prominent feature of the cell is the vacuole. The nucleus is usually in an off-center position. Since yeasts are eukaryotic cells, they have the usual complement of Golgi bodies, ribosomes, endoplasmic reticulum, and mitochondria.

Yeasts reproduce asexually by budding, fission, or arthrospore formation. In a budding yeast cell, enzymes degrade the cell wall, and the protoplast is blown out; a bud subsequently grows into a full-sized cell. Septum formation occurs at the isthmus between mother and daughter cells. In fission, the yeast cell elongates until a septum forms near the middle of the cell, dividing the yeast into two daughter cells. Arthrospores are produced by fragmentation of the hyphae.

Yeasts reproduce sexually by the fusion of two compatible haploid cells. Plasmogamy (fusion of cytoplasm) and karyogamy (fusion of the nuclei) result in a diploid zygote. The haploid ascospores are then produced by meiosis. Ascospore formation is triggered by a lack of food/nutrients and the accumulation of waste products. In the ascospore stage, yeasts can resist drying and other unfavorable conditions, allowing yeasts to overwinter in the soil.

Alcoholic Fermentation by Yeast

Under anaerobic conditions, yeasts obtain energy from sugar, including sucrose, fructose, and glucose by fermentation. The biochemical change effected by the yeast is:

C6H12O6 + yeast —> 2C2H5OH + 2C02

Glucose in the presence of yeast yields ethanol and carbon dioxide gas.

Anyone who has ever baked bread will, no doubt, have noticed the alcoholic odor of the raw bread dough, especially a sourdough or sourdough starter. In the bread-making process, the alcohol evaporates. The liberated by-product, carbon dioxide gas, causes the dough to rise, resulting in the “holes” in yeast breads. Other baked goods like cakes and biscuits rely on the chemistry of baking powder or baking soda for leavening.

Beer, whiskey, wine, and other alcoholic beverages are the products of yeast fermentation of a carbohydrate. These products differ from one another in original material fermented, the type of yeast utilized, and whether the final beverage undergoes a distillation process. Beer, wine, and cider are manufactured by fermentation alone. Commercial alcohol, whiskey, and rum are manufactured by distilling the fermentative product to increase the alcohol concentration. Winemaking requires fermentation of fruit juices, while beer is made from grains, mostly barley. Commercial fermentation processes depend mainly on different strains of Saccharomyces.

Learn more about the Chemistry of Beer.

Figure 1 Carbon dioxide, released by alcoholic fermentation, causes the balloon to expand.
Figure 1: Carbon dioxide, released by alcoholic fermentation, causes the balloon to expand.
Figure 2 Bromthymol blue changes from blue to yellow as carbon dioxide is dissolved in it. Control flask (left foreground) without yeast causes no color change in indicator (left background). Flask containing yeast (right foreground) causes color change in indicator (right background)
Figure 2: Bromthymol blue changes from blue to yellow as carbon dioxide is dissolved in it. Control flask (left foreground) without yeast causes no color change in indicator (left background). Flask containing yeast (right foreground) causes color change in indicator (right background)
Figure 3
Figure 3: Schizosaccharomyces octosporus ascus with the eight ascospores visible
Figure 4
Figure 4: A budding sprout cell of Saccharomycodes ludwigii
Figure 5
Figure 5 Budding Saccharomyces cerevisiae cells

Yeast Classroom Activities

  1. Fermentation by Yeast
    Fermentation is easily demonstrated in the classroom, even at the elementary level. One simple experiment involves letting the students mix a few grains of dry commercial yeast in warm water. Any source of sugar works well: table sugar, molasses, or honey. A small amount of the sugar source is mixed in with the yeast-water mixture. The resultant mixture can be aliquoted into test tubes. A balloon is placed over the mouth of the test tube, which is then left to incubate at room temperature. As carbon dioxide is released, the mixture begins to bubble, and the balloon expands (Fig. 1). The students can smell the odor of alcohol when the balloon has been removed.

    To demonstrate that this liberated gas is carbon dioxide, the gas can be bubbled into a flask containing bromthymol blue, 0.04 percent aqueous. As carbon dioxide is converted to carbonic acid (H2CO3) in the solution, the bromthymol blue indicator changes from blue to yellow (Fig. 2).

  1. Life Cycles of Yeast
    Yeasts are also useful in the classroom to demonstrate the various kinds of life cycles common to eukaryotes.
    1. The haplobiontic life cycle, with its long haploid phase and short diploid phase, is demonstrated by the fission yeast Schizosaccharomyces octosporus. The diploid phase is confined to the zygote. Meiosis occurs in the zygote to form an ascus with eight haploid ascospores (Fig. 3). The ascus ruptures, releasing the ascospores that act as somatic cells. S. octosporus forms ascospores more readily than do most yeasts.
      • Transfer a small amount of the yeast onto a plate of Sabouraud agar or yeast malt agar.
      • Prepare wet mount slides daily over a period of four days to observe fission.
      • To observe ascospore formation, continue to examine the culture over a period of a week.
    2. Saccharomycodes ludwigii exhibits a diplobiontic life cycle with a long diploid phase and a short haploid phase. The haploid phase is confined to the ascospores. A diploid vegetative cell, the sprout cell, undergoes meiosis, forming four haploid ascospores within the ascus. Two of the ascospores within the ascus fuse to form the diploid zygote. The zygote then forms a germ tube that penetrates the ascus wall and acts as a sprout mycelium from which yeast cells bud (Fig. 4). These buds separate from the mother cell and become independent sprout cells, completing the life cycle.
    3. Saccharomyces cerevisiae, the common bread yeast, demonstrates a haplodiplobiontic life cycle. The haploid and diploid phases are equal length. When two haploid cells of opposite but compatible mating types fuse, a diploid cell forms. This diploid cell may bud many times. Under stressful environmental conditions, diploid cells are transformed into asci where meiosis occurs, usually resulting in the formation of four ascospores, two of the “α” mating type and two of the “a” mating type. These haploid cells are liberated and, in turn, begin to reproduce by budding (Fig. 5).
    4. S. cerevisiae is heterothallic, requiring spores of opposite mating types “α” and “a” for fusion to occur. To see these ascospores in the classroom, S. cerevisiae is grown on sporulation medium consisting of 2.5 g tryptose, 0.62 g glucose, 0.62 g sodium chloride, 5 g sodium acetate, and 20 g agar per liter of distilled water. After autoclaving, S. cerevisiae is inoculated onto this agar. After 5 to 10 days’ growth, ascospores can be observed under oil immersion.
    5. In the heterothallic yeast Hansenula wingei, opposite mating types like strains 5 and 21 will agglutinate when mixed. Agglutination greatly increases zygote formation. The zygotes produce large numbers of diploid cells, only a few of which go on to develop into ascospores. This phenomenon can be demonstrated in the classroom. Prepare broth containing 30 g glucose, 7 g yeast extract, and 5 g mono-basic potassium phosphate per liter of distilled water. After autoclaving, one 250-mL flask containing 100 mL broth is inoculated heavily with strain 5 of H. wingei and another with strain 21. The flasks are incubated at room temperature for three days on a rotary shaker. The cultures are then concentrated by centrifugation down to 25 mL. Equal parts of each strain are mixed and centrifuged, along with a control tube for each strain. The sediment in each tube is mixed with a glass rod. The sediment in the control tubes can be easily dispersed, while the sediment in the experimental tubes repeatedly settles out, indicating agglutination (Fig. 6). Also, a concentrated suspension of each mating type can be mixed vigorously on a glass slide to demonstrate agglutination.
    6. To demonstrate the different modes of asexual reproduction, Schizosaccharomyces octosporus, Saccharomyces cerevisiae, and Geotrichum candidum can be used. As mentioned earlier, S. octosporus reproduces asexually by fission (Fig. 7). A wet mount of a 24-hour culture of S. cerevisiae will exhibit many budding cells (Fig. 5). G. candidum reproduces asexually by fragmentation to produce arthrospores (Fig. 8).
  1. Yeast Identification

    Yeasts can be used in the classroom to demonstrate using biochemical tests for identifying microorganisms (Table 1). Since many of the yeasts look very similar morphologically, biochemical tests help to differentiate among them. The clinical laboratory relies on these tests for identifying disease organisms.

    The student is given a challenging problem when asked to differentiate between Candida pseudotropicalis, Hansenula wingei, Saccharomyces cerevisiae, Saccharomycodes ludwigii, and Schizosaccharomyces octosporus because their colonies appear similar. S. octosporus can be differentiated from the others microscopically by asci formation on Sabouraud agar or yeast malt agar, while S. ludwigii is distinguished by the presence of sprout mycelia.

    H. wingei forms a pellicle (a film on the surface of the liquid) when grown in liquid broth. C. pseudotropicalis, S. cerevisiae, S. ludwigii, and S. octosporus all form sediments in broth.

    C. pseudotropicalis and S. cerevisiae can be distinguished by their differing abilities to ferment sugars. When inoculated into Durham tubes, C. pseudotropicalis ferments lactose, dextrose, and sucrose and forms acid and gas. S. cerevisiae will not ferment lactose but will ferment dextrose and sucrose. S. ludwigii and S. octosporus do not ferment any of these sugars (Fig. 9).

Figure 6
Figure 6 Agglutination of opposite mating types of Hansenula wingei. Strain 21 (left), strains 21 and 5 (center), strain 5 (right). All tubes were centrifuged and mixed well with a glass rod
Figure 7 Schizosaccharomyces octosporus demonstrating fission
Figure 8 Geotrichum candidum arthrospores formed by fragmentation of the hyphae
Figure 9 Candida pseudotropicalis ferments lactose. Saccharomyces cerevisiae, Saccharomycodes ludwigii, and Schizosaccharomyces octosporus do not ferment lactose

Table 1 Biochemical Identification of Yeasts

Organism Ascospore formation on Sabouraud agar Sprout mycelia formation Pellicle (P) or sediment (S) formation Lactose (Fermentation) Dextrose (Fermentation) Sucrose (Fermentation)
C. pseudotropicalis
No
No
S
Acid; gas
Acid; gas
Acid; gas
H. wingei
No
No
P
No
Acid
No
S. cerevisiae
No
No
S
No
Acid; gas
Acid; gas
S. ludwigii
No
Yes
S
No
No
No
S. octosporus
Yes
No
S
No
No
No

This article was originally published as “Yeasts in the Classroom” in Carolina Tips®, Vol. 48, No. 10 (print version, October 1985); it was revised May 2026.

References

Further Reading
Alexopoulos, C. S. and Mims, C. W., Introductory Mycology, 3rd edition, John Wiley and Sons, New York, 1979.

Ladder. J. and Kreger-Van Rij, N. J. W. The Yeasts: A Taxonomic Study, North-Holland Publishing Company. Amsterdam, 1967.

Monmaney, T., Microbes for hire, Science, 1985, 85, 30-46.

Neiman, AM 2005. Ascospore Formation in the Yeast Saccharomyces cerevisiae. Microbiol Mol Biol Rev 69:. https://doi.org/10.1128/mmbr.69.4.565-584.2005

Diane M. Calabrese, Ph.D.

22 Anderson Avenue,

Columbia, Missouri 65203-2673

About The Author

Carolina Staff

Carolina is teamed with teachers and continually provides valuable resources–articles, activities, and how-to videos–to help teachers in their classroom.

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