Detection of Gas Production by Yeast


Adapted from Bringing Life to Microbiology, an ASM Educational Publication


To demonstrate fermentation of different sugars by yeasts.


To observe the microscopic appearance of yeast cells.


Yeasts are microscopic fungi capable of fermenting different carbohydrates. In the process, carbon dioxide and ethyl alcohol are given off. Various industries, including the baking and brewing industries depend on selected yeast strains to add flavors, odors, and carbonation to their products.

Most yeasts reproduce asexually by budding, a process which involves the equal splitting of an organism's nucleus, but an unequal division of its cytoplasm. The development of the smaller cell or "bud" can be seen on one side of the parent cell.

The fermentative and microscopic features of the yeast Saccharomyces cerevisiae will be demonstrated in this exercise. The use of controls and variables in scientific investigation is also evident


Materials per pair of students:

  1. A plastic tube containing 10ml of sugar solution (Ten % sugar solutions or different kinds of juice are all useful.)
  2. A balloon that fits over the top of the tube
  3. One packet of baking yeast
  4. A 1/8 teaspoon or other means of measuring dehydrated yeast
  5. One wax marking pencil
  6. A piece of string
  7. A ruler or measuring tap

Materials for the class:

  1. Incubators

Optional Materials:

  1. Microscopes
  2. Microscope slides and methylene blue stain


  1. Record initial appearance of the liquid in each tube.
  2. Add 1/8 teaspoon of dehydrated yeast to each tube, replace cap and mix by inverting the tube.
  3. Remove the top and place a balloon over the top of each tube. Measure the circumference of the balloon in millimeters using the string and ruler. To determine the circumference for a deflated balloon, fold or roll up the balloon to minimize its volume.
  4. Incubate the tube at the conditions specified by the instructor.
  5. Observe the tubes at two and four hour intervals. The balloon will begin to inflate if gas is being produced. Measure the circumference of the balloon using the string and the ruler and record results on the chart.
  6. Reincubate the tubes.
  7. Observe at intervals. Measure and record findings.
  8. At the end of the specified incubation, make final observations and record final results.
  9. Remove the balloon from each tube and subject tubes to olfactory tests. Record odors detected and the appearance of the liquid in the tube.


  1. Use an inoculating loop to remove a sample from each tube. Spread the sample on a clean microscope slide.
  2. Add a tiny amount of methylene blue to the sample and mix thoroughly. Drop a cover slip over the stained sample.
  3. Observe microscopically at 10x, 40x, and 100x, if possible. Record observations by sketching representative views.

Yeast Fermentation of Carbohydrates


1. Complete the Table (Please record measurements in millimeters.)
Cir. 1 hr

Cir. 2 hr

Cir. 6-8 hr

Cir. 24 hr






Apple Juice

Grape juice

Cranberry Juice

2. Incubation temperature:__________________

3. Which sugar(s)/juice(s) were fermented?

4. When did fermentation stop?

5. In which carbohydrate/juice was the most gas produced?

6. List two commercial applications of yeast fermentation?

7. Which fermentation product is most important in each industry?