How to Measure Yeast Growth

Two Methods:Measuring Gas OutputDirect Observation

Yeasts are a type of fungus and are used to make bread and bread products and fermented beverages such as beer and wine. When yeast has the nutrients necessary for growth, such as sugar, it produces a natural gas, carbon dioxide, as a by-product of that growth. Yeast will stop growing when the nutrients are used up or the substance that the yeast is in, such as liquid, becomes too acidic and destroys the living cells. Yeast growth can easily be measured by performing an experiment using common household items or, if a laboratory is available to you, using laboratory equipment to directly measure yeast numbers.

Method 1
Measuring Gas Output

  1. 1
    Obtain several bottles of soda or water, depending on how many substances you wish to test.
  2. 2
    Label each bottle for contents, treatment and control.
  3. 3
    Drop 1 tsp. (4.7 g) of dry yeast in each bottle.
  4. 4
    Cap or otherwise seal each bottle tightly.
  5. 5
    Shake each bottle.
  6. 6
    Remove the caps or seals from each bottle.
  7. 7
    Place a balloon over each bottle's mouth, stretching so that it fits snugly and there are no leaks.
  8. 8
    Warm each bottle by placing them atop a warm surface such as the refrigerator, but do not place them in direct sunlight.
  9. 9
    Wait for 15 to 20 minutes or until the balloons begin to inflate.
  10. 10
    Measure the diameter of each balloon by either using a string or tape measure.
  11. 11
    Write down the time elapsed since first placing the bottles in the warm area.
  12. 12
    Continue to make additional measurements in 10-minute intervals and record the balloon diameter and time elapsed for each.
  13. 13
    Add different substances to some of the bottles, such as sugar or vinegar, and continue to measure the results comparing them with each other and the control bottle.

Method 2
Direct Observation

  1. 1
    Place 0.03 oz. (1 ml) of grape juice in each of 2 17 oz. (500 ml) beakers.
  2. 2
    Place a spatula of yeast into each beaker.
  3. 3
    Stir the contents of each beaker to mix the contents.
  4. 4
    Cover 1 beaker with paraffin; leave the other open.
  5. 5
    Place each beaker in an incubator at 77 degrees Fahrenheit (25 degrees Celsius).
  6. 6
    Remove some yeast from a beaker with a pipette.
  7. 7
    Place 0.03 oz. (1 ml) of the removed yeast into a measuring cup with a lid.
  8. 8
    Dilute the yeast by add enough liquid to make 3.4 oz. (100 ml); this a 100-fold dilution.
  9. 9
    Transfer a few drops of the diluted mixture onto a Rafter cell slide.
  10. 10
    Cover the slide carefully with a cover slip.
  11. 11
    Place the slide under a microscope.
  12. 12
    Examine the slide under low power first so as to not damage or heat up your yeast.
  13. 13
    Examine again under slightly higher power.
  14. 14
    Focus and find the yeast by locating the dots that appear round or oval.
  15. 15
    Count the dots within the squares of the slide.
  16. 16
    Record your counts and time elapsed.
  17. 17
    Calculate the density using the following formula: Number of cells per 0.03 oz. (1 ml) = average dots per slide square times 1 million x the dilution factor (100 if diluted once per above instructions).
  18. 18
    Perform this experiment several times to count different densities and growth over time from both the covered and uncovered beakers to compare yeast growth with and without oxygen.

Things You'll Need

  • Measuring tool (tsp. or g)
  • Marking pens
  • Active dry yeast
  • Several bottles of soda or water
  • Balloons
  • Sugar, vinegar, grape juice and/or ammonia
  • Microscope
  • Counting (Rafter) cell(s)
  • 2 17-oz. (500 ml) beakers
  • Incubator
  • Liquid paraffin
  • Pipettes
  • Stirring rods
  • Lab spatula
  • Paper, pencil or pen
  • Calculator
  • Safety goggles

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Categories: Science