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Baking Bread

Dissolving Yeast

VOCABULARY TO LEARN:

“YPD” = rich yeast growth media, like chicken soup for yeast.

 

  1. Check the expiration date of baker’s yeast and make a note of the date in your lab notebook.
  2. Label the flask with labeling tape and write “baker’s yeast” and your initials and today’s date on the label.
  3. Open the package of baker’s yeast and pour the contents into the sterile flask.
  4. Examine the bottle of YPD to confirm that it is clear and nothing fuzzy or cloudy is growing in it.
  5. Using the conical tube and sterile technique, measure 50 ml of YPD and pour the media into the flask.
  6. Repeat so the volume of YPD in the flask is 100 ml.
  7. Replace the cover of the flask.
  8. Place on the stir plate and set to stir gently until yeast is dissolved.
  9. Make notes on appearance of yeast solution at time = 0’, time = 1’, time = 5’, time = 15’

Subculturing Yeast

VOCABULARY TO LEARN:

“subculture” = to dilute a densely grown population of cells into fresh media to allow for active growth and to produce more cells.

 

  1. Label the flask with labeling tape and write “VitaYeast” and your initials and today’s date on the label.
  2. Mix the starting liquid overnight culture so it’s homogeneous. This can be done by inverting if the cap is tight, or by flicking or by vortexing if there is a vortex available.
  3. Examine the bottle of YPD to confirm that it is clear and nothing fuzzy or cloudy is growing in it.
  4. Using the conical tube and sterile technique, measure 50 ml of YPD and pour the media into the flask.
  5. Repeat so the volume of YPD in the flask is 100 ml.
  6. Using sterile technique, remove 1 ml of the overnight culture of yeast using your 5 ml serological pipet attached to a pipet bulb or pipet aid.
  7. Transfer the yeast into the 100 ml of YPD media in the flask.
  8. Dispose of the serological pipet in bio-hazardous or sharps waste.
  9. Replace the cover of the flask.
  10. Place on the stir plate and set to stir gently overnight.
  11. If there is time you may want to measure the starting density of the culture using 1 ml in a cuvette and a spectrophotometer set to 600 nm. Blank the spectrophotometer with YPD.

Serial Dilutions

  1. To prepare a series of 1:10 dilutions, you should add 900 ul of water to the cuvettes you’ll be using. If you are making these dilutions in glass tubes, you may need 2.7 ml of water. These volumes can be measured with a serological pipet and a bulb.
  2. Make sure that your starting liquid culture or solution is homogenous. You can mix by inverting if the cap is tight, or by flicking or by vortexing if there is a vortex available.
  3. Place a tip on the end of your pipetman and then remove 100 ul of the culture or solution and place it in the first tube with 900 ul of water (or remove 300 ul of culture if diluting to 2.7 ml water).
  4. Dispose of the pipet tip into bio-hazardous or sharps waste.
  5. Label that dilution “1:10” and then invert or gently vortex to mix completely.
  6. Repeat steps 3, 4 and 5, moving the 1:10 dilution to a fresh tube or cuvette with water to make a 1:100 dilution.
  7. Repeat using the 1:100 dilution to make a 1:1000 dilution.
  8. Using a tube or cuvette with water to blank the spectrophotometer, measure the optical density or absorbance of each dilution. Q: do you expect each to be precisely 1/10th of the last? Why or why not?

You may be asked to follow up this experiment with some statistics and analysis of your data.

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