CH 465 Biochemical Engineering
Spring 2009
Baker’s Yeast Fermentation
Introduction
The objective of this experiment is to learn about the kinetics of cell growth by
examining the growth rates of baker’s yeast.
A Zulauf or fed-batch process is generally employed by the baker’s yeast industry.
Nutrients and sugar are fed incrementally to the fermenter at an increasing rate during the
growth period so that the amounts of nutrients and sugar per cell remain roughly
constant. The temperature and pH are maintained at constant values throughout the
process. The concentration of cells is measured as a function of time.
If all of the sugar supply is added initially with the yeast, the yeast follows the typical
growth curve for batch cell cultivation: a lag phase, exponential growth phase, stationary
phase, and death phase. If, however, the yeast is activated and the sugar fed
incrementally, only the exponential growth phase is observed.
Apparatus
The fermentation apparatus consists of a jacketed fermenter with a stirrer, a constant
temperature circulation bath and an air pump. The total volume inside the jacketed
fermenter is 2 L. The working volume (i.e., the volume into which we can place the
fermentation broth) is 1.5 L. A spectrophotometer is used to measure the yeast
concentration. A calibration curve that relates cell concentration to the percent light
transmitted is attached. The wavelength has been set to approximately 660 nm. Do not
change the wavelength of turn off the spectrophotometer while you are conducting the
experiment.
Experimental Procedure
Lin Wu will prepare a nutrient medium for each group. The composition of the medium
is:
 10g/L ammonium sulfate
 6g/L potassium phosphate
 3g/L magnesium sulfate
 0.1 g/L calcium chloride
 0.1 g/L sodium chloride
 A multivitamin
First, 1000 mL of a solution containing the salts is placed in the reactor. Then 100 mL of
a solution of the multivitamin is added to the fermenter and brought to an appropriate
temperature (e.g., 37 C). Then, 100 mL of a suspension containing 0.8 grams of yeast is
prepared and added to the reactor. You can then measure light transmittance of the
suspension. Next, you will also need to prepare a 50 wt% of table sugar in water to add to
the reactor to initiate the fermentation; the sugar solution will contain 5 grams of sugar.
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After adding the sugar solution, turn on the air bubbler by plugging into the outlet (there
is no on/off switch). At equally spaced time intervals, samples should be collected and
analyzed to determine the yeast concentration. You can recalibrate the spectrophotometer
before each sample measurement using water. With water in the cuvette, press the
calibrate button and %T will be set to 100%. It is important to use the same cuvette for all
of your measurements to obtain consistent results. You should wash the cuvette out after
each measurement.
The total volume of the liquid suspension will be 1.2 Liters; this corresponds to 100 mL
of yeast suspension, 100 mL of multivitamin dissolved in water, and 1000 mL of water
with the salts dissolved in it. We will already have the 1000 mL of water and salts in the
reactor and at a temperature close to 37 C. We have a Mettler balance near the apparatus
that can be used to measure the amounts of yeast and sugar. The motor speed is set to 617
rpm. Please don’t change the motor speed.
The calibration curve relating yeast concentration to logarithm of % light transmitted
follows a linear relationship (Beer’s Law) for concentrations up to ~0.5 wt% (% light
transmitted > 2.5%. If the yeast concentration becomes greater than this (i.e., the % light
transmitted < 2.5%), you can dilute the collected sample with a known amount of
medium accurately monitor yeast concentration. The calibration curve is shown on the
following page along with a least squares fit.
Data Analysis
The collected data should be used to determine the specific growth rate during the
exponential growth phase, the yield and the doubling time. A growth curve (yeast
concentration as a function of time) should be prepared. By using the glucose meter,
monitor the sugar concentration as a function of time and calculate a yield coefficient.
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