Molasses Lab: A Study of Yeast Fermentation
Background Information: The yeast that you buy in a store contains living organismsinvisibly small ones, microorganisms. As long as they are kept cool and dry, they are not
active. But when they are given food, moisture, and warmth, they become active and do
many of the things larger organisms do.
Introduction: When we observe the results of feeding molasses to yeasts, we may raise
many biological problems. One of these has to do with the relationship between the
amount of food provided and the rate of yeast activity. We might hypothesize that the
less the amount of food, the less active yeast would be or in the form needed for an
experiment. If we reduce the amount of food given to yeasts, then they will be less active.
To test this hypothesis we need to set up a number of yeast cultures- groups of yeasts
growing under conditions that are known to be favorable for them. The cultures must all
be set up in the same way except for one thing—a Variable. Our hypothesis tells us what
the variable must be: We must vary the amount of food.
But how are we to make meaningful observations on yeast activity? We must have
some way to measure it. Among the activities that all organisms carry on is the giving off
of wastes. In yeasts, one of these wastes is a gas that can easily be collected. By
measuring the amounts of gaseous waste, a biologist can determine how rapidly the
invisible organisms are carrying on their activities.
.
Molasses, which you can also buy in a store, is a mixture of substances that are
obtained from a kind of giant grass plant, sugarcane. Through the substances do living
organisms, the sugarcane, make not alive they were plants. They are called organic
substances. Organic substances as well as organisms themselves are important to the
work of biologists. The organic substances in molasses can be used to feed the yeast.
Materials and Equipment: Per Lab Group
5 culture test tubes
Masking tape (labeling)
Molasses solution
Medicine dropper
5 fermentation tubes
Ruler in millimeters
test tube rack or beaker
50 ml graduated cylinder
small beaker (for water)
yeast suspension
cotton to plug
graph paper
Procedure: READ this carefully!
With glass marking pencil (or masking tape) number the large culture tubes from 1 to
5 and adds your team symbol or initials.
Using a graduated cylinder, measure ______ ml of molasses solution. Pour the
solution into Tube 1. Then measure 25 ml of molasses solution and add to it 25 ml of
water. Mix the 50 ml of diluted molasses solution thoroughly by holding the palm of your
hand over the top of the cylinder and shaking it. Pour ______ml of this 1st solution into
Tube 2. Now pour off the remaining diluted solution into a clean small beaker until 25ml
is left in the graduated cylinder. (BE CAREFUL… if you pour out too much adjust)
Next CAREFULLY add water back up to the 50 ml line of the graduated cylinder
containing the 25ml of diluted solution. Mix it again and pour_______ml of this diluted
solution into Tube 3. Again discard some of the remaining solution until you have
reached 25ml again. In the same way as before, make a 3rd dilution for Tube 4 and then a
fourth dilution for Tube 5. (NOTE: YOU NEVER DUMP OUT THE MOLASSES
SOLUTION IN THE 50 ML CYCLINDER)
Using a medicine dropper, add 10 drops of a yeast suspension to each of the 5 tubes.
CAUTION: Be sure to shake the yeast suspension thoroughly before you take out each
dropperful. After you have added the yeast suspension, shake each tube, holding your
thumb over the mouth of the tube. Into each tube place one of the small tubes upside
down. Remove bubbles of air from the small tubes by tilting the large tubes. (See figure
below). Plug each tube with a piece of cotton.
The next day examine the small tubes for the presence of gas. Using a ruler, measure the
length of the column of gas in each small tube. In your data book make a chart and record
the amount of gas in each small tube. Be sure to identify each tube by the number on the
tube.
Analysis: To answer the first analysis question you must remember the following
information; the molasses solution you used in Tube 1 was made by mixing 25ml of
molasses with 75ml of water---which is then a 25% solution of molasses.
1. Recalling the way you made your dilutions, what was the percentage of molasses in
each of your tubes?
2. Why do you think it is important to shake the yeast suspension just before you add
some to the tubes?
3. Millimeters are units of length but a gas occupies a volume. Why are millimeters
acceptable in this case to measure amounts of gas?
4. Scientists often put their numerical data into the form of a Line graph. On a piece of
graph paper, mark off a millimeter scale on the vertical axis and a percentage scale on
the horizontal axis. Plot your data on the gird and connect the points. (Class averages
for each tube can be collected and plotted using a different color pen.)
Conclusions:
5. Data from an experiment can either (a) support the hypothesis (b) fail to support the
hypothesis (c) do neither very clearly. Which of these 3 choices best describes your
data from this experiment. DEFEND your answer with data.
6. How could you obtain verification of your data?
7. If you were reporting this experiment to other scientists, what information would you
include?