Yeast and fermentation: the best kind of sugar
Yara Veenstra & Caroline Vuong
12 April 2010
Summary
Because the fossil fuels run out we have to find new fuel sources, for instance you can use ethanol.
Ethanol, together with carbon dioxide, is produced by fermenting sugar with yeast. This process is
called fermentation. Fermentation occurs in an oxygen free environment.
In the fermentation process you can use different kinds of sugar.
We made an inquiry where we tried to find the way to get as much ethanol as possible by using
different kinds of sugar.
We fermented sucrose with baker’s yeast and fructose with baker’s yeast. We found out that sucrose
produces a greater amount of carbon dioxide than fructose.
Introduction
Ethanol fermentation is a biological process in
which sugars are converted into cellular
energy and carbon dioxide and ethanol.
This process is for example responsible for the
rising of bread dough. There’s no alcohol in
baked bread, because all the ethanol
evaporated. Ethanol fermentation is also
important for wine-making.
Yeast cells are living
organisms who convert
sugar into carbon dioxide
and ethanol. They use
sugars as source of energy,
so they don’t need the sun to grow. Yeasts are
eukaryotes and classified in the kingdom of
Fungi.
Saccharomyces cerevisiae cells are known as
baker’s yeast or brewer’s yeast. In the alcohol
industry these organisms have been used to
ferment the sugars of rice, wheat and corn to
produce alcoholic beverages. In the baking
industry they’ve been used to expand or raise
dough. Commonly it’s used as baker’s yeast
and for some types of fermentation.
In this experiment we use S. cerevisiae cells.
Those cells use three major pathways to grow.
The first pathway is the fermentation of
glucose:
C6H12O6 (s)  2CH3CH2OH (l) + 2CO2 (g)
Second, the oxidation of glucose:
C6H12O6 (s) + 602 (g)  6CO2(g) + 6H2O(l)
Third, the oxidation of ethanol:
CH3CH2OH(l) + 3O2(g)  2CO2(g) + 3H2O(l)
If you look at these three pathways, you can
see that S. cerevisiae cells can grow in an
oxygen free and in an oxygen rich
environment. The first pathway is the most
interesting for our inquiry, because that
involves the production of ethanol and carbon
dioxide.
Sucrose, commonly known as table sugar, is a
disaccharide which first must be converted to
fermentable sugars . In figure 1 is sucrose
hydrolyzed to glucose and fructose by
invertase, an enzyme in yeast.
There’re different kind of sugars: sucrose, Dfructose etc. This raises the question:
with what kind of sugar (sucrose or fructose) is
the amount of produced ethanol the highest in
an oxygen free environment?
Our hypothesis is that with sucrose the
amount of produced ethanol is the highest,
because with sucrose the yeast cells have to
convert glucose and fructose. Sucrose has to
be hydrolyzed first. By using fructose the
yeasts only have to convert fructose.
Figure 1: Sucrose hydrolyzed to glucose and fructose
Experimental design
Before conducting the experiments we
calculated how much CO2 gas would be
produced. We assumed that both experiments
would have produced 80 ml of CO2. We put
the fructose solution in a flask of 1 liter, we
weighted the fructose with a scale to 3
significant digits. Then we divided the fructose
solution in three bottles of 250 ml. That way
the contents were the same and the results
more accurate. During the sucrose experiment
we made three separate solutions, we
weighted the sucrose with a accurate scale to
4 significant digits. Then one gram baker’s
yeast was added to each bottle. The yeast was
weighted with the scale to 4 significant digits.
To keep the temperature the same while
conducting the experiment we placed the
bottles in a tank with water at 35°C. We used
a gas measurement instrument to measure
the amount of produced carbon dioxide. To
keep the environment factors the same we
conducted the two experiments at the same
location. Also we did each experiment in
triplicate. That way we can be sure that the
measurements are reliable. We assume that
80 ml of CO2 will be formed.
the amount of gas didn’t rise anymore. The
results of the experiment is presented in table
1. On another day we did the fructose
experiment. The results of this experiment is
presented in table 2.
Sucrose experiment
Number Amount of
of bottle sucrose in
grams per
250 ml
Amount
of used
yeast in
grams
Produced
carbon
dioxide in
ml
Bottle 1
Bottle 2
Bottle 3
1,024
1,032
1,036
80
77
86
1,005
1,058
1,080
Table 1: Produced carbon dioxide (in ml) using sucrose
Fructose experiment
Number Amount of
of bottle sucrose in
grams per
250 ml
Amount
of used
yeast in
grams
Produced
carbon
dioxide in
ml
Bottle 1
Bottle 2
Bottle 3
1,020
1,008
1,004
62,5
60
62,5
0,28
0,28
0,28
Table 2: Produced carbon dioxide (in ml) using fructose
Data analysis
The temperature of the gas measurement
instrument was at room temperature.
That’s why we used the molar volume at 25ºC
= 298 K.
The molar volume = 2,45·10-2 m3 mol-1 =
24,5 L ·mol-1 = 24,5·103 mL ·mol-1.
Results
The experiment took two days. First we did
the sucrose experiment. Within a day we
observed that the amount of produced carbon
dioxide started to rise. At the end of the day
By using sucrose is:
- 80 mL carbon dioxide equal to
80mL/24,5·103 mL ·mol-1 = 3,3·10-3 mol CO2;
- 77 mL carbon dioxide equal to
77mL/24,5·103 mL ·mol-1 = 3,1·10-3 mol CO2;
- 86 mL carbon dioxide equal to
86mL/24,5·103 mL ·mol-1 = 3,5·10-3 mol CO2;
The average maximum amount of produced
CO2 is 3,3·10-3 mol CO2.
The formula of the fermentation of glucose:
C6H12O6 (s)  2CH3CH2OH (l) + 2CO2 (g)
The mol ratio 2CH3CH2OH : 2CO2= 2:2 = 1:1
So there’s 3,3·10-3 mol CH3CH2OH formed by
using sucrose.
By using fructose is:
- 62,5mL carbon dioxide equal to
62,5mL/24,5·103 mL·mol-1 = 2,6·10-3 mol CO2;
- 60,0mL carbon dioxide equal to
60,0mL/24,5·103 mL·mol-1 = 2,4·10-3 mol CO2;
- 62,5mL carbon dioxide equal to
62,5mL/24,5·103 mL·mol-1 = 2,6·10-3 mol CO2;
The average maximum amount of produced
CO2 is here 2,5·10-3 mol CO2.
So there’s 2,5·10-3 mol CH3CH2OH formed by
using fructose.
Conclusion and discussion
By using sucrose there is more carbon dioxide
produced than by using fructose. If the
amount of carbon dioxide is higher than the
amount of ethanol is also higher. This
concludes that you get the highest amount of
ethanol by using sucrose in the fermentation
process. The average of the produced carbon
dioxide using sucrose is 81 ml. That’s more
than we expected.
However the amount of produced ethanol
using fructose is less than 80 ml.
For the production of ethanol, using sucrose is
more effective than using fructose.
This conclusion raises the question what other
variables are having an influence at the
produced ethanol.
Evaluation
First we did an inquiry in which we tried to
find the optimal pH-values, however the
experiment completely failed. We used
balloons to measure the amount of CO2, but
after a while the balloons started to shrink.
The cause for the shrinking was probably the
heat of the bottles, which were kept at 35 °C.
After the failed experiment we started
another inquiry and used a different way of
measuring the amount of CO2. This time we
used three gas measurement instruments
instead of the balloons. The gas measurement
instrument are much more reliable and
accurate since the CO2 is unable to get out of
the gas measurement instrument.
The temperature of the bottles was different
from the temperature of the gas
measurement instruments. The temperature
of the bottles was kept at 35°C while the gas
measurement instruments were at room
temperature. This might have influenced the
results. Also since the room temperature
wasn’t exactly the same when conducting the
experiments, the results might have been
affected.
When weighing the fructose we used a less
accurate scale than for weighing the sucrose.
This may have lead to a less accurate measure
of fructose, which may have had influence on
the results. However because we filled the
three bottles of the fructose experiment with
the same fructose solution the concentration
fructose was exactly the same. So it was more
accurate than the sucrose experiment in
which we put the sugar individually in the
three bottles. Also the yeast was put
individually in all of the bottles, so not every
bottle had the exact same amount of yeast.
The two experiments weren’t conducted at
the same time. That was due to the gas
measurement instruments. We had just three
gas measurement instruments and couldn’t do
all the experiments at the same time. So there
might have been a difference in the
environmental factors which might have
affected the measurements. We turned the
gas measurement instrument off when we
thought there would be no more produced
CO2. This might have resulted in a less amount
of CO2 then what would have been produced
if the gas measurement instrument was
turned on for a longer time.
This concludes that our inquiry still has much
room for improvement.
Bibliography
1. BINAS havo/vwo, vijfde druk.
2. Practical Assignment Chemistry,
Fermentation
3. http://www.microbiologybytes.com/vi
deo/Scerevisiae.html
4. http://www.yeastgenome.org/VLwhat_are_yeast.html
5. www.sigmaaldrich.com/.../enzymatickits.html
ICD9_Yara_Caroline