Optimal conditions of ethanol production in yeast fermentation process
Joanna Bąk, Joanna Kasprzak
II Liceum Ogólnokształcące im. Tadeusza Kościuszki w Kaliszu
Introduction
Precious yeast, belonging to species
Saccharomyces cerevesiae have big
economic importance. They are used in
food industry, mainly fermentative.
Yeast’s ability to sugar fermentation, e.g.
glucose and production of ethyl alcohol
and carbon dioxide is used in production of
wine, beer, other drinks, also bread.
Fermentation is a anaerobic chemical
process, which consists of enzymatic
desintegration of sugars. Microorganisms
or produced by them enzymes are
necessary to the course of fermentation. In
yeast fermentation process glucose by
means of yeasts is transformed into ethyl
alcohol and carbon dioxide. It is
accompanied by emission of thermal
energy. Special species of yeasts are able
to produce even about 21% alcohol,
baker’s yeast, available in food shops, are
able to use maximum about 14% alcohol.
The progress of yeast fermentation
process
C6H12O6 ---> 2C2H5OH + 2CO2 + 24 kcal
Theoretically from this reaction it is clear
that every 180,5g sugar will give 92,1g
ethyl alcohol (or 100g sugar 51,1g
ethanol). However, in practice yeast use up
for other fermentation products, e.g.
for breathing process, growth of biomass.
A part of ethanol leaks with bubbling CO2.
Therefore, it is assumed that practical
productivity amounts to about 90% of
theoretical productivity, i.e. from 1,7 kg of
sugar we obtain 1 litre of ethyl alcohol.
.
Experimental procedure and approach
We added 270g of glukose to 1,5l of
destilled water. In that way we received
18% solution.
We poured obtained solution to three
plastic 0,5l mineral water bottles, filled
them ¾ full.
Then we added 40g compressed fresh
baker's yeast to each bottle. All bottles
were shaken for a few minutes. After that
each bottle was filled right up to the top
with the glucose solution. Then we labelled
every bottle and fitted balloons over the
necks of bottles. Bottles were placed in
different laboratory incubators which were
set at temperatures: 20, 35 and 40ºC. Each
bottle we left there for two days in constant
temperature. After that we took bottles out,
carefully stringed and removed balloons
from them and weighed the balloons. From
obtained masses of balloons we subtracted
masses of empty balloons. That is how we
calculated masses of obtained CO2. Using
filter centrifuge we cleaned content of
individual bottles from yeast to measure
amount of ethanol in individual bottles.
Conclusions
Measuring alcohol concentrations in
solutions we found out that yeast
fermented glucose in yeast fermentation
process. One of the products of this
reaction si the ethanol.
Results
Introducing to every filtered solutions
device measuring amounts of ethanol, we
measured amount of produced ethanol. We
have presented results in a table.
Temperatura
20C
35C
40C
Stężenie etanolu w
roztworze (%)
4
9
6
Table 1: The concentrations of ethanol in
particular solutions.
Weighing individual balloons with CO2,
we obtained the following results:
Temperatura
Masa CO2 (g)
20C
1,6
35C
4,8
40C
1,9
Table 2: mass of produced CO2 in individual
temperatures.
Most CO2 was produced in temperature
35ºC, least in temperature 20ºC.
Explanation
Yeasts are live organism, too high
temperature causes their death. Maximum
temperature which kills these mushrooms
is about 40ºC. But content of ethanol
causes that yeat become more sensitive. At
concentration of 14% alcohol they are
extinct in 33C, and at concentration of
20% alcohol in 25C. In 40C and with
sensivity for ethanol yeast was weakened,
and the most ethanol they produced in
35C – is the closest temperature to
optimal 37C, which is the best
temperature to growth of yeast as a living
organisms.
Apart from oxygen and azote, yeast needs
other nutrients, like : vitamins from group
B, mineral matters (magnesium, potassium,
calcium, sodium). These components are
called activators of fermentation process,
i.e. substances improving the course of this
process. The opposite to the activators are
inhibitors – substances affecting negative
the course offermentation process. Main
inhibitors in fermentating setting is ethyl
alcohol and sulfur compounds, carbon
dioxide, tannins, volatile acids (acetic acid)
and some heavy metals (mainly copper).
A time of fermentation depend largely on
species of yeast and a temperature and may
be from 24 hours to several or more days.
.
As we have already mentioned, the biggest
possible to obtain ethanol concetration
amounts to about 21%. In order to obtain
it, indispensable are however, specially
selected yeast with great tolerance for ethyl
alcohol.
Baker’s yeast can produce up to 14% of
alcohol. In our experiment we have
obtained maximum 9% of alcohol.
There may be a lot of reasons of that.
Among others a percent of ethanol which
we obtained depended on proportions of
used water and sugar. Too big sugar
concentration affects negative productivity
of yeast’s work. Theorethically, we
shouldn’t exceed amount 300g per 1 litre,
but in practice it is worth reducing it a bit
more and assume that limit amounts to
200-250g per litre, i.e. about 20%. Thus
concentration of glucose in our experiment
was correct, though it oscillated around
upper limit.
The only fermentation activators in our
experiment were oxygen and azote, which
were present only at the beginning. Then,
they were pushed out into a balloon by
carbon dioxide and they weren’t still
supporting the fermentation process. In our
experiment ethanol and carbon dioxide
were present, too. They were retarding
fermentation process, consequently
producing ethanol, too.
Another reason may be time necessary to
produce ethanol. It largely depends on
yeast species and the temperature and
amounts to from 24 hours to several or
more days. At beginning phase there are
little yeast cells, but they are in good
condition and have great conditions to live.
Loss of sugar associated with that is
minimal ( about 5g/ litre). When yeast cells
achieve population
number amounting to about 100 millions in
a millimetre, completely capturing
environment, as a matter of fact they stop
pullulating. Henceforth, until the end of the
fermentation process only already existing
yeast cells busily convert sugar in alcohol
Yeast of the species Saccharomyces
cerevisiae, both wild and precious, reveal
similar fermentation dynamics. That
process stops after six days. Our bottles
were left in a thermostat for two days.
Another factor is pH of a solution. Optimal
pH for the yeast fermentation process is
4,5-4,7. We didn’t estimate which pH
reaction our solution had. Summarizing, in
our experiment we didn’t obtain the
maximal concentration of ethanol, because
we hadn’t provided culture medium,
optimal pH reaction and maximal time for
growth of our yeast. Conentration of sugar
and ethanol in our solution were correct
(didn’t disturb fermentation process).
Temperature differed from optimal by
about 2C.
Bibliography:
1) E. Solomon, L. Berg, D. Martin
„Biologia”
2) G. Kupryszewski „Wstęp do chemii
organicznej”
3) www.wikipedia.org
4) www.bryk.pl
5) www.biotechnologia.uni.opole.pl
6) www.ar.krakow.pl
7) "Yeast and fermentation: the optimal
temperature" - Slaa, J., M., & Else, H.
Vrije Universiteit Amsterdam, The
Netherlands (October 2009)