STUDENT
PRACTICAL SHEET
NAME
FORM
DATE
Investigating pH and soft drinks
Introduction
Aim
Fruit drinks can contain more than juice . Food scientists can
add natural chemicals to change the pH of the drink so that
microbes are less likely to grow. Microbes can make the drink
unpleasant or unsafe to drink.
Citric acid is a natural chemical found in lemons which can be
used to lower the pH. Baking powder is safe to eat and can be
used to increase the pH.
To investigate the effect of changing pH on the growth of a
microbe in a chilled drink.
Equipment
1x boiling tube rack
Warm water bath (set at 40 °C)
Limewater (calcium hydroxide)
Activated yeast solution
4x bungs and delivery tubes
Stop watch
pH meter
Grape juice
2x Spatula
Citric acid powder
Baking powder
Splash proof eye protection
Safety: Splash proof eye protection must be worn
as limewater is an irritant.
See CLEAPSS Student Safety Sheet 32.
Image: Krones
Method
Questions
1. Using the 10cm3 measuring cylinder, put 5cm3 of limewater
into each test tube and place in a test tube rack.
1. What is the independent variable in this experiment?
2. Using the 50cm3 measuring cylinder add 20cm3 of grape juice
into each boiling tube and place in the boiling tube rack.
2. What is the dependent variable in this experiment?
3. Label each boiling tube A to C. Then add 5cm3 of activated
yeast to each tube. To test tube A add 1 spatula of citric acid and
to test tube C add 1 spatula of baking powder.
3. What are the control variables in this experiment?
4. Measure and record the pH of the liquid in each boiling tube.
5. Put the bung onto the boiling tubes and make sure that the
delivery tube end is going into the limewater. Ensure that the
boiling tubes are in the warm water bath.
6. Record how long it takes for the limewater to turn cloudy.
4. What microbe are we using in this experiment?
5. What do you predict will happen in this experiment?
6. Are your results reliable? Yes / No / Don’t know
(delete as appropriate).
Explain your answer.
Results
Tube
pH
Time (minutes)
A
B
C
Conclusion
Fill in the missing words:
Yeast is a fungus which is a type of …………………... It can be used to find out the effect of
…………………... on the growth of microbes in chilled drinks.
Yeast grew best in pH values closest to pH 7.
Grape juice is quite acidic with a pH value of ………………….... By adding citric acid, the
taste is not changed and the pH is …………………... reducing the growth of microbes and
increasing the shelf life. When baking powder is added the pH is …………………... .
The baking powdered will react with some of the natural acids in the grape juice and will affect
the ………………….... and appearance of the product.
© Chilled Food Association. This document may be copied freely for educational use.
TEACHERS
NOTES
Investigating pH and soft drinks
Teaching ideas
Allow students to set up the experiment and then
encourage them to complete the questions while
the yeast is fermenting. To increase the production
of carbon dioxide the yeast mixtures could be put in
a water bath set at 40 °C. This could be made with a
250cm3 beaker half filled with warm tap water.
Grape juice has a pH of about 3, with citric acid this
is lowered to about 1 and with baking powder raised
to about 7. At low pH values, the yeast will not respire
and the limewater will not change. With just grape
juice there is some fermentation occurring, and the
limewater is likely to change. Neutral solutions, the
colour of red grape juice will change (as this contains
anthocyanin a natural acid base indicator). Initially
effervescence will be observed as the citric acid reacts
with the baking powder. Then the yeast will respire
readily causing the limewater to change in about 20
minutes.
Answers
1.pH
2.time
3. control variables include: volume of yeast, volume of liquid, temperature, concentration of yeast.
4.yeast
5. prediction should be what they think will happen and why.
Conclusion
Yeast is a fungus which is a type of microbe. It can be used to find out the effect of pH on the growth
of microbes in chilled drinks. Yeast grew best in pH values closest to pH 7.
Grape juice is quite acidic with a pH value of (students own response). By adding citric acid, the taste
is not changed and the pH is lowered reducing the growth of microbes and increasing the shelf life.
When baking powder is added the pH is increased. But baking powdered will react with some of the
natural acids in the grape juice and will affect the taste and appearance of the product.
Extension information
Extension information
Extension
Yeast is a fastInformation
growing unicellular microorganism. They have a spherical shape and are classified within
Yeast
is a kingdom.
fast growing
unicellular
microorganism.
Theycell
have
a spherical
and are
classified
within
the fungi
Inside
the cytoplasm
of each yeast
there
are the shape
organelles
known
as
Yeast is a fast growing unicellular microorganism. They have a spherical shape and are classified
the fungi kingdom.
Insidethese
the cytoplasm
ofthat
each
yeast cell takes
there place.
are the organelles known as
mitochondria.
is inside
structures
respiration
within
the fungiItkingdom.
Inside
the cytoplasm
of each yeast
cell there are the organelles known as
mitochondria. ItIt isis inside
inside these
these structures
structures that
that respiration
respiration takes
place.
mitochondria.
takes
place. chemical equation:
Yeast can respire aerobically
and this can be
represented by
the following
Yeast can respire aerobically and this can be represented by the following chemical equation:
6O2 → 6CO
C6H12Ocan
6 + respire
2 + 6H2O
Yeast
aerobically
and this can be represented by the following chemical equation:
C6H12O6 + 6O2 → 6CO2 + 6H2O
This reaction is used in the baking industry to make bubbles of carbon dioxide in bread which then makes
This
reaction
used
in the
bakingthe
industry
makeand
bubbles
of carbon dioxide
in bread which then makes
it
rise.
When is
bread
is baked,
yeast to
istokilled
the respiration
This
reaction
isthe
used
in the
baking industry
make bubbles
of carbonstops.
dioxide in bread which then
it rise. itWhen
the bread
is baked,
the yeast
is killedisand
theand
respiration
stops. stops.
makes
theanaerobically
bread
is baked,
killed
thebyrespiration
Yeast canrise.
alsoWhen
respire
andthe
thisyeast
can be represented
the following
chemical equation:
Yeast
can
also
respire
anaerobically
and
this
can
be
represented
by
the
following
chemicalequation:
equation:
Yeast can also respire anaerobically and this can be represented by the following chemical
C6H12O6 →2CO
2CH3CH2OH
C6H12O6
2CO22++ 2CH3CH2OH
C6H12O6 →2CO2 + 2CH3CH2OH
This reaction
reaction is
This
is used
used in
in the
the brewing
brewingindustry
industrytotomake
makealcoholic
alcoholicdrinks.
drinks.The
Theethanol
ethanolproduced
producedcan
canthen
then
This
reaction
is
used
in
the
brewing
industry
to
make
alcoholic
drinks.
The
ethanol
produced
can
then
further
oxidised
to
make
ethanoic
acid
and
this
is
the
process
used
to
make
vinegar.
further oxidised to make ethanoic acid and this is the process used to make vinegar.
further oxidised to make ethanoic acid and this is the process used to make vinegar.
east can spoil food by releasing the enzyme mixture that it produces known as zymase. This is a
Yeast can spoil food by releasing the enzyme mixture that it produces known as zymase. This is a
east can spoil
food by releasing
the enzyme
mixture that
it produces
known as zymase.
This is a
biological
catalyst
ofof
biological
catalyst which
whichconsumes
consumesthe
thesugars
sugarsininthe
thefood.
food.This
Thischemically
chemicallychanges
changesthe
thecomposition
composition
biological
catalyst
consumes
the sugars
the food
food and
and
with
itit the
and
feel.
the
with which
the taste
taste
and mouth
mouth
feel. in the food. This chemically changes the composition of
the food
with found
it the taste
and
mouthoffeel.
Yeasts
areand
usually
on the
surface
foods and are resistant to common additives introduced to
Yeasts are usually found on the surface of foods and are resistant to common additives introduced to
foods to improve shelf life. Live yeast can be indicated using methylene blue.
Yeasts
are
usuallyshelf
foundlife.
on the
of foods
and areusing
resistant
to common
foods to
improve
Livesurface
yeast can
be indicated
methylene
blue. additives introduced to
foods to improve shelf life. Live yeast can be indicated using methylene blue.
Lesson plans, practical sheets and homework sheets in this series can be freely downloaded from
plans,sheets
practical
sheets and
homework
in this
series from
Lesson Lesson
plans, practical
and homework
sheets
in this seriessheets
can be freely
downloaded
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Author
Author
Sam Holyman, Bablake School, Coventry, UK
Sam Holyman, Bablake School, Coventry, UK
www.schoolscience.co.uk/partners/chilled-food-association
Author
Sam Holyman, Bablake School, Coventry, UK
© Chilled Food Association. This document may be copied freely for educational use.