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science activity three: eat your greens
CAN WE STOP FOOD FROM GOING OFF BUT STILL KEEP IT TASTY AND NUTRITIOUS?
Students observe what happens to the colour and texture of green beans when they are heated
in different pH conditions.
The students record their observations, use the data they have obtained to suggest which
conditions are best for canning and attempt some explanation of what has happened.
delivery
01 Introduction 5-15 minutes
02 Practical activity ~40 minutes
03 Plenary 10-15 minutes
It would be useful to give the school some information about yourself
in advance of the session. You may be able to bring photographs,
equipment or other means to illustrate your work. The session should
help students to recognise that the science they do in school relates
to science in the real world, scientists are real people, and that they
could be scientists too.
Throughout the session, try to refer to your own personal experience
as a scientist as much as possible.
Younger KS3 students are likely to work more slowly. They may need
more time to carry out the investigation than older students. If students complete the
work more rapidly than expected, more time can be spent on the plenary discussion.
You might also ask students to compare their beans and solutions with those found in
commercial cans of beans.
Students do not need to complete the questions before they are discussed in the plenary. Ask
students to stop and clear up in order to allow sufficient time for the plenary discussion.
During the practical work you will be able to walk round and talk to students to gain an idea of
what level you will need to adopt for the discussion and plenary.
Endorsed by the Society of Biology
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science activity three: eat your greens
01
introduction
Introduce yourself as a professional scientist.
Explain, using examples from your own experience, that:
●● Scientists collect data and make sense of it.
●● They learn how things work through observation and experiment. They try to explain
observations and test their explanations through experiments. When they have a good
explanation they can use it to solve problems.
The task
Background info
You could use the following information, or you may have your own ideas that relate more
directly to your own work and expertise.
Green vegetables grow best in the warmer and sunnier months. Microorganisms such
as bacteria and fungi (moulds) soon start to decay them once they have been harvested.
Therefore, to prevent them from ‘going off’, green vegetables have to be processed to
preserve them. You may wish to show specimens of fresh and decaying green vegetables.
The most common method used for preserving green vegetables is canning.
The Frenchman, Nicolas Appert, is known as the ‘father of canning’. At the beginning
of the nineteenth century, he won a competition to devise a new method of preserving
food for the French army and navy. He cooked and sealed food in glass jars. From his
observations he had noticed that food cooked inside a jar did not spoil unless the seals
leaked. This was 50 years before Louis Pasteur showed that microorganisms cause food to
go off.
Shortly afterwards, in the UK, Peter Durand developed the use of cylindrical metal ‘cans’ –
short for ‘canisters’. Unfortunately, the can opener was not invented for another 30 years!
Cans were cut open with knives or bayonets.
Originally, cans were hand made and very expensive. So canned food was more of a
novelty or a status symbol for middle class households. In the mid 1800s most canned food
was used by the army and navy.
Today vegetables are cleaned and prepared, packed into cans, sealed in and heated. High
temperatures around 240 oC are used to destroy microorganisms. Microorganisms cannot
re-enter the sealed cans, so the food will keep for a long time. However, the heat treatment
causes chemical changes and changes to the texture of the vegetables.
Getting started
How can we use science to improve something to make it work better?
In this activity you are going to work as scientists to try to discover how the canning
process affects green vegetables.
You are going to investigate the changes in heated green vegetables when the pH is altered.
You will collect data – working accurately and recording your observations as precisely as
you can.
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science activity three: eat your greens
Explain to the students that they will be working in pairs and briefly run through the
procedure, emphasising any safety precautions and pointing out any other important
information. For example, explain that the task will be divided up between groups, so each
group heats only one from water, hydrochloric acid or sodium hydroxide. Each group can
then compare their results with other groups.
You will need to make sense of your data - using your scientific knowledge to try to explain
what has happened and to suggest what conditions are best for canners to use to preserve
the quality of green vegetables.
02
practical activity
What you will need
Discuss this with the teacher and/or technician at the school. Make sure that everything
will be available on the day. You may wish to use a computer and data projector to
illustrate aspects of your work or the history of canning.
Optional
●● Examples of fresh and decaying green vegetables to show what happens if they are not
processed.
For each student
●● Eye protection
●● Student Instruction Sheet (may be printed separately, or you may wish to modify this
to suit your own circumstances)
For each group of students
●● About 50 g of fresh green beans cut into 2 cm lengths
●● Dilute hydrochloric acid (0.01M HCl) or dilute sodium hydroxide solution (0.01M NaOH)
●● Deionised or tap water
●● Timer or clock
●● 250 cm3 beakers x 2
●● 100 cm3 measuring cylinder
●● Glass rod
●● Access to top pan balance
●● Filter paper discs
●● Tile or board
●● Spatula
●● Bunsen burner, tripod, gauze, heat resistant mat
●● Heat resistant tongs
●● Marker pen
●● pH universal indicator paper
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science activity three: eat your greens
Health and safety
All activities have been trialled and tested by 4science (www.4science.org.uk) and every
reasonable effort made to ensure that the activities are safe when conducted as instructed.
However, a full risk assessment should be carried out before any practical work is
undertaken. The supervising teacher should be able to advise on this and any local
requirements, such as procedures recommended by Local Authorities or safety advisers
used by the institution.
4science assume no responsibility for any damage or injury caused or sustained while
carrying out these activities to the full extent permitted by law.
For this practical you may wish to discuss with the teacher issues such as the safe handling
of glassware and safe use of Bunsen burners. Students may need more guidance or a more
thorough reminding of safe working procedures when heating liquids using a Bunsen
burner, tripod and gauze. Eye protection should be worn.
Students must be supervised at all times during practical work by a teacher (and/or
responsible adult).
You may wish to seek more advice (for example, CLEAPSS Hazcards) on the use of
hydrochloric acid and sodium hydroxide.
03
plenary
What you will need
●● Internet connected computer/data projector or interactive whiteboard (optional)
Discussion
Ask students to report their findings. Ask them for their answers to the questions and
discuss their ideas. You may wish to project images about the canning process, or that
relate to your own work.
Explanations
Why were some beans left unheated?
Some beans were left unheated to act as a control to show what happened without heat
treatment. It answers the question: Does heating affect the colour and texture of the
beans, or would just soaking in water have the same effect?
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science activity three: eat your greens
What conditions were kept constant?
It is important that only one factor is changed. This allows you to observe the effect that
the changed factor has alone. Therefore, heating times, the volumes of the solutions and
mass of the beans must be kept constant.
Which pH best retains the colour and texture of the beans?
pH near neutral (6-7) generally minimises the adverse chemical reactions that cause loss
of colour or texture.
What pigment gives the beans their colour?
The green colour of green vegetables is due to the presence of chlorophyll which is
important in transferring light energy to chemical energy in photosynthesis.
What causes a loss of texture (firmness) in the beans?
The loss of texture (firmness) in the beans is caused by partial destruction of the cell walls
and cell membranes of the cells. Water in intact cells causes pressure on the cell walls
making the tissues firm. Loss of this makes cells soft. Acid will also dissolve the cell cement
between cell walls, softening tissues.
Mild heating causes chemical changes in fat-soluble chlorophylls due to the action of an
enzyme (chlorophyllase), which produces water soluble molecules which can leach out of
the cells. More severe heat treatment or an acid medium causes some chlorophylls to be
converted to olive brown pigments (phaeophytins).
How does the temperature of water used for heating your solutions compare to that used in
commercial canning?
The temperature of water used for heating in the experiment is 100 oC. In commercial
canning, 240 oC is used. Some bacteria (Clostridium botulinum) produce spores which are
not killed by boiling water.
More information on canning and Nicolas Appert can be found at:
http://en.wikipedia.org/wiki/Canning
http://en.wikipedia.org/wiki/Nicolas_Appert
http://www.britannica.com/EBchecked/topic/30573/Nicolas-Appert
Note: All websites cited in this resource were valid at October 2012
follow up
Students could attempt chromatography to compare the pigments in fresh green vegetables
with those heat treated at different pH.
The effect of heating for different lengths of time could also be compared.
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science activity three: eat your greens
>>
curriculum links
There is an opportunity for curriculum links to be made. You may like to discuss with the
supervising teacher whether they would like specific examples to be used or any issues to be
raised. For example, they may like a discussion of the role of chlorophyll in photosynthesis.
ks3 Science: Programme of Study for Key Stage 3
Key concepts
1.1 Scientific thinking
a. using scientific ideas and models to explain
phenomena and developing them creatively to
generate and test theories.
b. critically analysing and evaluating evidence from
observations and experiments.
1.2 Applications and implications of science
a. exploring how the creative application of
scientific ideas can bring about technological
developments and consequent changes in the way
people think and behave.
Key processes
2.1 Practical and enquiry skills
a. use a range of scientific methods and techniques
to develop and test ideas and explanations.
b. assess risk and work safely in the laboratory, field
and workplace.
c. plan and carry out practical and investigative
activities, both individually and in groups.
2.2 Critical understanding of evidence
a. obtain, record and analyse data from a wide range
of primary and secondary sources, including ICT
sources, and use their findings to provide evidence
for scientific explanations.
b. evaluate scientific evidence and working
methods.
2.3 Communication
a. use appropriate methods, including ICT, to
communicate scientific information and contribute
to presentations and discussions about scientific
issues.
Range and content
3.3 Organisms, behaviour and health
a. life processes are supported by the organisation
of cells into tissues, organs and body systems.
Curriculum opportunities
a. research, experiment, discuss and develop
arguments.
b. pursue an independent enquiry into an aspect of
science of personal interest.
c. use real-life examples as a basis for finding out
about science.
e. experience science outside the school
environment, including in the workplace, where
possible.
f. use creativity and innovation in science, and
appreciate their importance in enterprise.
i. prepare to specialise in a range of science subjects
at key stage 4 and consider career opportunities
both within science and in other areas that are
provided by science qualifications.
k. make links between science and other subjects
and areas of the curriculum.
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science activity three: eat your greens
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checklist

You will need to liaise closely with the teacher.
Have you checked:
●● that all the necessary materials and resources, including worksheets, will be available?
●● the meeting arrangements at the school (for example: time, place, people’s names,
contact numbers, parking)?
●● the length of the session?
●● that risk assessments have been made and any appropriate safety measures are in place?
●● how your experiences as a working scientist can be related to the activity?
●● if and how any curriculum links should be made to the activity (this might also be
related to your experience if you have provided the school with information about your
work and interests)?
●● what kind of follow up (if any) would be useful?
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science activity three: eat your greens
student sheet page 1 of 3
eat your greens
Can we stop food from going off but still keep it tasty and nutritious?
what you need to do
●● Work with a partner to investigate the effect of heat and pH on the
colour and texture of green beans.
Safety: Wear eye protection
Use the marker pen to label two beakers and filter paper discs as follows:
Unheated Water
... and one of these three (you’ll be told which one) ...
Heated Water
HEATED 0.01M HCl
heated 0.01m NaOH
Weigh about 20 g of green beans into each beaker.
Use a measuring cylinder to add 100 cm3 of water or solution into each
appropriately labelled beaker. Rinse if changing to a new solution.
Keep a beaker of beans in water unheated. Use a Bunsen burner, gauze and
tripod on a heat resistant mat to bring the beans in the other beaker to the
boil. Reduce the heat and simmer (gentle boil) for exactly 15 minutes.
Safety: Close the airhole of the Bunsen burner when not using it to give a
visible yellow flame. When heating, make sure the airhole is opened to give
a hotter blue flame. To simmer, partly close the airhole and turn down the
gas. Do not use the sooty yellow flame for heating.
Watch what is happening to the beans being heated
in different solutions by other groups.
Allow the heated beaker to cool and decant the solution back into the measuring
cylinder to leave the beans behind (decant – ‘dee-kant’ – means pour off some
liquid leaving the solid behind). If time is short, use tongs to remove the beans.
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science activity three: eat your greens
student sheet page 2 of 3
Tip the drained beans onto the
appropriately labelled filter paper discs.
Use the data table provided to record your results (see Results
and questions). Note: Use a 0-5 scale to record the colours and
textures (0 = least; 5 = most).
Use clean glass rods to transfer one drop of each solution/water
to small pieces of pH indicator paper. Record the pH of each.
Observe and record the colour and
colour intensity of each solution.
Observe the texture (firmness) of each of the samples of green
beans by crushing or cutting them with a spatula on a tile or other
hard surface, and record. Test several beans from each sample.
Note: To complete your results table, swap results with other groups
that have heated beans using different solutions to yours.
Discuss and attempt to answer the questions.
results and questions
Colour (0-5 scale)
Treatment
pH
Solution
Drained beans
Bean texture (0-5 scale)
Unheated water
Heated water
Heated dilute acid
Heated dilute alkali
Why were some beans left unheated?
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science activity three: eat your greens
student sheet page 3 of 3
For heated beans:
●● What conditions were kept constant?
●● Which pH retained the best colour for the beans?
●● Which pH retained the best texture for the beans?
●● Which pH gave the worst colour retention in the beans?
●● Which pH gave the worst texture for the beans?
●● Based on your results, what would you tell canners to do?
●● What pigment gives the beans their colour?
●● What causes a loss of texture (firmness) in the beans?
●● How does the temperature of water used for heating your solutions compare
to that used in commercial canning? Suggest why there is a difference.
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