Introductory Level

Preserving food

Introduction

Investigate the effect of different preservatives on frozen peas. Establish that decay is caused by the action of microbes, and therefore preservatives work by reducing microbe activity.

This practical is based on an investigation called Preserving food published in Practical Microbiology for Secondary Schools © Society for General Microbiology. The original protocol is available here (229 KB).

Lesson Organisation

Each group could prepare a tube with a different preservative. Or you could set up all the tubes in advance to demonstrate the different effects.

Apparatus And Chemicals

For each experimental set:

Test-tube rack, 1
Test tubes, 8
Non-absorbent cotton wool plugs, 8
Marker pen
Frozen peas, 24
Distilled water
Solutions of different preservatives, 5 cm³ of each

For the class – set up by technician/ teacher:

Enough of the following solutions for 5 cm³ per experimental set:
• sodium chloride solution (dilute) (See note 2)
• sodium chloride solution (concentrated) (See note 3)
• sugar solution (sucrose) (See note 4)
• vinegar (clear white table vinegar) (See note 5)
• sodium nitrite/ sodium nitrate (III) (NaNO₂) (See note 6)

Health & Safety

Read our health and safety guidelines

• Carry out a full risk assessment before planning any work in microbiology (see note 1 for more details).
• Food or drink should not be stored or consumed in a laboratory that is used for microbiology.
• Take care with vinegar and sodium nitrite solutions. Avoid contact with eyes and wash hands after handling.
• Leave the cotton wool plugs in when observing the peas next lesson.
• Do not eat the peas!

Technical Notes

1 Before embarking on any practical microbiological investigation carry out a full risk assessment. For detailed safety information on the use of micro-organisms in schools and colleges, refer to Basic Practical Microbiology – A Manual (BPM) which is available, free, from the Society for General Microbiology (email education@sgm.ac.uk) or go to the safety area of the SGM website (www.microbiologyonline.org.uk/safety.html) or refer to the CLEAPSS Laboratory Handbook, section 15.

2 Sodium chloride solution (dilute) is 1% w/v (See Hazcard 47B: Sodium chloride is described as ‘low hazard’)

3 Sodium chloride solution (concentrated) is 20% w/v (See Hazcard 47B: sodium chloride is described as ‘low hazard’)

4 Sugar solution (sucrose) is 20% w/v (described on Hazcard 40C as ‘low hazard’)

5 Vinegar is clear white table vinegar – approximately 1.0 mol dm^-3 ethanoic acid: Hazcard 38A says solutions from 1.7-4.0 mol dm^-3 should be labelled ‘irritant’)

6 Sodium nitrite is sodium nitrate (III) (NaNO₂) at 5% w/v (approximately 0.7 mol dm^-3: Hazcard 93 says solutions from 0.4-3.5 mol dm^-3 should be labelled ‘harmful’)

Procedure

SAFETY:
Label vinegar ‘irritant’ and sodium nitrite ‘harmful’ and ensure students wash their hands after handling and avoid contact with their eyes.

Preparation

Prepare solutions as described above

Investigation

a Make up sets of 8 test-tubes.

b Put 3 peas in each test-tube.

c Label the tubes A to H.

d Add nothing to tubes A and B.

e Refrigerate tube A.

f Add approximately 5 cm³ of liquid to tubes C to H as follows: distilled water, dilute salt solution, concentrated salt solution, concentrated sugar solution, vinegar (SAFETY: irritant), sodium nitrite solution (SAFETY: harmful).

g Plug each tube with cotton wool.

h Leave for at least 48 hours at room temperature (20-25 ºC), except tube A.

Teaching Notes

After 48 hours, some signs of decay may be visible in tube B – for example, discolouration of the peas, mould growing on them. The liquids in the tubes will become cloudy (or turbid) as microbe populations (mainly bacteria) develop. Turbidity just visible to the naked eye indicates around 106 microbes per cm³. Very dense turbidity indicates around 109 microbes per cm³.

There is scope to extend this by testing the effects of these preservatives in different combinations, of using other preservatives, or of sealing peas in tubes containing an atmosphere that does not support bacteria. You could also compare the peas after 48 hours with dried peas (from a fresh packet, and from a packet opened when the experiment was set up) and look at the ‘best before’ date on a can of peas.

If salt and sugar work equally well, ask students to think about which food we prefer to preserve in salt, and which in sugar. CLEAPSS do not advise tasting anything in the laboratory. However, tasting the salt and sugar solutions to find out if the students would like to eat peas preserved with them is interesting. Maybe you can work in a teaching room, or kitchen, or give students instructions for approximating the salt and sugar concentrations at home to do this.

Bacon is typically preserved with sodium chloride at concentrations of 3-6% and small quantities of nitrite (parts per million). Nitrite content is limited by law because of possible health dangers. Nitrite works better as an anti-microbial in the presence of sodium chloride, in heat-treated food, and at pH values of 5-6 (slightly acidic).

Downloads

Download a student sheet with questions and answers here (76 KB).

Web Links

www.microbiologyonline.org.uk/teachers/resources
Society for General Microbiology – source of Basic Practical Microbiology, an excellent manual of laboratory techniques and Practical Microbiology for Secondary Schools, a selection of tried and tested practicals using microorganisms. This protocol is based on Preserving food. The original protocol is available here (229 KB).

www.microbiologyonline.org.uk
MiSAC (Microbiology in Schools Advisory Committee) is supported by the Society for General Microbiology (see above) and their websites include more safety information and a link to ask for advice by email.

(Websites accessed July 2009)

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