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.
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
Frozen peas, 24
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) (Note 2)
- sodium chloride solution (concentrated)
- sugar solution (sucrose) (Note 4)
- vinegar (clear white table vinegar) (Note 5)
- sodium nitrite/ sodium nitrate (III) (NaNO2) (Note 6)
Health and Safety and Technical notes
Carry out a full risk assessment before planning any work in microbiology (see note 1 for 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!
2 Sodium chloride solution (dilute) is 1% w/v (See CLEAPSS Hazcard: Sodium chloride is described as 'LOW HAZARD’)
3 Sodium chloride solution (concentrated) is 20% w/v (See CLEAPSS Hazcard: sodium chloride is described as ‘LOW HAZARD’)
4 Sugar solution (sucrose) is 20% w/v (described on the CLEAPSS Hazcard as ‘LOW HAZARD’)
5 Vinegar is clear white table vinegar – approximately 1.0 M ethanoic acid: Hazcard 38A says solutions from 1.7-4.0 M should be labelled ‘IRRITANT’)
6 Sodium nitrite is sodium nitrate (III) (NaNO2) at 5% w/v (approximately 0.7 M: CLEAPSS Hazcard says solutions from 0.4-3.5 M should be labelled ‘HARMFUL’)
Label vinegar ‘irritant’ and sodium nitrite ‘harmful’. Ensure students wash their hands after handling, and avoid contact with their eyes.
Prepare solutions as described above.
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 °C), except tube A.
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).
Download the student sheet Preserving Food (76 KB) with questions and answers.
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: Preserving Food.
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 October 2011)