Experiment to show some methods of preserving food

Each team will be provided with 8 tubes with cotton wool bungs, all containing 3 peas.
The class will be provided with bottles containing various chemicals.

Your task today is to subject the peas to different chemical and physical treatments (by adding various liquids and using different temperatures).

Procedure

1 Label the tubes A-I, and add your identification mark.
2 Half fill the tubes with liquids, as shown in the table below.
3 Note the appearance (especially the clarity) of the liquids.
4 Hand in tube A for placing in the refrigerator, and tubes B-I for placing in the incubator.

You will see the effects next week.

Tube	Liquid added	Temperature treatment	Results after 1 week
A	none	cool - refrigerator
B	none	warm - incubator
C	distilled water	warm - incubator
D	dilute salt solution	warm - incubator
E	concentrated salt solution	warm - incubator
F	dilute sugar solution	warm - incubator
G	concentrated sugar solution	warm - incubator
H	sodium nitrite solution	warm - incubator
I	vinegar	warm - incubator

Next lesson

Look at the contents each of the tubes, paying special attention to the clarity/ cloudiness of liquids.

Record your results in the table above
(+++, ++, + for different degrees of cloudiness, and - for clear).

You may smell the tubes, but you must not open them or spill their contents.

THIS EXPERIMENT (A "CLOSED TOPIC") COULD BE QUITE EASILY EXPANDED INTO AN "OPEN" INVESTIGATION, IN WHICH THE PROCESSES OF PREPARING, CARRYING OUT THE INVESTIGATION, AND INTERPRETING THE RESULTS, CAN BE ASSESSED.

Questions - technique

The peas (originally frozen, but now defrosted!) were placed into the tubes using forceps.
Glassware was washed clean, but nothing was sterilised.
Why were forceps used?
> to minimise contact with skin microflora
Why were tubes provided already prepared?
> if touched by different people, different amounts of contamination would occur
What is the purpose of the cotton wool bungs?
> to allow air in & other gases out, and reduce entry of other (airborne) microbes
Why not use rubber bungs?
> could blow off if gas develops/prevent oxygen entering/can't smell if contaminated
Which tube/tubes may be called controls?
>A, B, C (& even D) each by comparison with the next
Why was sterilisation not necessary?
> bacteria are expected to grow in some but not others - according to the effectiveness of the treatment

Questions - results

Why has the liquid in some of the tubes gone cloudy?
> growth of bacteria causing turbidity
What is the original source of that?
> bacteria on the surface of the peas, in the freezer/packaging (suspended animation?)
What effect does temperature have on microbial growth?
> reduced temp slows, increased temp speeds up (within range)

What is the difference between the results in tubes D and E?
> D slightly cloudy , E clear (better preservative effect)
Which was the best preservation treatment?
> vinegar/sodium nitrite/ strong salt?
How does vinegar act as a preservative? (answer may have several levels)
> lowers pH/prevents enzymes action/kills bacteria
What other preservation technique has been applied to these peas?
> freezing (& blanching)

What sorts of foods are usually preserved by these chemicals ?

salt    > meat/fish

sugar    > fruits

sodium nitrite > meat

vinegar    > vegetables, e,g, onions, gherkins eggs etc

This worksheet is based on a protocol - Preserving Food - from the Society for General Microbiology: "Practical Microbiology for Secondary Schools" - now available via the Society of Biology.