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 ocur
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
