Every preservation method works by removing or changing one condition that microorganisms need to grow. To explain a method, name the condition and then say what happens to the microorganisms.
This lesson is part of nutrition and food technology. It follows reading nutritional data.
What do microorganisms need?
Bacteria and moulds need water, a suitable temperature, nutrients and a suitable acidity. Some also need air. Food supplies the nutrients, so preservation works on the other conditions.
| Method | Condition changed | Effect on microorganisms |
|---|---|---|
| Drying | Water removed | Cannot grow without water |
| Salting or sugar | Concentration raised | Water leaves cells by osmosis |
| Refrigeration or freezing | Temperature lowered | Growth slows or stops |
| Pickling in vinegar | Acidity raised | Unsuitable conditions for growth |
| Canning | Heated, then sealed | Heat kills them, sealing keeps new ones out |
Worked example: why does ikan masin keep?
Write the answer in three linked parts.
- Method: the fish is packed in a large amount of salt and dried.
- Condition changed: the salt makes the surroundings very concentrated, and drying removes water from the fish.
- Effect: water leaves the cells of bacteria by osmosis, so they dry out and cannot grow. The fish therefore spoils much more slowly.
A full-mark answer has all three parts. Naming the method alone earns one mark at most.
The mistake that costs marks
The common slip is to write that cold temperatures kill bacteria. It sounds reasonable, but freezing only slows growth, and the microorganisms recover when the food warms up.
| Step | Wrong | Right |
|---|---|---|
| Method | Freezing | Freezing |
| Condition | (not stated) | Temperature lowered |
| Effect | Bacteria are killed | Bacteria become inactive and grow very slowly |
| After thawing | (not stated) | Growth resumes, so eat the food soon |
Canning is the opposite case. The heat step does kill most microorganisms, which is why the two methods need different verbs.
Reading a data table about spoilage
A question may give an original table, such as the days a chicken stays fresh: 28 °C gives 1 day, 4 °C gives 5 days. The data supports the statement that a lower temperature slows spoilage.
It does not show why. For the reason, add the principle: lower temperature slows the growth of microorganisms. A claim plus a reason is the pattern used in the claim and evidence revision desk.
Check yourself
Strawberry jam is made with a large amount of sugar and stored in a sealed jar. Explain why it keeps for a long time.
Answer
The high sugar concentration makes the surroundings very concentrated, so water moves out of microorganism cells by osmosis. The cells dry out and cannot grow.
The sealed jar also stops new microorganisms and air from entering. The two conditions together explain the long storage.
What to study next
Continue with comparing processing methods using evidence, where each method is judged on nutrient and shelf-life data.
If you want a teacher to work on your explanations with you, see online one-to-one Science tuition.