Food preservation methods
Compare how pH, oxygen, available water and temperature affect microbes, and explain the limits of each preservation method.
On this page
Food Safety · about 25–35 min
What you need to understand
Method → changed condition → effect on microbes.
Definitions
- preservation
treating or storing food to delay spoilage
For example: Cooling slows microbial activity.- pasteurisation
controlled heating that reduces harmful or spoilage microbes without claiming complete sterilisation
For example: Milk can be pasteurised and still needs suitable storage.- dehydration
removing water from food to reduce the water available to microbes
For example: Drying fruit slows microbial activity.
Key idea
- LookFresh herbs must be stored for months without freezing.
- ThinkMethod → changed condition → effect on microbes.
- DoExplain a named method through the condition it changes rather than saying only that it keeps food fresh.
Explanation
Microbes need suitable conditions to grow. Preservation changes one or more of these conditions or uses heat to reduce microbial numbers. Start with the method, name the changed condition, then explain its effect.
| Changed condition | Method and example | Effect and limit |
|---|---|---|
| Lower pH | Pickling vegetables in vinegar makes the food more acidic. | Growth of many microbes is slowed. Some acid-tolerant microbes can remain. |
| Less oxygen | Air is removed before bottling and sealing, or by vacuum packaging. | Microbes that need oxygen have less available. Some microbes grow without oxygen, so removing air alone does not make food safe. |
| Less available water | Dehydration dries foods such as fruit or herbs. Freeze-drying removes water from frozen food. Sufficiently concentrated salt or sugar also reduces available water. | Microbial growth becomes more difficult. Salt and sugar need not remove water from the food as drying does; they reduce the water microbes can use. |
| High temperature | Controlled heating is used in pasteurisation, sterilisation and canning. | Heat reduces or destroys microbes. The temperature, treatment time and type of microbe affect the outcome. |
| Low temperature | Freezing foods such as vegetables keeps them at a low temperature during storage. | Microbial activity is greatly slowed. Some microbes survive and may grow again after thawing. |
Heating methods have different aims. Pasteurisation, for example of milk, reduces harmful and spoilage microbes but does not sterilise the food. Sterilisation aims to destroy viable microbes; commercial food treatments are designed to prevent microbial growth under the intended storage conditions. Canning combines a controlled heat treatment with a sealed container that prevents new contamination. These processes are not interchangeable, and their names alone do not specify a safe treatment.
Methods can be combined. Bottled food may also be heated or acidified; simply closing a lid does not remove all the air or preserve the food safely. Drying must be followed by suitable storage so moisture does not return. Freezing alone does not remove water from food as freeze-drying does. Follow the storage instructions for the product rather than assuming that any method guarantees safety indefinitely.
Pause and say it: Method → changed condition → effect on microbes.
Common mistake
Tempting wrong idea: Pickling preserves mainly by adding oxygen.
Why it fails: Pickling keeps food in an acidic liquid such as vinegar. The low pH slows the growth of many microbes. Adding oxygen would not do this, and it is not how pickling works.
Use this instead: Pickling lowers pH and makes conditions more acidic.
Practical work
What to show: Explain a named method through the condition it changes rather than saying only that it keeps food fresh.
Before you finish: Preservation delays spoilage; it does not guarantee safety indefinitely.
Practical: Evaluate storage evidence safely
Use the supplied illustrative data to compare colour changes during storage. What can this comparison tell you, and what can it not establish?
Safety: This is a data investigation. Use the supplied records; do not prepare, open, smell or taste spoiled samples or culture unknown microbes.
- Change
- storage temperature in the supplied comparison
- Measure
- colour-change score using the same scoring rule
- Keep the same
food batch, amount, container, observation times and conditions for viewing the colour
View the apparatus, method and sample results
Apparatus
Supplied records, the scoring rule below, and a pencil or spreadsheet.
Method
- Identify the two storage conditions and the factors kept the same.
Read the colour-change scale: 1 = none, 2 = slight, 3 = clear and 4 = strong.
- Compare the scores at each matching observation time.
- Describe the pattern, using a pair of values as evidence.
State one limitation and how a further supplied comparison could improve the evidence.
| Day | Refrigerated colour score | Room-temperature colour score |
|---|---|---|
| 0 | 1 | 1 |
| 2 | 1 | 2 |
| 4 | 2 | 4 |
Interpretation: On day 4, the refrigerated sample has a score of 2 and the room-temperature sample has a score of 4. The room-temperature record shows a greater colour change. These are categories, not counts of microbes: a score of 4 does not mean twice as many microbes as a score of 2.
Limitation: Colour judgement can vary, and colour can change for non-microbial reasons such as oxidation. These records do not identify microbes or prove that either sample is safe.
Improvement: Compare records from repeated matched samples and have observers score supplied images without knowing the storage condition. This would improve the colour comparison; it would still not turn colour into a direct measurement of microbial numbers or food safety.
6. Worked Examples
Modelled example 1
Choose a preservation method
Problem
A producer wants to store herbs for several months without freezing them. Suggest a preservation method and explain the condition it changes.
Study the worked solution
Reason from the evidence
Method
Use the storage time and no-freezing condition to select a workable method.
Reason
A preservation explanation must name the microbial condition changed and how that delays spoilage.
Working
- Microbes require available water for growth.
- Drying removes much of that available water.
- A sealed dry container then limits moisture returning.
State the conclusion
Working
Dehydration followed by dry storage can delay spoilage.
Common misconception 2
Does freezing sterilise food?
Learner response
“Frozen food contains no living microbes, so it stays safe forever after thawing.” Spot and correct the error.
Distinguish slower activity from destruction
View solution step by step
State what low temperature does
Method
Explain that freezing greatly slows microbial activity.
Reason
Some microbes can survive and become active again as food warms.
Working
Freezing delays spoilage; it does not guarantee complete sterilisation or unlimited safe storage.
Challenge 3
Choose methods for three foods
Problem
Choose and explain one suitable preservation approach for sliced fruit kept without refrigeration, vegetables stored in an acidic liquid, and milk treated briefly with controlled heat. Name the microbial condition or effect in each case.
Use method → condition → effect
Hints
Hint 1: match the contexts
Think dehydration, pickling and pasteurisation.
Hint 2: avoid one-word answers
State water, pH or microbial numbers after heating.
View solution step by step
Reduce water or lower pH
Method
Dehydrate the fruit and pickle the vegetables.Reason
Drying reduces water available to microbes; pickling lowers pH.
Working
fruit: less available water; vegetables: more acidic conditions
Use controlled heating
Method
Pasteurise the milk and store it as directed.Reason
Pasteurisation reduces harmful or spoilage microbes without claiming complete sterilisation.
Working
controlled heat → reduced microbial numbers
Guided practice
Try it with support
Explain how pickling, vacuum packaging, dehydration, pasteurisation and freezing each delay spoilage.
For each method, name the changed condition or microbial effect.
Check the guided answer
Answer: Pickling lowers pH; vacuum packaging reduces oxygen; dehydration reduces available water; pasteurisation uses heat to reduce microbes; freezing greatly slows microbial activity.
Check: Bottling with air removal reduces oxygen; canning also uses controlled heating and a sealed container. State each method’s effect rather than treating all methods as the same process.
Practise and continue
Practise this
Choose a suitable method for herbs stored for months without freezing and explain why it works.
Need a hint?
Target a condition microbes need.
Check your answer
Answer: Dehydrate the herbs and keep them sealed and dry. Removing available water makes microbial activity more difficult.
Check: Preservation delays spoilage; it does not guarantee safety indefinitely.
Think like a scientist
Why does a lower visible-change score not prove complete sterilisation?
Check the reasoning
The observation shows that visible change was delayed, but it does not test for every living microbe or toxin.
Remember: Use only conclusions supported by the measured indicator.
One-minute check
How do pickling, vacuum packaging and dehydration change conditions for microbes?
Explain why pasteurisation and freezing should not be described as complete sterilisation.
Correct this common mistake: “Pickling preserves mainly by adding oxygen.”
Syllabus and review details
- SEC G1 Science 2027 · 2027
Content structure and syllabus content, PDF pages 8–17
Last reviewed: