Food preservation and useful microorganisms
Compare preservation methods by the condition they control, and explain milk processing and microbial production of cheese and yoghurt.
Exam scope and assessment notes
This original StudyForA lesson maps Cambridge IGCSE Food & Nutrition 0648 topic 11 for examinations in 2026–2029. These are not Cambridge questions, mark schemes or official grades. The practical examination is a real, centre-administered cooking test with a planning session and assessed preparation, time management, results and quality. These screen-based lessons can teach the theory and help rehearse planning and evaluation, but they cannot provide a kitchen, verify safe food handling or practical skill, replace teacher-supervised cooking, award the practical mark, or guarantee an exam grade. Follow your centre’s instructions, local food-safety rules, medical advice and emergency procedures. Any meal examples are for syllabus practice, not individual dietary or medical advice.
Before you begin
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before you start: What is one reason to preserve food?
What you will learn
- Match a preservation method to the condition it changes
- Compare advantages and limits of preservation methods
- Distinguish milk treatments and products
- Explain controlled bacterial and enzyme action in fermented dairy foods
Why and how foods are preserved
Preservation extends usable life, reduces seasonal loss, supports storage and transport and can improve convenience. Methods control microorganisms or chemical and enzymic change. Heating during canning or bottling destroys many microorganisms when the process is correctly controlled; removal of moisture by dehydration limits the water available for growth; freezing slows growth and reactions but does not necessarily kill all microorganisms.
Sugar, salt or vinegar can reduce conditions needed for growth when used in a suitable concentration and process. Modified-atmosphere packaging changes gases around a food to slow some spoilage processes. Irradiation uses controlled ionising radiation to reduce microorganisms or pests; it does not make food radioactive. Each method has limits and must be used with correct processing, packaging and storage.
Milk processing, cheese and yoghurt
Pasteurisation uses a controlled heat treatment to destroy many harmful microorganisms, followed by appropriate storage; it is not the same as sterilisation. Sterilisation uses a more intensive treatment to produce a longer-lasting product when sealed and handled correctly. UHT milk receives very high-temperature treatment for a short time and is packaged aseptically. Evaporated milk has some water removed and is sterilised; condensed milk is concentrated and sweetened; dried milk has most water removed.
Yoghurt is made when suitable bacteria ferment lactose to lactic acid, lowering pH and changing flavour and texture. In cheese-making, enzymes such as rennet can coagulate milk protein; starter bacteria acidify the milk and contribute to flavour and texture. The organisms and processes are controlled. Follow product labels and local storage directions; preservation does not excuse unsafe handling after opening.
Sources for this lesson
Pause and explain
How does dehydration help preserve food?
Worked example
A household needs milk with longer unopened storage before refrigeration. Compare UHT milk with dried milk.
Show the worked solution
- UHT milk is heat-treated and aseptically packaged; unopened shelf stability depends on packaging.
- Dried milk has most water removed, limiting conditions for microbial growth.
- After opening or reconstitution, storage and use instructions still apply.
Answer UHT milk is treated at very high temperature briefly and packaged aseptically, while dried milk has most water removed. Both can be stored unopened as directed, but once opened or reconstituted, the product’s label instructions and safe storage conditions must be followed.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Choose a suitable explanation for why freezing slows food spoilage and state one reason it does not make handling irrelevant.
Give me a hint
Consider temperature effects and what can happen during thawing or storage.
Compare my reasoning
- Explain that low temperature slows many microbial and chemical processes.
- Do not claim that freezing sterilises or kills every microorganism.
- Continue to follow packaging, freezer, thawing and cooking directions.
Freezing slows the growth of many microorganisms and the rate of some reactions, extending storage life. It does not sterilise food or remove the need for safe packaging, storage, thawing and cooking. Follow the product’s directions and local food-safety rules.
Look for these in your work
- Explains slowed processes
- Avoids a sterilisation claim
- Includes safe handling after freezing
A learner confuses condensed milk with evaporated milk. Explain one process difference and the likely result.
Give me a hint
Both have water removed, but consider whether sugar is added.
Compare my reasoning
- State that both products are concentrated by removing water.
- Identify that condensed milk is sweetened.
- Explain how added sugar affects taste and use.
Both are concentrated milk products, but condensed milk is sweetened while evaporated milk is unsweetened. Added sugar makes condensed milk much sweeter and changes how it is used in recipes; the product label confirms its ingredients and storage directions.
Look for these in your work
- Identifies concentration
- Identifies added sugar
- Explains a product consequence
Yoghurt becomes thicker and more acidic during manufacture. Explain the role of bacteria and identify two process controls.
Give me a hint
Connect lactose fermentation to lactic acid, then consider conditions that affect cultures.
Compare my reasoning
- Explain that suitable bacteria ferment lactose to lactic acid.
- Link increased acidity to changes in flavour and milk-protein texture.
- Name controls such as temperature, time, culture and clean equipment.
Suitable bacteria ferment lactose and produce lactic acid. The increasing acidity changes flavour and helps alter the milk’s protein structure, producing a thicker texture. Temperature, time, culture quality and clean equipment must be controlled so the intended process occurs safely.
Look for these in your work
- Explains fermentation
- Links acid to texture and taste
- Names relevant controls
Common mistakes
- Claiming freezing sterilises food
- Treating pasteurisation, sterilisation and UHT as identical
- Assuming preserved food never needs storage instructions
What can you explain now?
How does dehydration help preserve food?
Compare with the explanation
It removes moisture and reduces water available for growth
Use the syllabus concept, connect it to the stated evidence or needs, and explain any limit on your conclusion.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
For two preservation methods, state which condition each controls and one limit of the method.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.