Why does food go off, and how do preservation methods make it last longer?
Food spoilage and preservation: the signs and causes of spoilage (micro-organisms, enzymes, moisture, warmth), date marks, and how preservation methods such as chilling, freezing, drying, canning, vacuum packing and using acid, salt or sugar extend shelf life.
A focused answer on food spoilage and preservation for Eduqas GCSE Food Preparation and Nutrition (C560), covering the causes and signs of spoilage, use-by and best-before dates, and how preservation methods such as chilling, freezing, drying, canning, vacuum packing and using acid, salt or sugar extend shelf life.
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What this dot point is asking
Eduqas wants you to explain why food spoils, the signs and causes, the meaning of date marks, and how preservation methods extend shelf life by controlling micro-organisms and enzymes. Link each method to what it does to bacteria or enzymes.
Why food spoils
Signs of spoilage include off or sour smells, visible mould, a slimy surface, colour changes and a sour or bad taste. Note that spoilage usually gives signs, while dangerous pathogenic bacteria can grow with no visible sign, which is why use-by dates matter.
Date marks
Preservation methods
Preservation works by removing one of the things micro-organisms need (warmth, moisture, time or a suitable environment):
- Chilling (fridge below 5 degrees C) slows bacteria, giving a few days' extra life to perishable food.
- Freezing (minus 18 degrees C) makes water unavailable and stops bacteria multiplying (they are dormant, not killed), keeping food for months.
- Drying / dehydrating removes the water micro-organisms need, so pasta, pulses, dried fruit and powdered milk keep for a long time.
- Canning / bottling heats sealed food to a high temperature to kill micro-organisms, then seals it from air, keeping it for a long time unopened.
- Vacuum packing and modified-atmosphere packing remove or change the air (especially oxygen) so micro-organisms grow more slowly.
- High acid, salt or sugar create conditions micro-organisms cannot grow in: pickling in vinegar (acid), salting or curing (salt), and making jam (sugar).
Try this
Q1. Which date mark is about safety and appears on high-risk foods? [1 mark]
- Cue. The use-by date.
Q2. Explain how drying preserves food. [2 marks]
- Cue. It removes the water that micro-organisms need to grow, so they cannot multiply and the food keeps for longer.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20194 marksExplain the difference between a 'use by' date and a 'best before' date.Show worked answer →
A 4-mark question on date marking.
A use-by date is about safety. It is found on perishable, high-risk foods (such as meat, fish, dairy and ready meals); after this date the food may be unsafe to eat even if it looks and smells fine, because harmful bacteria may have grown, so it should not be eaten or sold after it.
A best-before date is about quality, not safety. It is found on longer-life foods (such as tinned, dried and frozen foods); after this date the food is usually still safe but may have lost some flavour, texture or appearance.
Markers reward use-by as a safety date on perishable food (do not eat after) and best-before as a quality date on longer-life food (still safe but past its best).
Eduqas 20216 marksExplain how three different preservation methods extend the shelf life of food, referring to what they do to micro-organisms.Show worked answer →
A 6-mark extended-response question. Mark it for three methods each linked to controlling micro-organisms.
Freezing (minus 18 degrees C) makes the water in food unavailable and stops bacteria multiplying (they are dormant, not killed), so the food keeps for months.
Drying or dehydrating removes the water that micro-organisms need to grow, so dried foods (pasta, pulses, dried fruit, powdered milk) keep for a long time at room temperature.
Canning heats the sealed food to a high temperature to kill micro-organisms and then seals it from the air, so it keeps for a long time unopened. Using acid (pickling), salt or sugar in high amounts also preserves food by creating conditions micro-organisms cannot grow in.
Top-band answers (5 to 6 marks) name three methods and explain how each controls micro-organisms (cold, removing water, heat and sealing, or hostile conditions).
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Sources & how we know this
- WJEC Eduqas GCSE Food Preparation and Nutrition specification (C560) — WJEC Eduqas (2016)