What makes food go off, what conditions do micro-organisms need, and how can we preserve food?
Microbiology and food spoilage: the micro-organisms that spoil food, the conditions they need to grow, signs of spoilage, enzymic action, and methods of food preservation.
A focused answer to the WJEC Food Preparation and Nutrition science of food topic on microbiology and food spoilage, covering bacteria, yeasts and moulds, the conditions micro-organisms need, signs of spoilage, enzymic action, and methods of preservation.
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What this dot point is asking
You need to know the micro-organisms that spoil food, the conditions they need to grow, the signs of spoilage, the role of enzymes in spoilage, and the main methods of preserving food.
What spoils food
Food is spoiled by micro-organisms and by the food's own enzymes.
- Bacteria: tiny organisms that multiply quickly in the right conditions; some spoil food, some cause food poisoning.
- Yeasts: spoil sugary and acidic foods by fermentation (also used helpfully in bread and brewing).
- Moulds: fungi that grow as fuzzy patches on bread, cheese and fruit.
- Enzymes: natural in food, they cause ripening and then over-ripening and enzymic browning (for example a cut apple turning brown).
The conditions micro-organisms need
Signs of spoilage
- off or sour smell,
- slime on the surface,
- visible mould,
- changes in colour and texture,
- a sour or unusual taste.
Spoiled food should not be eaten. Note that food can carry harmful bacteria without looking or smelling off, which is why use-by dates and safe handling matter.
Methods of preservation
Each method removes a condition micro-organisms need:
- Chilling (fridge below 5 degrees Celsius): slows growth; microbes are not killed.
- Freezing (minus 18 degrees Celsius): stops growth by removing warmth and available water; microbes become dormant, not killed.
- Heat treatment (canning, bottling, pasteurisation): kills microbes, and sealing keeps new ones out.
- Drying / dehydrating: removes water, so microbes cannot grow.
- Salting or sugaring: a high concentration of salt or sugar draws out water (jam uses sugar).
- Pickling: the acid in vinegar stops microbes growing.
- Vacuum packing: removes air, slowing many microbes.
- Modified atmosphere packaging (MAP): the air in the pack is replaced with a gas mixture that slows spoilage and keeps food fresh for longer.
Helpful micro-organisms
Not all micro-organisms are harmful. Some are used deliberately to make food: yeast makes bread rise and brews beer, and bacteria turn milk into yoghurt and cheese and are used to make foods such as sauerkraut. The same fermentation that spoils some foods is used helpfully in others, under controlled conditions.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style6 marksExplain the conditions that bacteria need to grow, and describe how chilling and freezing help to keep food safe.Show worked answer →
A 6-mark question. Mark it for the conditions and how cold preservation works.
Bacteria need warmth, moisture, food and time to grow, and most grow fastest in the danger zone of 5 to 63 degrees Celsius, with body temperature around 37 degrees Celsius being ideal. Chilling food in a fridge below 5 degrees Celsius slows bacterial growth greatly, so food stays safe for longer, though bacteria are not killed and will grow again if the food warms up. Freezing at minus 18 degrees Celsius stops bacterial growth almost completely by removing available water and the warmth they need, putting the bacteria into a dormant state; again the bacteria are not killed, so food must be used soon after thawing and not refrozen.
A top answer lists the conditions (warmth, moisture, food, time) and the danger zone, then explains that chilling slows and freezing stops growth without killing the bacteria. Reward the temperatures.
WJEC style4 marksDescribe four methods of preserving food and explain briefly how each works.Show worked answer →
A 4-mark question. Award one mark per method with how it works.
Freezing: low temperature stops micro-organisms growing by removing warmth and available water. Canning or bottling: heat kills micro-organisms and the sealed container keeps new ones out. Drying or dehydrating: removing water means micro-organisms cannot grow. Adding salt or sugar (and pickling in vinegar): a high concentration of salt or sugar, or the acid in vinegar, stops micro-organisms growing. Jam-making uses a high sugar concentration to preserve fruit.
Markers reward any four methods (freezing, canning/bottling, drying, salting/sugaring, pickling, vacuum packing) each with a correct reason: remove warmth or water, kill and seal, or create conditions microbes cannot grow in.
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Sources & how we know this
- WJEC Eduqas GCSE Food Preparation and Nutrition specification (from 2016) — WJEC Eduqas (2016)