What makes food go off and how do bacteria cause food poisoning?
The signs and causes of food spoilage by micro-organisms and enzymes, the conditions bacteria need to grow, the main food-poisoning bacteria and their sources, and the difference between use-by and best-before dates.
A focused answer to AQA GCSE Food Preparation and Nutrition on food spoilage by micro-organisms and enzymes, the conditions bacteria need to grow, the main food-poisoning bacteria and the difference between use-by and best-before dates.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
AQA wants you to know what causes food to spoil, the conditions bacteria need to multiply, the main food-poisoning bacteria, and how date labels protect consumers. This is the science behind safe food handling.
Causes of food spoilage
Signs of spoilage include off smells, sliminess, mould growth, discolouration, softening and souring. Enzymic browning, such as a cut apple going brown, is an enzyme action.
Conditions bacteria need
This is why we chill food below 5 degrees C, cook it above 75 degrees C and keep hot food above 63 degrees C, and why we do not leave food out for long.
The main food-poisoning bacteria
- Salmonella - raw poultry, eggs, meat.
- E. coli - undercooked beef, raw vegetables, unpasteurised milk.
- Campylobacter - raw and undercooked poultry; a very common cause.
- Listeria - soft cheeses, pate, chilled ready-to-eat foods (dangerous in pregnancy).
- Staphylococcus aureus - spread from people's skin, nose and cuts.
These bacteria cause food poisoning, with symptoms such as diarrhoea, vomiting, stomach cramps, nausea and fever, usually within hours to a few days. High-risk foods are moist, high-protein and ready to eat (cooked meat and poultry, dairy, eggs, cooked rice, shellfish, gravy and sauces), because they give bacteria the food and moisture they need and are not cooked again before eating. Low-risk foods (dry foods such as biscuits, and high-acid, high-sugar or high-salt foods such as jam and pickles) do not support rapid growth. Some bacteria, including certain strains, form heat-resistant spores or produce toxins that may survive cooking, which is why prevention (good hygiene and temperature control) matters as much as cooking.
Use-by and best-before dates
Try this
Q1. State the four conditions bacteria need to grow. [2 marks]
- Cue. Warmth, moisture, food and time.
Q2. Explain the difference between a use-by date and a best-before date. [2 marks]
- Cue. Use-by is a safety date on high-risk food; best-before is a quality date after which food is usually still safe.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20186 marksExplain the conditions that bacteria need to grow and how a cook can use this knowledge to keep high-risk food safe. (Paper 1, Section B)Show worked answer →
Bacteria need four conditions to multiply: warmth (fastest in the danger zone of 5 to 63 degrees C, best around body temperature 37 degrees C), moisture, food (especially high-risk, high-protein foods) and time.
A cook uses this by keeping high-risk food out of the danger zone: storing it in the fridge below 5 degrees C to slow growth, cooking thoroughly to above 75 degrees C at the centre to destroy bacteria, holding hot food above 63 degrees C, and not leaving food at room temperature so bacteria have less time to multiply.
Top-band answers (5 to 6 marks) give the four conditions and link each to a control the cook uses.
AQA 20214 marksName two food-poisoning bacteria, give a likely food source for each, and state one symptom of food poisoning. (Paper 1, Section A)Show worked answer →
For 4 marks, give two named bacteria with sources plus a symptom.
Salmonella is associated with raw poultry, eggs and meat. Campylobacter is associated with raw and undercooked poultry and is a very common cause. Other valid examples are E. coli (undercooked beef, unpasteurised milk) and listeria (soft cheese, pate, dangerous in pregnancy).
A symptom of food poisoning is diarrhoea, vomiting, stomach cramps, nausea or fever. Markers reward two correctly matched bacteria and sources plus at least one valid symptom.
Related dot points
- The principles of food safety including personal, kitchen and equipment hygiene, preventing cross-contamination, the four Cs (cleaning, cooking, chilling, cross-contamination), and using temperature control and probes correctly.
A focused answer to AQA GCSE Food Preparation and Nutrition on the principles of food safety, covering personal and kitchen hygiene, preventing cross-contamination, the four Cs and correct temperature control.
- How to buy, store, prepare, cook, cool, reheat and freeze food safely, including correct storage of different foods, defrosting, the rules for reheating, and following food safety from purchase to consumption.
A focused answer to AQA GCSE Food Preparation and Nutrition on buying, storing, preparing, cooking, cooling, reheating and freezing food safely, including correct storage, defrosting and reheating rules.
- Why food is cooked - to make it safe, palatable, digestible and varied - and how cooking and preparation affect the nutritional value of food, including the effect of heat, water and air on vitamins.
A focused answer to AQA GCSE Food Preparation and Nutrition on the reasons food is cooked - safety, palatability, digestibility and variety - and how heat, water and air affect the nutritional value of food.
- The methods of heat transfer (conduction, convection and radiation), the main water-based, fat-based and dry cooking methods, and how to select an appropriate method for a given food and outcome.
A focused answer to AQA GCSE Food Preparation and Nutrition on heat transfer by conduction, convection and radiation, the main water-based, fat-based and dry cooking methods, and how to choose a suitable method.
- How food is processed and produced, including primary and secondary processing, the production of staple foods such as flour, milk and cheese, fortification, and methods of food preservation.
A focused answer to AQA GCSE Food Preparation and Nutrition on how food is processed and produced, including primary and secondary processing, staple foods such as flour, milk and cheese, fortification and food preservation.
Sources & how we know this
- AQA GCSE Food Preparation and Nutrition (8585) specification — AQA (2016)