How does food spoil, and how do preservation and safe handling keep food fit to eat?
Explain how food spoils and how preservation methods and safe food handling protect food quality and prevent foodborne illness
Food spoils through the action of microbes and enzymes. Preservation methods control these by removing the conditions microbes need, while safe handling and the temperature danger zone prevent foodborne illness.
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What this dot point is asking
You need to explain the causes of food spoilage, how preservation methods prevent it, and how safe food handling protects against foodborne illness.
How food spoils
Food deteriorates in two main ways:
Microbial spoilage. Bacteria, moulds and yeasts grow on food, breaking it down and sometimes producing toxins. Microbes need warmth, moisture, food (nutrients) and time to multiply; remove any of these and growth slows. Some microbes spoil food; others, the pathogens, cause illness.
Enzymic and chemical spoilage. Natural enzymes in food continue to act after harvest, causing browning, softening and ripening. Fats can also turn rancid through reaction with oxygen. These changes reduce quality even without microbes.
How preservation works
Preservation methods all work by removing a condition microbes need or by inactivating enzymes:
- Chilling and freezing: lower temperature slows microbial growth and enzyme activity. Freezing stops most growth but does not kill all microbes.
- Drying and dehydration: removing water denies microbes the moisture they need.
- Salting and sugaring: high salt or sugar draws water out of microbes, preventing growth.
- Canning and bottling: heat destroys microbes and the sealed container prevents recontamination.
- Pasteurisation: controlled heating kills most pathogens while keeping quality, as in milk.
- Acidifying (pickling) and chemical preservatives: create conditions microbes cannot tolerate.
- Vacuum and modified atmosphere packaging: remove or change the gases microbes need.
Different methods suit different foods and have trade-offs for nutrient content, texture and flavour, which links to food processing and technologies.
Safe food handling and the danger zone
Even good food can cause illness if handled poorly. Foodborne illness comes from eating food contaminated with harmful microbes or toxins. The key control is temperature.
The temperature danger zone is the range in which bacteria multiply fastest, roughly 5 to 60 degrees Celsius. Safe handling means keeping cold food below 5 degrees and hot food above 60 degrees, cooking thoroughly, cooling food quickly, and avoiding cross-contamination between raw and ready-to-eat foods through clean hands, surfaces and utensils.
Why this matters
Preservation and safe handling protect both the quantity of food, by reducing waste and spoilage, and its safety and quality. This connects to food security, since reducing loss makes more food available, and to the food technologies that extend shelf life.
In short, food spoils through microbes and enzymes that need warmth, moisture, food and time, and preservation removes those conditions while safe handling, centred on the temperature danger zone, prevents foodborne illness.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 SACE Stage 22 marksExplain the relationship between food poisoning and the temperature of food.Show worked answer →
For 2 marks, link temperature to bacterial growth and then to food poisoning.
Most food-poisoning bacteria multiply fastest in the temperature danger zone, which is between 5 degrees C and 60 degrees C. When food is kept in this range, bacteria can double roughly every 20 minutes, quickly reaching numbers high enough to cause illness (1 mark).
Keeping food cold (below 5 degrees C) slows bacterial growth, and cooking food thoroughly (above 60 degrees C, typically to 75 degrees C) kills most bacteria. So the risk of food poisoning rises sharply when food is held at warm temperatures in the danger zone (1 mark).
2019 SACE Stage 22 marksSally has leftover pizza that she plans to eat the next day. Explain two ways in which she could ensure that the leftovers are safe to eat.Show worked answer →
Two marks for two correct, explained food-safety practices (1 mark each).
Way 1 - Refrigerate promptly: Cool the leftovers quickly and store them in the fridge below 5 degrees C, out of the temperature danger zone, so bacteria cannot multiply to unsafe levels overnight (1 mark).
Way 2 - Reheat thoroughly: When eating the leftovers, reheat them until steaming hot all the way through (to at least 75 degrees C) to kill any bacteria that may have grown during storage (1 mark).
Other accepted points include covering the food and not leaving it out at room temperature for long periods.
2018 SACE Stage 22 marksBacteria can double in number every 20 minutes when exposed to favourable conditions. State two conditions that would increase bacterial growth in pre-packaged fresh fruit salads.Show worked answer →
One mark for each correct condition (two required).
Condition 1 - Warm temperature: Storing the fruit salad in the temperature danger zone (5 to 60 degrees C), such as at room temperature, lets bacteria multiply rapidly (1 mark).
Condition 2 - Moisture: Fresh fruit salad is moist, and bacteria need water (and the available nutrients and sugars in the fruit) to grow (1 mark).
Time is also an accepted condition - the longer the salad is left in favourable conditions, the more the bacteria multiply.