Why does food go off, what causes spoilage and contamination, and how can food be preserved?
Explain the causes of food spoilage and contamination and describe methods of food preservation
A focused answer on why food spoils - micro-organisms and enzymes - the types of contamination, the signs of spoilage, and how preservation methods such as chilling, freezing, drying and canning slow it down.
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What this dot point is asking
The syllabus wants you to explain why food goes off, distinguish spoilage from contamination, recognise the signs, and describe how preservation slows decay. The central idea is that food spoilage is caused mainly by living micro-organisms and by enzymes, both of which need certain conditions to act, so preservation works by removing one of those conditions.
The answer
What causes spoilage
Food spoilage has two main causes:
- Micro-organisms: bacteria, moulds and yeasts land on food and grow, breaking it down. They cause off smells, slime, mould growth and souring.
- Enzymes: these are naturally present in the food and keep acting after harvest or slaughter. They cause fruit to ripen and then over-ripen and brown, and meat and fish to deteriorate.
Both speed up in warm, moist conditions and slow down in cold or dry ones, which is the basis of preservation.
Contamination versus spoilage
Spoilage makes food unpleasant - off smell, slime, mould - and is usually obvious. Contamination is the presence of harmful bacteria that cause illness, and dangerously, contaminated food can look, smell and taste completely normal. So food can be safe-looking but unsafe. Contamination can be by bacteria, by physical objects (hair, glass), or by chemicals (cleaning products).
Signs of spoilage
- An unpleasant, sour or "off" smell.
- A change in appearance: mould, discolouration, or a dull look.
- A change in texture: slimy, soft, mushy or dried out.
- A sour or off taste (though tasting suspect food is risky).
Methods of preservation
Preservation works by removing a condition that micro-organisms or enzymes need:
- Chilling and freezing keep food cold, slowing or stopping microbial growth and enzyme action. Freezing does not kill bacteria; they restart when the food thaws.
- Drying removes water, so micro-organisms cannot grow (for example dried fish, dried mushrooms).
- Canning and bottling heat food to destroy micro-organisms and seal it airtight so none can enter.
- Using salt or sugar in high amounts (salting fish, making jam) draws water out of micro-organisms so they cannot grow.
- Using acid (pickling in vinegar) creates conditions micro-organisms dislike.
Examples in context
Example 1. Dried ikan bilis. Drying anchovies removes almost all their water, so micro-organisms cannot grow and the fish keeps for a long time at room temperature. This traditional method, common in Singaporean cooking, is preservation by removing moisture in action.
Example 2. Canned versus fresh pineapple. Canned pineapple is heated to destroy micro-organisms and sealed airtight, so it keeps for months in the cupboard, while fresh pineapple spoils within days as micro-organisms and enzymes act. The contrast shows how canning extends shelf life by destroying and excluding micro-organisms.
Try this
- Cue. State the two main causes of food spoilage and the conditions that speed them up. Recall micro-organisms and enzymes, faster in warmth and moisture.
- Cue. Explain the difference between spoilage and contamination. Recall that spoilage is usually obvious, while harmful contamination can leave food looking and smelling normal.
- Cue. Describe how drying and canning each preserve food. Recall drying removes water so microbes cannot grow, and canning heats to destroy microbes and seals out air.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original6 marksExplain the main causes of food spoilage. Describe three signs that a food has spoiled.Show worked answer →
Food spoils mainly because of micro-organisms (bacteria, moulds and yeasts) that grow on it and break it down, and because of enzymes naturally present in the food that continue to act and cause changes such as ripening and then over-ripening. Spoilage is faster in warm, moist conditions.
Three signs of spoilage: an unpleasant or sour smell; a change in appearance such as mould growth or discolouration; and a change in texture such as becoming slimy, soft or mushy. The taste also becomes sour or off.
What markers reward: micro-organisms and enzymes named as the causes of spoilage, the link to warm, moist conditions, and three clear, distinct signs of spoilage.
Original5 marksDescribe how three different methods of food preservation work to make food last longer, giving a food example for each.Show worked answer →
Chilling and freezing: keeping food cold in the fridge or freezer slows down or stops the growth of micro-organisms and the action of enzymes, so the food lasts longer; for example, fresh fish kept in the fridge, or peas frozen.
Drying: removing the water from food means micro-organisms cannot grow without moisture; for example, dried fish (ikan bilis) or dried mushrooms.
Canning or bottling: the food is heated to destroy micro-organisms and sealed in an airtight container so none can enter; for example, canned pineapple or canned sardines.
What markers reward: three different preservation methods, a correct explanation of how each prevents spoilage (cold slows growth, drying removes water, canning destroys and seals out micro-organisms), and a food example for each.
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