Food Science and Food Safety
FoundationMicroorganisms such as bacteria and mould multiply quickly in warm, damp places and spoil food; chilling, freezing, drying, curing and canning all extend storage life. Reading the ingredient list, dates and nutrition labels, and following the five keys to food safety — choose wisely, keep clean, separate raw and cooked, cook thoroughly, keep safe temperatures — makes eating both safe and healthy.
- Microorganisms multiply fastest in the danger zone between four and sixty degrees Celsius and spoil food.
- Chilling slows and freezing nearly stops bacteria; drying, curing, canning and vacuum packing each work differently.
- Ingredient lists run from most to least; food past its 'use by' date must not be eaten.
文體小知識：食物安全科普說明文解釋食物變壞的原因和保存的原理，再逐點教人安全處理食物的方法。
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1Food goes bad over time mainly because microorganisms are at work. Bacteria and mould multiply extremely fast in warm, damp conditions and are most active between about four and sixty degrees Celsius — the 'danger zone'. Spoiled food gives off odd smells, grows mould, changes colour or turns slimy, but the toxins of some bacteria have neither smell nor visible sign, so judging by nose and eye alone is not reliable.
2Most ways of preserving food work by making life hard for microorganisms. Chilling lowers the temperature below four degrees Celsius to slow bacteria down; freezing, below minus eighteen degrees, nearly stops their multiplication. Drying removes water so microorganisms struggle to grow — dried squid and shiitake mushrooms are examples. Curing uses lots of salt or sugar to hold bacteria back, as with salted eggs and preserved fruit. Cans are sterilised at high heat and sealed against microbes, and vacuum packing draws out the air so bacteria that need oxygen cannot live.
3Labels on packaged food carry key information. The ingredient list runs from most to least, so the first item is the main one. 'Best before' means the food is at its best quality before that date — it may not be harmful just after, but flavour and texture decline. 'Use by' is about safety: past that date, do not eat it. Nutrition labels list energy and protein, carbohydrate, sugar, fat, saturated fat, trans fat and sodium, so comparing products helps us choose wisely. Food additives such as preservatives, colourings and sweeteners are safe within legal limits, but too much may affect health.
4The World Health Organization's five keys to food safety are: first, choose wisely — buy fresh food with a clear source from reliable shops; second, keep clean — wash hands with soap before and after handling food, and clean utensils and surfaces; third, separate raw and cooked — use different boards and knives and store them apart; fourth, cook thoroughly — food should reach at least seventy-five degrees Celsius at its centre, and reheating must be thorough too; fifth, keep safe temperatures — hot food above sixty degrees, cold food below four degrees, and no more than two hours at room temperature. Following the five keys greatly lowers the risk of food poisoning.
1Summary 重點摘要
Microorganisms such as bacteria and mould multiply quickly in warm, damp places and spoil food; chilling, freezing, drying, curing and canning all extend storage life. Reading the ingredient list, dates and nutrition labels, and following the five keys to food safety — choose wisely, keep clean, separate raw and cooked, cook thoroughly, keep safe temperatures — makes eating both safe and healthy.
2Vocabulary cards Vocabulary
Tiny living things too small to see, such as bacteria and mould; some of them spoil food.
When food changes in smell, colour or quality because microorganisms have grown, and is no longer fit to eat.
Storing food below four degrees Celsius to slow the multiplication of bacteria.
Substances added to food to preserve it, colour it or flavour it, such as preservatives and colourings.
An additive that holds back microorganisms and lengthens a food's storage life.
The list on packaged food arranged from the largest to the smallest amount.
The date by which food is at its best quality — about quality, not safety.
The safety deadline of a food; past it, the food must not be eaten.
The label listing a food's energy and main nutrients, helping us compare and choose.
When bacteria from raw food transfer to cooked or ready-to-eat food, such as cutting raw meat and fruit with the same knife.
3Step by step Steps
- Step 1: Know the danger zoneExample: a carton of fresh milk left at room temperature on a summer day turns sour within hours, because bacteria multiply extremely fast in the 'danger zone' between four and sixty degrees Celsius — their numbers can double about every twenty minutes. In a fridge below four degrees they slow down greatly, and thorough cooking kills most bacteria. Remembering the danger zone explains why food must not sit at room temperature for long.
- Step 2: Spot the signs of spoilage — and their limitsExample: bread with mouldy spots and leftover rice smelling sour have clearly spoiled and must not be eaten. However, some poisons made by bacteria have no colour and no smell, so the nose and eyes alone are not reliable. Besides observing, check the storage temperature and the date labels as well; putting the clues together gives a much better idea of whether the food is safe.
- Step 3: Compare how preservation methods workExample: dried squid keeps because removing the water leaves bacteria little chance to multiply; salted eggs rely on heavy salt to hold bacteria back; cans are sterilised at high temperature and then sealed against new bacteria; frozen meat stays at minus eighteen degrees Celsius, where bacteria almost stop moving. The methods differ, but the principle is the same: create surroundings where bacteria cannot live or multiply, and the food lasts longer.
- Step 4: Read ingredient lists and datesExample: on a biscuit packet the ingredient list runs from the largest amount to the smallest, so wheat flour listed first is the main ingredient. For dates, food past its 'best before' has simply lost some quality, but food past its 'use by' must be thrown away because it may no longer be safe. Reading these two pieces of information means shop advertising can no longer mislead us.
- Step 5: Compare nutrition labels to choose wiselyExample: two kinds of biscuits contain eight grams and thirty-two grams of sugar per hundred grams; comparing and choosing the lower one reduces the risk of tooth decay and obesity over time. Nutrition labels usually list energy, sugar, sodium and fat, and drinks can be compared for energy and sugar too. When similar products are compared side by side, the numbers speak — a basic skill of a smart consumer.
- Step 6: Separate raw and cooked in the kitchenExample: using the same knife to cut salad greens right after cutting raw chicken transfers bacteria onto ready-to-eat food — this is cross-contamination. The correct way is to prepare two chopping boards and two knives, handle raw and cooked food separately, and wash everything at once with detergent and clean water. When storing, keep raw meat covered on the lower shelf of the fridge so its juices cannot drip onto other food, making the kitchen much safer.
- Step 7: Master safe cooking and storage temperaturesExample: a chicken thigh is safe only when its centre reaches at least seventy-five degrees Celsius, the heat that kills salmonella hiding in the meat; a food thermometer pushed into the thickest part gives the most accurate reading. Put leftovers into the fridge within two hours, reheat until steaming throughout, and avoid reheating more than once. Mastering these temperature rules lets us eat hot food with confidence and store it with peace of mind.
4Diagrams Diagrams
5Worked example Example
Example: Ling is going on a picnic with classmates, bringing rice balls, cut watermelon, roast chicken wings and fresh milk. She leaves at ten in the morning with all the food in her backpack, and they eat at one in the afternoon. Point out the food safety risks and suggest improvements.
- Risk one: the rice balls and chicken wings sit in the danger zone for about three hours in hot weather, letting bacteria multiply and risking food poisoning.
- Risk two: the cut watermelon lies in the heat where surface bacteria multiply faster, and it cannot be washed again.
- Risk three: the fresh milk is never chilled, so as it warms it spoils easily.
- Improvement one: carry hot food such as the wings in an insulated flask above sixty degrees Celsius, or heat them thoroughly just before leaving.
- Improvement two: put the milk and watermelon in a cooler bag with ice packs to stay below four degrees, and have everyone wash hands with soap or wet wipes before eating.
- Answer: In hot weather, food left at room temperature for more than two hours risks food poisoning; keep hot food above sixty degrees, pack perishables with ice packs below four degrees, and keep hands and utensils clean.
6Common mistakes Mistakes
7Quick check Quick check
Q1In what temperature range do bacteria multiply fastest, and what is it called?
Q2How do drying and canning each preserve food?
Q3What is the difference between 'best before' and 'use by'?
Q4Name the five keys to food safety.
Q5What is cross-contamination? Give an example of how to avoid it.
8Recap Recap
- Microorganisms multiply fastest in the danger zone between four and sixty degrees Celsius and spoil food.
- Chilling slows and freezing nearly stops bacteria; drying, curing, canning and vacuum packing each work differently.
- Ingredient lists run from most to least; food past its 'use by' date must not be eaten.
- Nutrition labels let us compare sugar, fat and sodium and choose wisely.
- Five keys to food safety: choose wisely, keep clean, separate raw and cooked, cook thoroughly, keep safe temperatures.