Technology and Living Advanced: Core Concepts in Depth
AdvancedThis lesson deepens the food-science strand: it explains the three causes of food spoilage — microbial growth, enzymatic action and oxidation — then analyses the principles and limits of chilling and freezing, dehydration, curing, canning and vacuum packing, and evaluates how processing affects nutrition and eating quality, so that home storage decisions rest on principles.
- Spoilage has three causes: microbial growth, enzymatic action and oxidation.
- Low temperature suppresses rather than kills micro-organisms; handle defrosted food promptly.
- Dehydration and curing control moisture; canning relies on exhausting, sealing and heating.
文體小知識：科技知識說明文——以食物變壞的三大原因為起點，逐項說明低溫、脫水、醃漬、罐頭與真空包裝等保存方法的原理與限制，並評價加工對營養與品質的影響。
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1At junior level you only need to know that food should be kept in a cool, dry place; at senior level you must explain why. Food spoilage has three main causes: microbial growth, enzymatic action and oxidation. Understanding the causes reveals which link each preservation method targets, giving both answers and household decisions a firmer grounding.
2Micro-organisms include bacteria, yeasts and moulds, which multiply rapidly in warm, moist, nutrient-rich conditions. Moisture and temperature are the key factors: in summer room heat a dish can spoil within hours, while a dry environment leaves micro-organisms struggling to act. Controlling temperature and moisture is the starting point of most preservation methods.
3The other two causes are unrelated to microbes: enzymes are substances contained in the food itself that turn fruit brown and soft — a cut apple darkening is the classic case — while oxidation mainly affects fats and oils, giving fatty foods off-flavours. Preservation therefore must also consider blocking air and slowing enzymes, not only fighting microbes.
4Common preservation methods each have a principle: chilling lowers temperature to slow microbes; freezing suppresses their activity further; dehydration and curing remove or bind water; canning exhausts, seals and heats the container to shut out contamination; vacuum and modified-atmosphere packing reduce oxygen inside the pack. Whatever the method, the principles come down to controlling temperature, moisture, oxygen and routes of contamination.
5Processing changes eating quality: high-temperature treatment may destroy part of the vitamins, and long cooking affects texture; many processed foods add extra salt, sugar or fat for taste and shelf life. Students should learn to judge from information: processing is not inherently bad — the method, the amount and whether it fits the need are what matter.
6Hong Kong's humid, hot climate makes food spoil faster, so home purchasing and storage must adapt: in damp spells dried goods grow mould easily and should be sealed; in summer heat, food should be processed and refrigerated soon after purchase. Linking preservation principles to the local climate and habits is exactly the applied ability this subject stresses.
1Summary 重點摘要
This lesson deepens the food-science strand: it explains the three causes of food spoilage — microbial growth, enzymatic action and oxidation — then analyses the principles and limits of chilling and freezing, dehydration, curing, canning and vacuum packing, and evaluates how processing affects nutrition and eating quality, so that home storage decisions rest on principles.
2Vocabulary cards Vocabulary
A tiny living thing invisible to the naked eye, such as bacteria, yeast or mould.
Single-celled micro-organisms that can multiply in food and spoil it.
A fungus that grows in damp conditions and forms mould spots on food.
A substance contained in food that catalyses changes such as browning or softening.
The reaction of food with oxygen in the air, giving fats off-flavours.
Storing food at low temperature to slow microbial activity.
Storing food below freezing point to suppress microbial and enzymatic activity.
Removing water from food by sun- or oven-drying to preserve it.
Treating food with salt or sugar to reduce the water available to microbes.
Processing that exhausts, seals and heats food so it keeps at room temperature.
3Step by step Steps
- Step 1: Explaining the three causes of spoilageWhen answering why food spoils, cover all three causes with explanation: microbes multiply in warm, moist conditions and break food down; enzymes are the food's own substances that soften and discolour it; oxidation gives fats off-flavours. For example, bread growing mouldy spots in humid weather is mould growth; a cut apple turning brown is enzymatic action; fried snacks developing a rancid smell after long storage is fat oxidation — three examples, one for each cause.
- Step 2: Analysing low-temperature preservationChilling and freezing differ in temperature and effect: chilling slows microbial activity and suits short-term storage; freezing turns water to ice and strongly suppresses microbes and enzymes, suiting longer storage. Low temperature does not kill micro-organisms — once the temperature rises, their activity resumes. For example, fresh meat keeps only a few days in the chiller but much longer in the freezer; defrosted food should be cooked promptly and not thawed and refrozen repeatedly, or quality will decline.
- Step 3: Removing or binding waterMicrobes and enzymes need water to work, so dehydration and curing start there: dehydration removes water by sun- or oven-drying to make dried vegetables and fish; curing uses salt or sugar to reduce the water available to microbes — salted eggs and jam belong to this family. For example, candied fruit keeps because sugar binds water so microbes cannot act; dried vegetables must be soaked and washed thoroughly before use, since drying also costs some vitamins.
- Step 4: Understanding canningIn canning, food is packed into containers, exhausted and sealed, then heat-treated to destroy spoilage micro-organisms and shut out outside contamination, so it keeps at room temperature. The protection comes from the seal: once opened, it disappears. For example, an unopened can of soup can be stored in a cool place, but after opening the rest must go into a covered container in the fridge and be eaten soon; a bulging or damaged can should not be eaten at all, as the contents may already be contaminated.
- Step 5: Comparing preservation methodsCompare methods with criteria: length of storage, effects on nutrition and flavour, cost and convenience. Low-temperature methods best retain original flavour but depend on a fridge and electricity; dehydration and canning store long and travel well but change texture and lose some nutrients. For example, compared with fresh vegetables, frozen ones keep fibre and minerals reasonably well but taste softer; for a long trip, canned and dried foods are far more dependable than fresh meat.
- Step 6: Evaluating processing effectsProcessing is neither good nor bad in itself; judge by method and purpose: blanching and freezing largely retain nutrients; prolonged high-temperature processing loses more vitamins; and for taste and shelf life many processed foods carry extra salt, sugar or fat, which is unwise in excess. For example, comparing the ingredient labels of fresh milk and milk drinks, or fresh fruit and juice drinks, shows the difference between processing and additives, and choice becomes a matter of weighing needs.
- Step 7: Linking climate and household decisionsHong Kong's hot, humid weather means preservation principles must be applied to real conditions: in damp seasons dried goods grow mould easily and should be sealed with desiccant; in summer heat cooked food should not sit long at room temperature; purchasing plans can follow shelf life, eating leafy vegetables first and roots later. For example, stocking up on canned food, dried noodles and frozen items before a typhoon is exactly a decision made on preservation principles, balancing safety, nutrition and convenience.
5Worked example Example
Example question: A family buys in bulk at the market every Sunday and puts all the meat and vegetables in the lower compartment of the fridge, yet some food has spoiled by midweek. Using preservation principles, analyse the cause and suggest improvements.
- Analyse the cause: the lower compartment is chilled storage, which only slows microbial activity; food still spoils after several days, and overloading blocks cold-air circulation.
- Allocate by shelf life: leafy vegetables keep only a few days and should be eaten first; hardy roots follow; portion the meat per meal and freeze it.
- Improve shopping habits: replace one bulk purchase with smaller, more frequent trips, or supplement with frozen and dried foods to reduce spoilage.
- Build routines: check the fridge temperature and stock regularly, use older items first, and prevent food piling up and spoiling.
- Answer: The cause is a mismatch of method and need: chilling only keeps food fresh for days, and bulk-bought meat and vegetables cannot be finished in time, so spoilage is inevitable. Three improvements follow: portion and freeze meat per meal, eat leafy vegetables early and schedule hardier produce later; switch to smaller, more frequent purchases; supplement with frozen and dried foods. Each suggestion must state the preservation principle behind it — writing 'put it in the freezer' without explanation will not earn top marks.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the three main causes of food spoilage?
Q2How do chilling and freezing differ?
Q3Which condition do dehydration and curing both target?
Q4Why can canned food keep at room temperature?
Q5How should an opened can be handled?
8Recap Recap
- Spoilage has three causes: microbial growth, enzymatic action and oxidation.
- Low temperature suppresses rather than kills micro-organisms; handle defrosted food promptly.
- Dehydration and curing control moisture; canning relies on exhausting, sealing and heating.
- Compare preservation methods by shelf life, nutrition, texture and cost.
- Household storage strategies should follow the local humid climate and shelf life.