Why food spoils: microbes and what they need
A forgotten lunch box is opened after a weekend: furry coloured patches and a bad smell. Something tiny has been living on the food.
Microbes are living things so small that we need a microscope to see them. They are everywhere: in soil, water, air, plants, animals and inside us. When they grow on food they change its look, smell and taste, so the food spoils. Coloured patches are mould, a colony of thousands of microbes. Microbes need moisture, air and the right warmth to grow.
In the figure: Take away any one thing microbes need and their growth is stopped.
Key point 1: What microbes are
- Easy : Microbes are too small to see with eyes alone.
- Medium : Mould is a visible patch made of thousands of microbes.
- Exam-type : Some microbes spoil food, some are useful, and some cause illness.
Key point 2: Why some foods spoil faster
- Easy : Bread left in the open grows mould in two days.
- Medium : A pickle covered with oil stays good for months because oil keeps air away.
- Exam-type : Food with more moisture, left in a warm place, spoils fastest.
Spoiled food looks, smells or tastes different. Never taste it to check.
Thinking that mould is dust or a stain that can be wiped off a food that is still safe.
To stop spoilage, remove moisture, air or warmth. Memory clue: "M-A-W: Moisture, Air, Warmth – microbes love all three".
Spot the difference
| Point | Fresh food | Spoiled food |
|---|---|---|
| Smell | normal | bad smell |
| Look | even colour | coloured patches |
| Safe to eat? | yes | no |
Practice questions
- 1What do we need to see microbes?
- 2Name two places where microbes live.
- 3What are the coloured patches on old bread?
- 4Name two things microbes need to grow.
- 5Why does an oil-covered pickle stay good?
- 6Which will spoil faster: dry biscuit or cut fruit? Why?
- 7Why should we not eat food with mould, even if only a small patch shows?
- 8A cut mango is kept in a closed box in a fridge. Which of the three needs of microbes are reduced?
Show answers (try them yourself first)
- 1A microscope.
- 2Soil, water, air, plants, animals, our body (any two).
- 3Mould, made of many microbes.
- 4Moisture, air, warmth (any two).
- 5Oil keeps air away from the pickle.
- 6Cut fruit, because it has more moisture.
- 7Microbes may already be spread through the food and can make us ill.
- 8Air (closed box) and warmth (cold fridge).
Ways of preserving food
Grandmother dries chillies in the sun, adds oil to pickle and keeps milk in the fridge. Each trick stops microbes in a different way.
To preserve food is to keep it from spoiling. Drying (dehydration) removes moisture. Oil in pickle keeps air out. Cold in a fridge slows microbes down. Salt, sugar and spices help too, and factories use air-tight cans and packs.
In the figure: Each method removes one of moisture, air or warmth.
Key point 1: Methods and what they remove
- Easy : Sun-drying chillies, tomatoes or fish removes moisture.
- Medium : Oil on a pickle stops air; a fridge removes warmth; ghee lasts because its water has been boiled off.
- Exam-type : Salt and sugar pull water out of food; canned foods are sealed so air and microbes cannot enter.
Key point 2: Choosing the right method
- Easy : Raw milk and butter need the fridge.
- Medium : Wheat and rice stay good if kept dry in closed containers.
- Exam-type : For a two-day trip, take dry or well-packed food and keep water-rich food cool.
Preserved food still lasts only for some time. Check the date and store it as advised.
Thinking that any food kept in a fridge will stay good for ever.
Dry = no moisture, Oil/Seal = no air, Fridge = no warmth. Memory clue: "Dry, Seal, Chill".
Spot the difference
| Method | Removes | Example |
|---|---|---|
| Drying | moisture | chilli powder, papad |
| Oil / sealing | air | pickle, canned juice |
| Fridge | warmth | milk, butter |
Practice questions
- 1What does drying remove from food?
- 2Why is oil poured on a pickle?
- 3Name one food kept in a fridge.
- 4Why does ghee last longer than butter?
- 5Name two things used long ago to preserve food.
- 6Match: papad – ?, pickle – ?, milk – ? (dry / oil / fridge).
- 7You pack lunch for a long journey. Name two items and how you keep them safe.
- 8A tin of juice is opened. Why should it now go in the fridge?
Show answers (try them yourself first)
- 1Moisture (water).
- 2Oil keeps air out.
- 3Milk, butter, vegetables (any one).
- 4Water has been boiled off from ghee.
- 5Salt, sugar, spices (any two).
- 6Papad – dry, pickle – oil, milk – fridge.
- 7Example: puri (dry, packed) and fruit (cut late, kept in a closed box).
- 8After opening, air and microbes enter; cold slows them down.
Good microbes and digestion
A spoonful of curd with lunch, soft fluffy idlis for breakfast – some microbes are friends who make our food and help our tummy.
Not all microbes are bad. Good microbes turn milk into curd, make idli and dosa batter rise and help us digest food. Curd and buttermilk carry friendly microbes that join those in our stomach. Eating wrong or spoilt food can cause an upset stomach; if it is serious, we must see a doctor.
In the figure: Good microbes make curd from milk, and curd helps our stomach.
Key point 1: Microbes that help us
- Easy : Curd is made from milk with the help of microbes.
- Medium : Idli and dosa batter rises and becomes fluffy because of microbes.
- Exam-type : Fermented rice and buttermilk are cool, healthy foods in many parts of India.
Key point 2: Taking care of the stomach
- Easy : Eat fresh food and wash hands before eating.
- Medium : A glass of buttermilk can soothe a mildly upset stomach.
- Exam-type : If vomiting or pain is bad, go to a doctor and take medicine as advised.
Home remedies help in small problems; a serious problem needs a doctor.
Believing that all microbes are harmful and must be killed.
Good microbes help make curd, idli and digestion. Memory clue: "Curd–Idli–Digest: three C-I-D friends".
Spot the difference
| Point | Harmful microbes | Helpful microbes |
|---|---|---|
| Effect on food | spoil it | make curd, idli batter |
| Effect on us | can make us ill | help digestion |
| Example | mould on bread | microbes in curd |
Practice questions
- 1Name one food made with the help of microbes.
- 2Why is idli batter fluffy?
- 3What does curd give our stomach?
- 4Are all microbes harmful?
- 5What can buttermilk help with?
- 6Give one habit that prevents an upset stomach.
- 7When must you see a doctor for an upset stomach?
- 8Why is it wrong to say "kill all microbes"?
Show answers (try them yourself first)
- 1Curd (also idli/dosa, bread, buttermilk).
- 2Microbes in the batter make it rise.
- 3Good microbes that help digestion.
- 4No. Some are very helpful.
- 5Digestion, an upset stomach.
- 6Wash hands, eat fresh food (any one).
- 7When pain or vomiting is serious or does not stop.
- 8Many microbes are useful, like those in curd.
Eating right: seasonal food, teeth and chewing
Litchis arrive only in the rains, so the whole family waits and enjoys them. Good eating is also about how we eat – with clean teeth and slow chewing.
Fruits and vegetables taste best and are fresh in their season, especially when grown nearby. We have four kinds of teeth: incisors cut, canines tear, premolars crush and molars grind. Chewing well mixes food with saliva and helps digestion. Brushing and rinsing prevent cavities, and eating slowly prevents choking.
In the figure: Each kind of tooth has its own job in breaking food into small pieces.
Key point 1: Teeth and chewing
- Easy : Front teeth (incisors) bite and cut an apple.
- Medium : Pointed canines tear a piece of roti or a chikki.
- Exam-type : Premolars and molars crush and grind food; saliva then softens it so it digests easily.
Key point 2: Healthy eating habits
- Easy : Eat fruits and vegetables of the season; local ones are freshest.
- Medium : Brush every day, rinse after eating and have fewer sweets.
- Exam-type : Eat slowly, take small bites and do not talk or laugh with food in the mouth, to avoid choking.
A cavity is a hole in a tooth; see a dentist if a tooth aches.
Gulping food quickly to save time.
Teeth jobs – Cut, Tear, Crush, Grind (incisor, canine, premolar, molar). Memory clue: "I-C-P-M = Cut-Tear-Crush-Grind".
Spot the difference
| Tooth | Job | Where in mouth |
|---|---|---|
| Incisor | cuts | front |
| Canine | tears | beside incisors |
| Premolar / Molar | crush / grind | back |
Practice questions
- 1Which teeth cut food?
- 2Which teeth grind food?
- 3What does saliva do?
- 4What is a cavity?
- 5Name two seasonal fruits of your region.
- 6Why is eating slowly important?
- 7Which tooth would you use to tear a piece of chikki?
- 8Give two habits that keep teeth healthy.
Show answers (try them yourself first)
- 1Incisors.
- 2Molars.
- 3It softens food and helps digestion.
- 4A hole in a tooth.
- 5Example: mango (summer), amla (winter).
- 6To chew well and avoid choking.
- 7Canines.
- 8Brush daily, rinse after meals, eat fewer sweets (any two).