Food — Class 5 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 5 Science. Why food goes bad, how we stop it, and what a body needs from what it eats.
1. Why food spoils
Food spoils for two kinds of reason.
Biotic factors are living things. Bacteria, fungi (mould) and yeast are microscopic organisms that grow on food, break it down, and leave waste that makes it smell, taste and look bad. Insects and rodents spoil stored grain in a cruder way.
Abiotic factors are non-living conditions that help those organisms along:
- Temperature — warmth speeds spoilage
- Humidity — moisture encourages mould
- Air — oxygen turns oily food rancid
Notice that every abiotic factor works by making life easier for the microorganisms. That is the key to the whole chapter.
2. How food is preserved
Every preservation method removes one of the things microorganisms need.
| Method | What it removes or does | Examples |
|---|---|---|
| Drying | Takes away the moisture they need | Vathal, vadagam, dry fish, papad |
| Refrigeration | Cold slows their growth | Milk, vegetables |
| Freezing | Deep cold stops growth altogether | Peas, meat |
| Pickling | Salt, oil or vinegar creates conditions they cannot survive | Mango and lime pickle |
| Canning | Sealed and heated so none can get in | Tinned fruit, tomato puree |
| Adding sugar | High sugar draws water out of them | Jam, murabba |
| Vacuum packing | Removes the air | Packed snacks, coffee |
Sun-dried vathal and vadagam are the Tamil version of this science, worked out centuries before anyone had seen a bacterium.
Refrigeration does not kill bacteria. It only slows them, so food in a fridge still spoils, just more slowly. Freezing stops growth but does not sterilise either.
3. Best before and expiry date
These two labels do not mean the same thing.
Best before is about quality. The food is at its best until that date, but it is usually still safe afterwards — perhaps less crisp or less flavourful.
Expiry date is about safety. After it, the food should not be eaten.
4. The nutrients
| Nutrient | Job | Sources |
|---|---|---|
| Carbohydrates | Give energy | Rice, wheat, potato, sugar |
| Proteins | Build and repair the body | Dal, milk, egg, fish, groundnut |
| Fats | Store energy and keep the body warm | Oil, ghee, butter, nuts |
| Vitamins | Protect against disease | Fruits and vegetables |
| Minerals | Build bones, teeth and blood | Milk, greens, salt |
Water and roughage are needed too. Roughage is the fibre in vegetables and whole grains; it supplies no nutrients but keeps digestion moving.
5. Deficiency diseases
Going without a nutrient for a long time causes a deficiency disease.
| Missing | Disease | Signs | Sources |
|---|---|---|---|
| Vitamin A | Night blindness | Cannot see in dim light | Carrot, papaya, mango, greens |
| Vitamin B1 | Beriberi | Muscle weakness, nerve damage | Whole grains, pulses, nuts |
| Vitamin C | Scurvy | Bleeding gums, slow healing | Amla, guava, citrus fruits |
| Vitamin D | Rickets | Weak, bent bones | Sunlight, fish, egg yolk |
| Iron | Anaemia | Tiredness, pale skin | Greens, dates, jaggery |
| Iodine | Goitre | Swelling in the neck | Iodised salt, sea fish |
| Protein | Kwashiorkor | Poor growth, swollen belly | Dal, milk, egg, fish |
Amla is among the richest natural sources of vitamin C anywhere in the world.
Early signs are worth catching. Frequent tiredness with pale skin can mean anaemia; gums that bleed while brushing can mean too little vitamin C. Both are easy to correct through food if noticed early.
6. A balanced diet
A balanced diet supplies every nutrient in the right proportion. Roughly, carbohydrates should make up the largest share, then fats, then proteins, with vitamins, minerals and fibre from fruits and vegetables.
An ordinary South Indian meal comes close on its own: rice for carbohydrate, sambar and dal for protein, vegetables and greens for vitamins and minerals, a little oil or ghee for fat, and curd for protein and calcium.
7. Worked examples
Example 1. Name two biotic and two abiotic factors that spoil food.
Solution: Biotic: bacteria and fungi. Abiotic: warm temperature and humidity.
Example 2. Why does drying preserve food?
Solution: It removes the moisture that microorganisms need in order to grow.
Example 3. Does refrigeration kill bacteria?
Solution: No. It only slows their growth, so refrigerated food still spoils eventually.
Example 4. What is the difference between best before and expiry date?
Solution: Best before is about quality and the food is usually still safe afterwards. Expiry date is about safety, and after it the food should not be eaten.
Example 5. Name the disease caused by lack of vitamin B1.
Solution: Beriberi, causing muscle weakness and nerve damage.
Example 6. A child has a swelling in the neck. Which nutrient is missing and what should be used?
Solution: Iodine. Iodised salt should be used in cooking.
Example 7. What is roughage and why is it needed?
Solution: The fibre in vegetables and whole grains. It supplies no nutrients but keeps digestion moving.
8. Practice
- Name three microorganisms that spoil food.
- How does humidity help food to spoil?
- Name six methods of food preservation.
- Why does pickling preserve food?
- Give two Tamil examples of sun-dried preserved food.
- Name the five nutrients and the job of each.
- Which vitamin deficiency causes night blindness, and which foods prevent it?
- Name the deficiency disease caused by lack of protein.
- What is a balanced diet?
9. Answers
- Bacteria, fungi or mould, and yeast.
- Moisture encourages mould and other microorganisms to grow.
- Drying, refrigeration, freezing, pickling, canning and adding sugar (also vacuum packing).
- Salt, oil or vinegar creates conditions in which microorganisms cannot survive.
- Vathal and vadagam.
- Carbohydrates give energy, proteins build and repair, fats store energy and give warmth, vitamins protect from disease, minerals build bones, teeth and blood.
- Vitamin A. Carrot, papaya, mango and green leafy vegetables prevent it.
- Kwashiorkor.
- A diet containing all the nutrients in the right proportions.
10. Summary
- Biotic factors (bacteria, fungi, yeast, insects, rodents) and abiotic factors (temperature, humidity, air) spoil food.
- Every abiotic factor works by making life easier for the microorganisms.
- Preservation removes one thing they need: drying takes moisture, pickling makes conditions hostile, vacuum packing removes air.
- Refrigeration slows bacteria but does not kill them.
- Best before is about quality; expiry date is about safety.
- The five nutrients are carbohydrates, proteins, fats, vitamins and minerals, plus water and roughage.
- Deficiencies cause night blindness (A), beriberi (B1), scurvy (C), rickets (D), anaemia (iron), goitre (iodine) and kwashiorkor (protein).
- Amla is among the richest natural sources of vitamin C.
- A balanced diet supplies every nutrient in the right proportion.
