Food — Class 4 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 4 Science. Nutrients and what happens without them, balanced diet, and food preservation.
1. Why we eat
Food does three jobs: it gives energy, it helps the body grow and repair, and it protects us from disease. Different foods do different jobs, which is why no single food is enough.
2. The nutrients
| Nutrient | Job | Found in |
|---|---|---|
| Carbohydrates | Give energy | Rice, wheat, potato, sugar |
| Proteins | Build and repair the body | Dal, milk, egg, fish, groundnut |
| Fats | Store energy, keep the body warm | Oil, ghee, butter, nuts |
| Vitamins | Protect from disease | Fruits and vegetables |
| Minerals | Build bones, teeth and blood | Milk, green leafy vegetables, salt |
Water and roughage are needed too. Roughage is the fibre in vegetables and whole grains; it carries no nutrients but keeps digestion moving.
3. What happens when a nutrient is missing
A deficiency disease is caused by going without a nutrient for a long time.
| Missing | Disease | What it does | Good sources |
|---|---|---|---|
| Vitamin A | Night blindness | Cannot see in dim light | Carrot, papaya, mango, greens |
| 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 worth singling out — it is among the richest natural sources of vitamin C anywhere.
Notice the early signs. Feeling tired often with pale skin can mean anaemia. Gums that bleed while brushing can mean too little vitamin C. Caught early, both are easy to correct through food.
4. A balanced diet
A balanced diet contains all the nutrients in the right amounts. A South Indian meal is naturally close to one: 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.
5. Why food spoils
Food spoils because microorganisms — bacteria and fungi — grow on it. They need warmth, moisture and food to multiply. Take away any one of those and spoiling slows down. That is exactly what every preservation method does.
6. Six ways to preserve food
| Method | How it works | Examples |
|---|---|---|
| Drying | Removes the water microorganisms need | Vathal, dry fish, dry chillies, raisins, papad |
| Refrigeration and freezing | Cold slows the growth of bacteria | Milk, vegetables, meat |
| Pickling | Salt, oil or vinegar makes conditions bacteria cannot survive | Mango pickle, lime pickle |
| Canning | Sealed and heated so no microorganisms can get in | Tinned fruit, tomato puree |
| Adding sugar | High sugar draws water out of the microorganisms | Jam, murabba |
| Vacuum packing | Removes the air the microorganisms need | Packed snacks, coffee |
Sun-dried vathal and vadagam are the traditional Tamil version of this science, worked out long before anyone had heard of bacteria.
One important warning. Refrigeration does not kill bacteria. It only slows them down, so food in a fridge will still spoil eventually, just more slowly.
7. Worked examples
Example 1. Which nutrient builds and repairs the body?
Solution: Protein.
Example 2. A child cannot see well in dim light. Which vitamin is missing, and what should she eat?
Solution: Vitamin A. Carrot, papaya, mango and green leafy vegetables.
Example 3. Name the disease caused by a lack of iron, and its signs.
Solution: Anaemia — tiredness and pale skin.
Example 4. Why does drying preserve food?
Solution: It removes the water that bacteria and fungi need in order to grow.
Example 5. Does keeping food in a refrigerator kill the bacteria on it?
Solution: No. Cold only slows their growth, so the food still spoils eventually.
Example 6. Which salt is used to prevent goitre?
Solution: Iodised salt.
8. Practice
- Name the five nutrients.
- Which nutrient gives the most energy?
- Name the disease caused by a lack of vitamin C and one of its signs.
- Which fruit is among the richest sources of vitamin C?
- What is a balanced diet?
- Why does food spoil?
- Name any four methods of preserving food.
- Give two Tamil examples of sun-dried preserved food.
9. Answers
- Carbohydrates, proteins, fats, vitamins and minerals.
- Carbohydrates.
- Scurvy, which causes bleeding gums and slow healing.
- Amla, the Indian gooseberry.
- A diet containing all the nutrients in the right amounts.
- Because microorganisms such as bacteria and fungi grow on it.
- Drying, refrigeration, pickling and canning (also adding sugar, vacuum packing).
- Vathal and vadagam.
10. Summary
- Food gives energy, builds and repairs the body, and protects from disease.
- The five nutrients are carbohydrates, proteins, fats, vitamins and minerals, plus water and roughage.
- Deficiencies cause night blindness (A), 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 amount.
- Food spoils because bacteria and fungi grow on it, needing warmth, moisture and food.
- Drying, refrigeration, pickling, canning, sugar and vacuum packing each remove one of those needs.
- Refrigeration slows bacteria; it does not kill them.
