Food (Classes VI-VIII) • Topic 4 of 4

Food Spoilage and Preservation

Food spoils mainly because of the growth of micro-organisms - bacteria, fungi (moulds) and yeast - helped by warmth, moisture and air; spoiled food changes in smell, taste, colour and texture and can cause illness. Preservation works by stopping or slowing this microbial growth. CTET expects you to match each method to how it works. Heating and boiling kills microbes (pasteurisation of milk heats then quickly cools it). Cooling and freezing (refrigeration) slows microbial growth. Drying/dehydration removes the moisture microbes need (sun-dried fish, papad, dried mango). Adding salt or sugar in high amounts draws out water and preserves food (pickles, jams, salted fish) - this is osmosis at the everyday level. Adding oil and vinegar, and using chemical preservatives like sodium benzoate or potassium metabisulphite, also prevent spoilage. Common salt is the oldest preservative; sealing in airtight cans (canning) keeps air and microbes out. The classic misconceptions a teacher must address: that food in the fridge 'never' spoils (it only slows down), that boiling makes food preserved forever, or that all microbes in food are harmful (fermentation by useful microbes makes curd, idli and bread). Strong CTET pedagogy answers connect preservation to children's home kitchens - why mother makes pickle with extra salt and oil, why milk is boiled, why we keep grains dry - so the science is grounded in lived experience.

✅ Solved examples

1. Why does adding a large amount of salt help preserve fish or pickle? Explain the principle.
High salt concentration draws water out of the food and out of any micro-organisms (osmosis), leaving too little moisture for microbes to grow. Without enough water, bacteria and fungi cannot multiply, so the food is preserved.
2. A child claims food kept in the refrigerator can never go bad. Correct this and name what refrigeration actually does.
Refrigeration does not stop spoilage permanently; it only slows the growth of micro-organisms by keeping food cold. Food will still spoil eventually, just more slowly, so the claim is wrong.
3. Name the preservation method in each case: sun-dried fish, boiled-then-cooled milk in cartons, mango jam.
Sun-dried fish - drying/dehydration (removing moisture); boiled-then-cooled packaged milk - pasteurisation (heat treatment); mango jam - preservation with sugar (high sugar concentration).
4. Are all micro-organisms in food harmful? Give one example a teacher could use to show otherwise.
No. Some micro-organisms are useful: for example, bacteria turn milk into curd, and yeast is used to make bread and idli batter rise (fermentation). So not all microbes spoil food - some help make it.

✏️ Practice — try these, take hints as needed

1. State two conditions that speed up the spoilage of food.
Microbes love a certain environment.
Think warmth and water.
Warmth (heat) and moisture (and presence of air) speed up microbial growth and so speed up spoilage.
2. How does drying or dehydration preserve food such as papad or dried mango?
What do microbes need to grow?
Removing one thing stops them.
Drying removes the moisture (water) from the food; without water, micro-organisms cannot grow and multiply, so the food is preserved.
3. What is pasteurisation, and which common food is preserved this way?
Involves heating then rapid cooling.
Think of the dairy.
Pasteurisation is heating a food (milk) to a high temperature and then cooling it quickly to kill harmful microbes; milk is the common food preserved this way.
4. Why is common salt called the oldest method of food preservation, and name one food preserved with it today.
It removes water from microbes.
Think pickles or salted fish.
Salt preserves by drawing water out of food and microbes (osmosis) so microbes cannot grow; it has been used for centuries. Pickles or salted/dried fish are preserved with salt today.

📝 Topic test — 8 questions

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