Vegetables
Senior 1 · Agriculture
Vegetables
Lesson 5 of 5
22 min
Let's begin
At the peak of the harvest, tomatoes flood the market and a basin sells for very little; some are left to rot. A few months later the same basin costs several times as much. The farmer who can keep part of the harvest in good condition, or turn it into a product that lasts, earns from both seasons.
Preservation means treating or storing food so that it stays safe and good to eat for longer. This lesson explains why vegetables spoil so quickly and the methods that slow spoilage down.
Every method works by one or more of these: removing water, lowering the temperature, making conditions too acid or too salty for microbes, killing microbes with heat and sealing out air, and stopping enzymes.
| Method | How it works | Examples |
|---|---|---|
| Cool storage | Low temperature slows respiration, enzymes and microbes. Keep produce in shade; use an evaporative cooler (such as a clay pot-in-pot cooler with wet sand between the pots, or a charcoal cooler) or a refrigerator. | Tomatoes, leafy greens, carrots for a few days or weeks |
| Drying | Removes most of the water, so microbes cannot grow. Sun drying on raised racks, or a solar dryer (an enclosed box or tent heated by the sun) which is faster and keeps out dust, insects and rain. | Amaranth and other leaves, chilli, tomato slices, onion, carrot |
| Blanching (before drying or freezing) | Dipping in boiling water for 1 to 3 minutes, then cooling quickly in cold water, stops the enzymes that spoil colour and flavour. | Leafy vegetables, carrots, French beans |
| Salting and fermenting | Salt draws water out of the vegetable and out of microbes; in brine, useful lactic acid bacteria ferment the sugars and the acid preserves the food. | Salted cabbage (as in sauerkraut) |
| Pickling | Soaking in vinegar makes the vegetable too acid for spoilage microbes. | Onions, cucumbers, chillies |
| Canning and bottling | Heating in sealed jars or cans kills microbes and keeps air out. | Tomato sauce and paste |
| Freezing | Very low temperature stops microbes (blanch first). Needs electricity. | French beans, peas |
| Curing and dry storage (no processing) | Dry the outer skins in the sun or shade for one to two weeks, then store in a cool, dry, airy place. | Onions and garlic in net bags; pumpkins |
Question: 10 kg of fresh tomatoes contain 94% water (illustrative). They are dried until the dried product contains only 10% water. (a) What mass of dry matter do the tomatoes contain? (b) What is the mass of the dried product? (c) Fresh tomatoes sell for 150 Frw per kg at the harvest glut, and dried tomatoes for 3,000 Frw per kg (illustrative prices). Compare the value.
(a) Dry matter = 6% of 10 kg = 0.06 × 10 = 0.6 kg. Drying removes water only, so this 0.6 kg stays.
(b) In the dried product, dry matter is 90% of the mass. Dried mass = 0.6 ÷ 0.9 ≈ 0.67 kg. So 10 kg of fresh tomatoes shrink to about two thirds of a kilogram.
(c) Fresh: 10 × 150 = 1,500 Frw. Dried: 0.6 ÷ 0.9 × 3,000 = 2,000 Frw. Drying adds about 500 Frw of value, before the cost of labour and packaging, and the product keeps for months instead of days.
Tip
Memory hook: microbes need water, warmth and a friendly (not acid, not salty) home, so preservation takes away Water (drying), Warmth (cooling) or the Welcome (salt, vinegar, heat and sealing).
Exam technique: for each method, state the principle it uses. "Pickling: vinegar makes the food too acid for microbes" scores more than "pickling" alone.
Give three reasons why vegetables spoil quickly after harvest.
Solution: Any three: high water content supports microbes; they continue to respire and lose water; enzymes keep acting; microbes enter through bruises; pests attack them.
Explain why leafy vegetables are blanched before drying.
Solution: Brief heating in boiling water stops the enzymes that would otherwise destroy the colour, flavour and vitamins during drying and storage.
Give two advantages of a solar dryer over drying on the open ground.
Solution: Any two: faster drying; produce is protected from dust, insects, animals and rain; a cleaner, better-coloured product that sells for more.
State the principle used in (a) pickling, (b) an evaporative cooler.
Solution: (a) Vinegar makes the food too acid for spoilage microbes. (b) Evaporating water takes heat away, lowering the temperature and slowing respiration and microbial growth.
20 kg of fresh amaranth leaves contain 90% water. What mass of dry matter do they contain?
Solution: Dry matter = 10% of 20 kg = 0.1 × 20 = 2 kg.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
