Food Hygiene and Safety Techniques
Senior 1 · Home Science
Food Hygiene and Safety Techniques
Lesson 3 of 5
22 min
Let's begin
Water goes into almost everything a cook makes: drinks, porridge, juice, the rinse for salad tomatoes, and the water used to wash plates and hands. Water that looks clear can still carry the germs of cholera, typhoid and diarrhoea. The same is true of food bought at the market: a fish or a tin can look fine and still be unsafe.
In this lesson you will learn four ways to make water safe at home, how to store it so it stays safe, and how to choose and prepare raw food so that it does not bring hazards into the kitchen.
Household water treatment means making water safe at the point where it is used, in the home or the kitchen.
| Method | How to do it | Advantages | Limitations |
|---|---|---|---|
| Boiling | Bring the water to a rolling boil (large bubbles that keep coming even when stirred). The US Centers for Disease Control and Prevention advise 1 minute of rolling boil, or 3 minutes above 2 000 m altitude, where water boils at a lower temperature. Cool it covered. | Kills bacteria, viruses and parasites; needs no chemicals | Uses fuel; does not remove chemicals or mud; boiled water can taste flat (pour it between two clean containers to add air) |
| Chlorination | Add liquid chlorine or a chlorine tablet made for drinking water, in the dose printed on the label for that volume. Mix, cover, and wait at least 30 minutes before drinking. | Cheap; a little chlorine stays in the water and keeps protecting it in storage | Works poorly in cloudy water, so let it settle or filter it first; some people dislike the taste |
| Filtration | Pour water through a ceramic or other household filter. A folded clean cloth removes mud and particles, and is a useful first step before another method. | Removes mud and most bacteria and parasites; no fuel | Most household filters do not remove all viruses; the filter must be cleaned and replaced as the maker says |
| Solar disinfection (SODIS) | Fill clear plastic (PET) bottles of 2 litres or less with clear water, shake, close, and lay them in full sun, for example on a metal roof, for at least 6 hours; cloudy weather needs longer. | Free; uses sunlight's heat and ultraviolet rays; the water is drunk from the same bottle | Water must be clear; needs sunshine and many bottles; coloured or scratched bottles work poorly |
Is the water clear enough for SODIS? Stand the full bottle on a printed headline and look down through the opening. If you can read the letters, the water is clear enough. If not, filter it or let it settle first.
| Food | Signs that it is safe and fresh | Reject it if |
|---|---|---|
| Fruit and vegetables | Firm, bright, clean | Mouldy, rotting, deeply bruised, cut open, or laid on muddy ground |
| Fresh fish (tilapia, sambaza) | Clear, bulging eyes; bright red gills; firm flesh; fresh smell | Cloudy sunken eyes, grey gills, soft flesh, strong smell |
| Meat | From an inspected butchery; firm; kept cool and covered | Slimy, bad-smelling, or hanging in the sun among flies |
| Eggs | Clean, uncracked shells | Cracked or dirty with droppings |
| Dry grain, beans and flour | Dry, clean, free-flowing | Damp, lumpy, mouldy, or with weevils or holes |
| Packed and tinned food | Seal unbroken; date not passed | Tin bulging, leaking, rusty or deeply dented; packet torn |
Date labels. A "use by" date is about safety: do not eat the food after it, even if it looks fine. A "best before" date is about quality: after it the food may be less fresh or crisp, but it is usually still safe if stored correctly.
Task: The Habimana family of 5 lives at about 2 200 m on the slopes of the volcanoes. Each person drinks about 2 litres of water a day. (a) How many 1.5-litre bottles must they fill for SODIS each day? (b) If they boil the water instead, how long should the rolling boil last?
Solution:
(a) Water needed = 5 × 2 = 10 litres. Bottles = 10 ÷ 1.5 = 6.67. A part-bottle is not possible, so round up: 7 bottles. (Rounding down to 6 bottles gives only 9 litres, not enough.)
(b) They live above 2 000 m, where water boils below 100 °C, so the CDC advice is a rolling boil for 3 minutes, not 1 minute.
Tip
Memory hook: "Boil, Bleach, Filter, Sun" are the four home methods, and "pour, don't dip" keeps treated water safe.
Exam technique: when you compare methods, give one advantage AND one limitation of each. For date labels: "use by" = safety, "best before" = quality.
Give one advantage and one limitation of boiling water.
Solution: Advantage: it kills bacteria, viruses and parasites without chemicals. Limitation: it uses fuel (or: it does not remove chemicals or mud).
Why must water be clear before SODIS or chlorination is used?
Solution: Mud and particles block the sun's rays in SODIS and use up the chlorine in chlorination, so germs hidden in cloudy water may survive. Filter or settle the water first.
A tin of beans is bulging at both ends. Should it be bought? Explain.
Solution: No. A bulging tin may mean that bacteria inside are producing gas, and some of these make dangerous poisons.
A packet of biscuits says "best before" last week. A carton of milk says "use by" yesterday. Which should be thrown away?
Solution: The milk. "Use by" is a safety date. The biscuits are past their "best before" date, which is about quality, so they are probably still safe if the packet is sealed and dry, though they may be less crisp.
A school needs 300 litres of treated drinking water a day and uses chlorine. How many 20-litre jerrycans is this?
Solution: 300 ÷ 20 = 15 jerrycans.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
