Why food makes people ill
Microbes are living things too small to see. Some are useful (yeast in bread, bacteria in curd). Others are pathogens: they cause disease.
Food poisoning happens when we eat food with harmful microbes or their poisons (toxins). Signs: stomach pain, vomiting, loose motions, fever.
Common culprits: Salmonella (raw eggs, chicken), E. coli (dirty water, undercooked meat), Listeria (soft cheese, cold ready meals), and moulds on old bread or nuts.
Hazards in food come in three kinds:
- Biological: bacteria, viruses, moulds, parasites.
- Chemical: pesticide residues, cleaning liquids, too much colour or preservative.
- Physical: hair, glass, stones, plastic.
The four rules: clean, separate, cook, chill
- Clean. Wash hands with soap for 20 seconds before cooking and after the toilet. Wash fruit and vegetables in clean water. Keep surfaces clean.
- Separate. Keep raw meat, fish and eggs away from food that will not be cooked again. Use different boards and knives. Moving germs from one food to another is called cross-contamination.
- Cook. Heat food until the centre is at least 75 °C (steaming hot all through). Reheat leftovers only once.
- Chill. Keep the fridge at 5 °C or below. Put leftovers in the fridge within 2 hours. Thaw frozen food in the fridge, not on the counter.
The temperature danger zone
Bacteria grow fastest between 5 °C and 60 °C. This is the danger zone. Near body temperature (about 37 °C) some can split in two every 20 minutes.
So 1 bacterium → 2 → 4 → 8... After 3 hours (9 doublings): 2⁹ = 512.
- Below 5 °C (fridge): growth slows or stops. Germs are not killed.
- Below −18 °C (freezer): growth stops. Germs wake up when thawed.
- Above 60 °C: most bacteria start to die; 75 °C in the centre is the safe cooking target.
Some bacteria make toxins that heat does not destroy. So reheating old food does not always make it safe.
Choosing and storing food safely
- Check the label: use-by date (safety: do not eat after it) and best-before date (quality: may be less fresh after).
- Look for a food-safety licence or mark on packets (in India, the FSSAI number).
- Avoid dented or swollen cans, torn packets, and cut fruit left open.
- In the fridge: cooked food on top shelves, raw meat at the bottom in a closed box so juices cannot drip.
- First in, first out: use older food first.
- Farms produce safe food by using clean water, the right amount of pesticide, and waiting the set days after spraying before harvest.
Hygiene in kitchens and food factories
Food businesses use HACCP (Hazard Analysis and Critical Control Points):
- List the hazards at each step.
- Find the critical control points (steps where a hazard can be stopped, like cooking or chilling).
- Set limits (e.g. cook to 75 °C).
- Check and record (thermometer readings).
- Fix problems, review the plan and keep records.
Cleaning removes dirt and grease with detergent. Disinfecting then kills most remaining germs. Order: remove food bits → wash with detergent → rinse → disinfect → dry. Also: pest control, covered bins, clean uniforms, hair nets, no work when sick.
Food labs test samples: they grow microbes on agar plates and count the colonies, check pH and temperature, and always work with clean, sterile tools.
Try it
In the 3D, set 30 °C and 2 hours. Predict the number of germs, then check. Now set 4 °C. At home: find a thermometer or the fridge dial, and check where raw and cooked food are kept. Read the dates on three packets in your kitchen.
Key formulas and definitions
- Danger zone: 5 °C – 60 °C
- Cook to 75 °C in the centre; fridge ≤ 5 °C; freezer ≤ −18 °C
- Doubling: N = N₀ × 2ⁿ, n = time ÷ doubling time
- 2-hour rule: refrigerate cooked food within 2 hours
- Use-by = safety; best-before = quality
Worked examples
1. One bacterium doubles every 20 minutes. How many after 2 hours?
2 hours = 120 min = 6 doublings. N = 1 × 2⁶ = 64.
2. A cooked dish has 50 bacteria and is left out for 1 hour (doubling every 20 min). How many now?
3 doublings: 50 × 2³ = 50 × 8 = 400.
3. Name the hazard type: (a) a hair in soup, (b) bleach in a cup, (c) Salmonella in an egg.
(a) physical, (b) chemical, (c) biological.
4. A chef cuts raw chicken and then slices salad with the same knife. What went wrong and how do we fix it?
Cross-contamination: germs from raw chicken moved to food that will not be cooked. Use a separate knife and board, or wash and disinfect them in between, and wash hands.
5. In a HACCP plan for making rice, which step is a critical control point and what is its limit?
Cooking (limit: steaming hot, at least 75 °C in the centre) and cooling (cool quickly and refrigerate within 2 hours at ≤ 5 °C).
Common mistakes
- Thinking the fridge kills germs. It only slows them down.
- Thawing frozen meat on the kitchen counter for hours (it sits in the danger zone).
- Washing raw chicken in the sink: the splashes spread germs.
- Mixing up use-by (safety) and best-before (quality).