Food Safety Techniques
Senior 2 · Home Science
Food Safety Techniques
Lesson 3 of 4
20 min
Let's begin
Many Rwandan homes cook once and eat the same pot of beans or isombe at more than one meal, often without a fridge. That is a sensible way to save fuel and time, but it is also when most home food poisoning happens. Knowing how temperature and time control bacteria lets you store and reuse food safely.
In this lesson you will learn the temperature danger zone, the two-hour rule, how to store dry, fresh and cooked food, and how to reheat leftovers safely.
Food-poisoning bacteria grow fastest at warm temperatures. The temperature danger zone is the range in which they multiply quickly. Food safety agencies give slightly different limits (between about 4 °C and 63 °C); in this course we use 5 °C to 60 °C.
The two-hour rule: cooked food, milk, meat and fish should not stay in the danger zone for more than two hours in total. After that, food should be thrown away, not saved.
Question: Under ideal warm conditions some bacteria divide every 20 minutes. A spoonful of cooked beans holds 100 bacteria. How many could there be after (a) 2 hours and (b) 4 hours on a warm kitchen shelf?
Solution: The number doubles once every 20 minutes, so there are 3 doublings per hour.
Doubling the time does not double the bacteria: it multiplies them by 64. This is why the two-hour limit matters so much.
| Food | How to store it |
|---|---|
| Dry foods: beans, rice, maize flour, sugar | Cool, dry, clean store; in sealed containers or hermetic bags, raised off the floor on pallets; away from chemicals. Grain must be fully dry before storage to prevent mould and aflatoxin. |
| Fruit and vegetables: tomatoes, avocado, amaranth, cabbage | Cool, airy place out of the sun; leafy vegetables used within a day or two; throw away rotten or mouldy items. |
| Milk | Boil raw milk, cool it quickly, and keep it covered in a clean container, in a fridge if possible. Use within a day if there is no fridge. |
| Fresh fish and meat | Keep on ice or in a fridge (0 to 5 °C) and cook the same day, or freeze at −18 °C. Fish such as sambaza can also be dried or smoked to keep longer. |
| Cooked food | Either keep it hot (above 60 °C) or cool it quickly and refrigerate within two hours. |
First in, first out (FIFO): put new stock behind old stock and use the old stock first. Label containers with the date they were filled or cooked.
Tip
Memory hook: "5 to 60, 2 hours, 75 to reheat". Keep cold food below 5 °C, hot food above 60 °C, no more than 2 hours in between, and reheat to 75 °C.
Exam technique: bacteria growth questions are doubling questions. Count the number of doubling periods first, then multiply by 2 that many times.
Beans are cooked at 13:00, lunch is served at 13:30, and the leftover beans stay on the counter until 16:30. Are they safe to keep for supper? Explain.
Solution: From 13:00 to 16:30 is 3.5 hours. Once the beans cooled below 60 °C they were in the danger zone, almost certainly for more than two hours. They should be thrown away, not saved.
A trader stores new bags of rice in front of old bags. Name the rule she is breaking and the risk.
Solution: First in, first out. The old rice at the back may be kept too long and become stale, damp or infested with insects.
State the correct temperature for (a) a fridge and (b) a freezer.
Solution: (a) 0 to 5 °C; (b) −18 °C or colder.
One bacterium divides every 20 minutes. How many are there after 1 hour?
Solution: 1 hour = 3 doublings, so 1 × 2 × 2 × 2 = 8 bacteria.
Summary
Sources and credits
The bacteria-growth worked example, the storage table and the practice questions are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
