Agriculture and Food 7–12 · Years 9–10

Cooling cooked rice safely: the two-stage cooling rule

Food Service and Catering

Practical, model not builtLow risk

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The idea

Cooked rice is a potentially hazardous food, so it must cool from 60 °C to 21 °C within 2 hours and from 21 °C to 5 °C within a further 4 hours, and a shallow tray cools far faster than a deep pot.

Safety card

Low riskAn adult supervises

Hazards

burns from hot rice and pots
food-poisoning bacteria in rice held in the danger zone

Controls

oven mitts and a teacher present
the rice is thrown away after the trial

Note

Food handling follows the FSANZ food safety standards (Standard 3.2.2) as explained on the FSANZ food temperature and thermometers page; check class allergy and anaphylaxis records before any food is prepared; the Food Technology Years 7–10 Syllabus reminds teachers that food allergies can cause anaphylaxis. Heat: complete a RiskAssess or school risk assessment for hot food handling.

What you need

freshly cooked rice, two batches of about 1 kg
a deep container (rice about 10 cm deep) and a shallow tray (rice about 3 cm deep), both metal
two probe thermometers checked in an ice slurry, or temperature data loggers with probes
refrigerator with a wire rack, and an air thermometer inside it
clock or spreadsheet for readings every 15 minutes

How to do it

Check both probes in an ice slurry (0 °C within 1 °C).
Cook the rice, divide it between the deep container and the shallow tray and record the rice depth in each.
Put a probe in the centre of each, where the rice cools last, and start the clock when each centre falls to 60 °C.
Place both containers uncovered on the refrigerator rack so air can flow around them, and record the fridge air temperature.
Record both centre temperatures every 15 minutes for 6 hours (a data logger can do this unattended).
Find the time each container took to reach 21 °C and 5 °C and compare with the FSANZ limits; enter the first 30-minute reading into the model to predict the rest of the curve and compare.
Discard the rice; it is not eaten.

What you should see

The shallow tray reaches 21 °C and then 5 °C sooner than the deep container. In the model, a container whose centre falls from 60 °C to 40 °C in its first 30 minutes in a 4 °C fridge (k = 0.0147 per minute) reaches 21 °C at 81 minutes and 5 °C at 273 minutes, passing both stages (81 minutes within 120; 192 minutes within 240). Stage 2 in the model depends strongly on the fridge air temperature, because 5 °C is within about 1 °C of it: the same container gives 153 minutes of stage 2 in a 3.0 °C fridge and 235 minutes at 4.5 °C, so a change of a degree in the fridge air can change the stage 2 verdict, and the measured probe readings decide it. A container that falls only to 50 °C in the first 30 minutes (k = 0.0066 per minute) needs 182 minutes to reach 21 °C and fails the 2-hour stage. Measured curves may depart from the single-exponential model because a thick mass of rice cools from the outside in, and the learner reports both. The learner knows it worked when the probes were checked in ice and the deep container's centre lags the tray's throughout.

What changes

What you change: container depth (deep or shallow)
What you measure: centre temperature every 15 minutes; time to reach 21 °C and 5 °C (minutes)
What you keep the same: same rice batch and starting temperature
same refrigerator and rack position
probe in the centre
containers uncovered

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Hot food must cool on the bench to room temperature before it goes in the fridge.
A big pot cools as fast as a small tray in the same fridge.
Rice is a dry food, so bacteria cannot grow in it.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Food Technology Years 7–10 Syllabus (2019)FT5-1 FT5-5
Food Technology 7–10 Syllabus (2025), new syllabus, implementation from 2028FT5-SAF-01
Australian Curriculum v9AC9TDE10P03AC9TDE10K05

Sources

The pages the author read to write this activity.

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