Agriculture and Food 7–12 · Years 7–8

Checking a food thermometer and the temperature danger zone

Food and agricultural practices

PracticalLow risk

The idea

Food-poisoning bacteria can multiply between 5 °C and 60 °C, so potentially hazardous food is kept at 5 °C or colder or at 60 °C or hotter, checked with a probe thermometer proven in an ice slurry.

Safety card

Low riskAn adult supervises

Hazards

burns from hot food and oven surfaces
cross-contamination carried on the probe
eating food held too long in the danger zone

Controls

oven mitts and a teacher present at the oven
sanitise the probe between every food
follow the 2-hour/4-hour decision; nothing from the timed sandwich is eaten after 4 hours

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 handling hot food.

What you need

digital probe thermometer (Standard 3.2.2 clause 22 requires accuracy to plus or minus 1 °C)
crushed ice, about 500 mL, and a little cold tap water in a 600 mL beaker or cup
alcohol wipes and paper towel
the food room refrigerator
a hot dish held in an oven or bain marie, for example soup
a cheese sandwich made in the lesson, and a clock

How to do it

Clean the probe, then sanitise it with an alcohol wipe and let it dry.
Mix ice with just enough cold water that the ice does not float, stir into a slurry, insert the probe and wait about three minutes before reading.
Repeat the ice-slurry reading three times and record all three. Compare every reading with 0 °C, not just with the other two.
Measure the refrigerator at its warmest point (often the door shelves) by placing the probe between two packs of food and waiting for a steady reading.
Sanitise the probe, then measure the centre of the hot-held dish.
Leave the sandwich on the bench, record the time it left the fridge and how long it stays between 5 °C and 60 °C, and decide what may be done with it using the 2-hour/4-hour rule.

What you should see

A thermometer fit for use reads 0 °C in the ice slurry, and both halves of the FSANZ check matter. FSANZ says of the ice-slurry reading "Check and note the temperature. It should read 0°C", and then "Do this three times and compare the temperatures recorded. If they vary by more than 1°C, get your thermometer checked by the supplier." Agreement between the three readings is only repeatability: a probe that reads 3.0, 3.0 and 3.0 °C in an ice slurry agrees with itself perfectly and is still 3 °C out, so it fails the accuracy of plus or minus 1 °C that Standard 3.2.2 clause 22 requires and is not fit to prove a refrigerator or a hot-held dish. The refrigerator should read 5 °C or colder and the hot-held food 60 °C or hotter. For the sandwich, less than 2 hours between 5 °C and 60 °C means it may be used or refrigerated; 2 to 4 hours means use it now and do not refrigerate it again; more than 4 hours means throw it out. The learner knows it worked when all three ice readings sit within 1 °C of 0 °C, not merely within 1 °C of one another; a probe that is repeatable but off zero goes back to the supplier to be checked or is replaced.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

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

Food that looks and smells fine is safe to eat.
A refrigerator keeps food safe whatever temperature it is set to.
A digital thermometer never needs checking.
A probe that gives the same reading every time must be accurate.

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)FT4-1
Technology 7–8 Syllabus (2023), focus area Food and agricultural practicesTE4-SAF-01
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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