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Appendix II

How to Select and Use a Food Thermometer

Temperature Checking

Temperature checking is a critical measurement for ensuring the safety and quality of many food products especially potentially hazardous foods. Suitable, reliable and accurate food thermometers are essential for monitoring temperatures at receiving, production or final product storage and distribution stage.

Common Types of Food Thermometers

There are several types of food thermometers on the market. Some are used to measure the temperatures of food and some are to measure temperatures of equipment and storage areas. Since temperature control is essential to safe food production, choosing an appropriate food thermometer and using it properly are of great importance.

When choosing a thermometer, consider (but is not limited to) the following:

  1. the product type or the environment to be monitored;
  2. sensing area. For example, a bi-metallic stemmed thermometer is good for measuring temperatures of thick food (any food more than 76 mm thick) such as beef roasts because its sensing area extends in the stem from about 6 mm to about 76 mm up the stem;
  3. the expected accuracy of the thermometer. Select a thermometer that is accurate to at least ±1oC. For example, if the thermometer reads 60oC, the actual temperature of the food should be between 59oC and 61oC.
  4. the temperature range to be measured. Thermometers with a narrower temperature range will provide greater accuracy at a cheaper price. In general, a thermometer that has a range of -50oC to 150oC is all that is required for measuring the temperature of food;
  5. the sensitivity (some thermometers can detect a 1.0 degree temperature change while some can detect a 0.1 degree temperature change);
  6. response time. Some thermometers take less than 1 second while others may take 2 minutes to measure the temperature; and
  7. the calibration methods (some thermometers can be calibrated while others cannot).
  1. Bi-metallic Stemmed Thermometers

    Diagram 2: Bi-metallic
    Stemmed Thermometer

    Bi-metallic stemmed thermometers are the commonest type of food thermometers. These thermometers read the temperature from the tip and up the stem for 50 mm to 76 mm and the measured temperature is the average of the temperatures along the sensing area.

    They are, therefore, more suitable for measuring the core temperature of thick food (any food more than 76 mm thick) because the entire sensing area of the thermometer must be inside the food. Depending on the types, these thermometers give readings within 20 seconds to 2 minutes.

  2. Thermocouple Thermometers and Thermistor Thermometers

    Thermocouple thermometers and thermistor thermometers measure temperatures through a sensor in the tip of the stem. They give readings quickly (within 10 seconds) and since the sensor is in the tip, these thermometers can measure temperatures in thin and thick foods conveniently. These thermometers may also be called "digital food thermometers" because measured temperatures are normally indicated on a digital display.

    Diagram 3: Digital Food Thermometer Diagram 4: Digital Food Thermometer
  3. Infrared Thermometers

    Hand-held, portable infrared thermometers measure the surface temperature, in less than 1 second, of food and packages of food without contact by measuring the amount of radiant energy emitted from the surface.

    Infrared thermometers are useful for checking the storage temperature in freezers and refrigerators, temperature of food held in hot holding equipment and temperature of incoming food products. They are, however, unsuitable for measuring the centre temperatures of food during cooking or cooling as only the surface temperature can be measured. Besides, they cannot accurately measure the temperature of metal surfaces and reflective foils.

    Diagram 5: Infrared Thermometer

Glass thermometers are filled with mercury or spirits. They are not recommended to be used for measuring the temperature of food because the glass and the inner liquid pose physical and chemical hazards.

There are other thermometers designed specifically to measure certain food items or food processes. These include candy / jelly / deep fry thermometers, oven thermometers, single-use temperature indicators, etc.

Checking Thermometer Accuracy

Food thermometers need constant checking / calibration in order to make sure that their readings are accurate and reliable. The manufacturer or distributor should be asked to calibrate them at least once every year but self-checking on the accuracy of food thermometers should be done at least quarterly using the following methods:

  1. Ice Point Method

    1. fill a container with finely crushed ice. Add clean tap water to the top of the ice and stir well;
    2. let the mixture stand for 5 minutes to allow the temperature of the mixture to become stable;
    3. immerse the tip (a minimum of 50 mm) of the food thermometer into the mixture without touching the sides or the bottom of the container;
    4. wait for approximately 2 minutes and record the temperature; and
    5. the temperature should read 0oC. If the food thermometer reads more than ±1oC, it should be adjusted, repaired, or replaced, if adjustment is not possible.
  2. Boiling Point Method

    1. boil clean tap water and immerse the tip (a minimum of 50 mm) of the food thermometer into the boiling water;
    2. wait for a few minutes to allow the temperature to stabilize;
    3. write down the temperature of the food thermometer; and
    4. the temperature should read 100oC at sea level (if your premises locate at a higher attitude, adjust the boiling point accordingly). If the food thermometer differs more than ±1oC, it should be adjusted, repaired, or replaced, if necessary.

Making a Measurement

Before using a food thermometer, read the manufacturer's instructions first. The instructions should tell how far the food thermometer must be inserted into the food to give an accurate reading. Observe the following guidelines when taking temperature measurements:

  1. store the food thermometer in a clean and hygienic manner;
  2. check that the battery low sign, if any, is not showing. Replace with a new battery if required;
  3. wash with warm water and detergent, sanitize and air dry the food thermometer before use;
  4. sanitize the food thermometer by putting the probe in hot water (at least 77oC or above) for 6 seconds, using alcohol swabs or other appropriate sanitizers as recommended by the manufacturer;
  5. for soups and sauces, stir prior to checking their temperature;
  6. take core temperature of food by inserting the probe into the centre (or thickest part) of the food;
  7. do not let the tip of the thermometer touch the bottom or sides of food containers;
  8. wait for 10 seconds (or the time recommended by the manufacturer) and take the reading;
  9. if the food thermometer is used to measure hot and cold food, wait for the reading to return to room temperature between measurements; and
  10. in taking the surface temperature measurement of packed or frozen food, place the food thermometer probe between two packages of packed / frozen food items and avoid damaging the packaging of packed food.