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Destructive and non-destructive temperature measurement: how to get it right

Which method allows you to most accurately measure the temperature of your product?

Temperature measurement instruments such as thermometers come in a huge variety of styles and brands. Depending on the type of frozen or chilled food you produce or transport, certain types of thermometers will be better suited to your product. 

Probe thermometers are some of the most commonly used temperature monitoring devices. Consisting of a hygienic metal probe attached to a digital display, probe thermometers are generally accurate to +/- 0.5 degrees. This allows cold chain operators to get an accurate reading and maintain cold chain integrity throughout the product’s lifecycle. 

However – having a well-functioning instrument is only part of the story. 

It’s vitally important to understand how to properly use a probe thermometer or any other type of temperature monitoring device. 

When taken incorrectly, temperature measurements can vary enough to warrant throwing away a whole container of fresh produce, so it’s critical that anyone on the front line of a cold chain business understands exactly how to monitor a product’s temperature.

Perfecting your placement

The accuracy of temperature measurement depends on how and where the probe thermometer is placed. If placed in the wrong spot, the reading will be inaccurate. 

Probes can be placed on the outside of boxes or packages, but the only truly accurate food temperature is core or pulp temperature, where a probe is inserted into the food. This is because the surface temperature may be warmer or cooler than the temperature in the rest of the food. 

Both non-destructive and destructive temperature monitoring have their place in the cold chain. Let’s take a closer look at these two methods and how to ensure you’re taking the most accurate readings possible. 

Using thermometers for non-destructive temperature monitoring

The non-destructive method of checking the temperature of products involves using a probe

thermometer. This method is fast and can be done without unduly disturbing the food product. 

However, because the temperature being measured is only registering the outside of the package or carton, there could be up to 2°C difference between that and the true product temperature inside.

For best results, a probe thermometer must be placed between boxes on a pallet or between packages inside a carton. Use sufficient pressure to ensure good thermal contact, and sufficient length of the inserted probe to minimise conductivity errors. 

Choose a probe with a flat profile for good surface thermal contact, low thermal mass, and high thermal conductivity. It should also be waterproof and easy to sterilise. 

Destructive temperature measurement

With the destructive method, a pointed probe thermometer is inserted into the product or pressed firmly into its side. 

Probe thermometers are not designed to penetrate quick-frozen foods – first, you need to make a hole in the product the same size as the probe you are using. This is easy to do with a pre-cooled sharp metallic device such as an ice punch, hand drill or auger.

To give a proper reading, the probe needs to penetrate the product to a minimum depth of 25mm from the product’s surface. For smaller products, the probe should be inserted to a minimum depth from the surface of three or four times the diameter of the probe.

Where it is not possible to make a hole in certain foods, such as diced vegetables, the internal temperature of the food package should be determined by inserting a suitable sharp-stemmed probe.

In general, the probe thermometer should be accurate at the preferred temperature range, instantly readable, and fitted with a thin probe that slides easily into the product.

Best practice use of a thermometer

To ensure the most accurate temperature measurement, first you should:

  • Ensure the device has been properly calibrated
  • Verify the display
  • Check the battery and if you’re replacing it, always re-calibrate your instrument
  • Stabilise the temperature of the sensor and pay attention to the temperature delay and radiation heat
  • Pre-cool the thermometer prior to the test to equalise the temperature of the probe to that of the product’s surrounding air temperature. This helps avoid heat being conducted from the probe to the product, which can result in inaccurate temperature measurement.
  • Clean the thermometer before and after the test by washing it with cool soapy water, or clean with alcohol wipes and then allow it to air dry or wipe it dry with a clean cloth or paper towel. 

What happens when the temperature reading isn’t clear? 

Temperatures that are in dispute can only be proven by a destructive temperature check. 

This should be done by using a probe thermometer for which calibration, accuracy and limitation tolerances can be confirmed. Having a paper trail that confirms the accuracy of the thermometer could help you avoid load rejection and potential liability claims down the track. 

3 Tips for better thermometer accuracy

1. Beware meat and poultry

Meat and poultry are the most refrigeration-intensive foods, and are subject to the most stringent standards and regulatory scrutiny. There are traps for the unwary when checking the temperature of these foods, including the following: 

  • Bone, fat and gristle have different thermal properties and heat transfer rates
  • If the food is irregularly shaped, the temperature should be checked in several places
  • The probe should not penetrate the packaging because this could contaminate the contents and damage the sensor to the point where it delivers incorrect readings. 

2. Look after your probe 

The probe should always be disinfected before and after measuring products. Use dedicated disinfecting cloths or hold the sensor in boiling water and wipe with clean, disposable paper.

3. Always take temperature readings inside a refrigerated space

At critical control points, such as a loading dock that is exposed to ambient airflow, the temperature of the outer surface of the package can’t be relied on. Correct temperatures can only be taken inside a refrigerated space. 

At loading docks, temperature measurement can be taken with a probe placed between two packs while they are still in the truck or trailer. Leave the probe in place for at least one minute before reading the results. 

Build your confidence in temperature measurement.

Take a look at our online training course – ‘Thermometers and the Cold Chain Practitioner‘, or contact us for more details on how to become a pro at temperature measurement.