The label is printed with a special ink strip that remains invisible until it changes to black at the specified temperature (70ºC), ensuring that the food has been properly heated.
The change is permanent and irreversible. If desired, the rest of the label can be used to show essential information, such as batch number, date and other variable data, which is printed and applied with a simple one or two-line hand-held labeller. This takes the guesswork out of reheating food and provides proof that it was correctly heated before serving.
The special ink has been developed in the US, where it is already widely used in food processing. Mark Hurst, sales director for Positive ID Labelling, said: "We looked at various materials and processes on behalf of our customers in the food and catering sector before we selected this option. We found it the simplest, most economic and reliable solution to this important labelling and quality control task."
Another ink is available for use in food testing laboratories where pipettes and other equipment need to be sterilised in a steam autoclave. This changes colour at 120°C. In both cases the whole label, material, adhesive, standard and temperature-sensitive inks, is specified so that it remains secure and legible after the food or equipment has been heated or treated.