Thermochromic crystals may provide a non-invasive method of determining whether foods have been exposed to destructive temperatures.
To be able to accurately gauge the temperature of an object by sight, rather than touch, is a useful method of heat detection and it can also be an excellent safety precaution.
Such a phenomenon does exist in the form of thermochromic crystals which transition in colour when subjected to heat. The practical application of such substances is diverse, ranging from colour changing baby bottles and novelty mugs to medical devices.
Josephine Mueller and Taylor Lauster, of North Central College Illinois, USA, conducted thermal microscopy experiments of thermochromic crystals and highlighted a potential application for such crystals in food packaging.
The students worked alongside The McCrone Group, our partners in the US, who graciously donated a Linkam FTIR600 stage.
Although designed for infra-red analysis, the students used the heating feature of the FTIR600 to conduct simple heating and cooling experiments on the thermochromic crystals, with the aim of finding potential applications for thermochromism in industry. The quartz window also allowed image capture of the crystals during the experiment.
The study focused on the idea that if a crystal was to be permanently deformed after exposure to a given temperature, this would be a good method of monitoring thermal conditions of food items in transit. Such crystals could be placed within food packaging and their permanent deformation would indicate exposure to destructive temperatures.
To test the suitability of thermochromic crystals for such an application, the students first grew their own thermochromic crystal - (DEA)2CuCl4.
For their thermal analysis study, the crystals were heated from room temperature up to 70°C and cooled back down to room temperature. Below are images captured during the heating and cooling of the thermochromic crystal (courtesy of Taylor and Josephine).
Initial sample of (DEA)2CuCl4 using a 50X objective. The sample has a strong green colour.
Melted sample of (DEA)2CuCl4 at 50°C, note a change in colour from green to an orange-green.
Melted sample of (DEA)2CuCl4 at 56°C. Most of the sample has now lost the green colouring.
Fully melted sample of (DEA)2CuCl4 at 70°C, now an orange colour.
Sample of (DEA)2CuCl4 cooled to room temperature, returning to the original green colour.
Their results showed the thermochromic change to be reversible for (DEA)2CuCl4 . There appears to be slight structural integrity loss after cooling, but the discrepancies are not obvious to the eye and would require microscopic analysis.
Although these particular crystals are not suitable as temperature determinants in food packaging, their study is a great step forward into improving the transportation of goods. Work must now be done in developing a non-toxic thermochromic crystal which has permanent deformation at destructive temperatures.
We would like to thank Josephine, Taylor and The McCrone group for their innovative study and for kindly sharing their findings with us.
By Tabassum Mujtaba