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Packaging Technology

Nano tag changes colour
to show freshness

A colour-coded smart tag could tell consumers whether a carton of milk has turned sour or a can of vegetables has spoiled without opening the containers, according to researchers at Peking University in Beijing, China, which has patented the technology. In addition the tag, which would appear on the packaging, could be used to determine if medications and other perishable products were still active or fresh, they said.

The tag has a gel-like consistency, is inexpensive and safe and can be widely programmed to mimic almost all ambient-temperature deterioration processes in foods according to  Chao Zhang, Ph.D., the lead researcher of the study. Even when manufacturers, grocery-store owners and consumers do not know if the food has been unduly exposed to higher temperatures, the tag can give a reliable indication of the quality of the product, he confirmed.

Tags, which are about the size of a kernel of corn, could appear in various colour codes on packaging. In the test configuration, red, or reddish orange, would mean fresh. Over time the tag changes its colour to orange, yellow and later green, which indicates the food is spoiled.

The colours signify a range between 100 per cent fresh and 100 per cent spoiled. The researchers developed and tested the tags using E. coli bacteria in milk as a reference model. The tags could also be customized for a variety of other foods and beverages.

Containing tiny metallic nano-rods made of gold and silver the tags, at different stages and phases, can have a variety of colours: red, orange, yellow, green, blue and violet. Silver chloride and vitamin C are also in the tags, reacting slowly and controllably. Over time, the metallic silver gradually deposits on each gold nano-rod, forming a silver shell layer. That changes the particle's chemical composition and shape enabling the tag colour to evolve.

The researchers say tags would be very inexpensive with all the chemicals in it costing much less than one cent — $0.002 is estimated. In addition, all of the reagents in the tags are nontoxic, and some of them (such as vitamin C, acetic acid, lactic acid and agar) are even edible. 

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  • Modified 23 Dec 2015
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