Smart tag keeps tabs on food spoilage

Forget use-by dates - what if the packaging could tell you if a product was safe to eat? Researchers have developed a smart tag that can tell whether the product inside has spoiled without the need to open the container.

“This tag, which has a gel-like consistency, is really inexpensive and safe, and can be widely programmed to mimic almost all ambient-temperature deterioration processes in foods,” said Chao Zhang, PhD, the lead researcher of the study, which was presented at the 247th National Meeting & Exposition of the American Chemical Society (ACS).

Use of the tags could potentially solve the problem of knowing how fresh packaged, perishable foods remain over time, he added. And a real advantage, Zhang said, is that even when manufacturers, grocery-store owners and consumers do not know if the food has been unduly exposed to higher temperatures, which could cause unexpected spoilage, “the tag still gives a reliable indication of the quality of the product”.

About the size of a corn kernel, the tags could appear in various colour codes on packaging. “In our configuration, red - or reddish orange - would mean fresh,” said Zhang, a researcher at Peking University.

“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% fresh and 100% spoiled. For example, if the label says the product should remain fresh for 14 days under refrigeration, but the tag is orange, it means the product is roughly only half as fresh. This would indicate to the consumer that the product is good for another seven days if kept refrigerated.

To develop and test the tags, the researchers used E. coli in milk as a reference model. “We successfully synchronised, at multiple temperatures, the chemical evolution process in the smart tag with microbial growth processes in the milk,” Zhang said. The tags could also be customised for a range of other foods and beverages, and even other perishable items like medication.

The tags contain tiny metallic nanorods which can have a variety of colours, from red through to blue and violet. “The gold nanorods we used are inherently red, which dictates the initial tag colour,” Zhang said.

“Silver chloride and vitamin C are also in the tags, reacting slowly and controllably. Over time, the metallic silver gradually deposits on each gold nanorod, forming a silver shell layer. That changes the particle’s chemical composition and shape, so the tag colour now would be different. Therefore, as the silver layer thickens over time, the tag colour evolves from the initial red to orange, yellow and green, and even blue and violet.”

Despite containing gold and silver, the tags would be relatively inexpensive, Zhang said. All the chemicals in the tag cost around US$0.002. “In addition, all of the reagents in the tags are non-toxic, and some of them (such as vitamin C, acetic acid, lactic acid and agar) are even edible,” said Zhang.

The technique has been patented in China and some preliminary results have been published in the journal ACS Nano. The researchers now plan to approach food manufacturers with the concept and explain how it would be useful for them and their customers.