A team at Kyushu University has developed a self-healing freshness indicator material using purple sweet potato anthocyanins and metal-organic frameworks.

Researchers at Kyushu University recently unveiled a hydrogel packaging film that changes color as it spoils. When used in meat packaging, the film gradually changes from purplish-red to yellowish-green as bacteria multiply and alkaline substances accumulate, allowing consumers to detect spoilage without opening the package. The research has been published in the journal *Chemical Engineering. 

In supermarket refrigerated display cases, the appearance of meat products often fails to reflect microbial activity. Meat is initially slightly acidic, but as bacteria break down proteins, they release alkaline compounds, causing the pH level to rise before visible spoilage. The research team utilized this change to make packaging a "display" of freshness.

Natural anthocyanins are ideal candidates. They are found in plants such as purple sweet potatoes and red cabbage, turning purplish-red in acid and yellowish-green as the pH increases. However, anthocyanins are sensitive to light and heat, and their color signals are prone to shift. The Kyushu University team extracted anthocyanins from inexpensive and readily available purple sweet potatoes, prepared them into freeze-dried powder, and then stabilized them using a UiO66-NH₂ metal-organic framework. Anthocyanin molecules attach to the framework surface through various chemical interactions, restricting their activity and thus making them more resistant to oxygen, light, and temperature disturbances, while still maintaining pH sensitivity.

In pork testing, the complex pigment exhibited a stable response: as the meat sample decomposed, the packaging color transitioned steadily from purplish-red to yellowish-green, with readable signals at each stage. The team then incorporated anthocyanin-loaded metal-organic frameworks into the hydrogel, creating a soft, malleable, and biodegradable film. Compared to untreated samples, this film extended the shelf life of pork by approximately 12 hours.

This material also possesses self-healing capabilities. One of the paper's authors, Professor Fumihiko Tanaka of the Faculty of Agriculture at Kyushu University, stated that after the film was cut and re-attached, the scratches almost disappeared within minutes, and the tensile strength recovered to 99% within two hours. Traditional packaging, once cracked, often allows these cracks to become channels for bacteria; however, this material can re-bond itself, and its protective properties are restored after healing.

The paper's corresponding author, Xirui Yan, a researcher at the Japan Society for the Promotion of Science in the Faculty of Agriculture at Kyushu University, mentioned that metal-organic frameworks (MOFs) have attracted much attention due to their porous structure and multifunctionality, and she hopes to bring these materials to everyday scenarios such as food preservation. She recalled that Professor Susumu Kitagawa's speech on the sustainable use of functional porous materials at the "Hope Conference," which was attended by Nobel laureates, prompted her to think about the value of MOFs in food systems. 

The research team is currently exploring a companion mobile application to help manufacturers, logistics companies, and consumers assess food quality in real time and objectively. Tanaka Fumina, Associate Professor at the Faculty of Agriculture, Kyushu University, added that these smart materials, composed of natural ingredients and nanotechnology, may not only serve food packaging but also have many unforeseen applications in the future.

Source: Kyushu University