Meat in supermarket freezers may look fresh, but bacteria may have already begun to multiply inside the packaging. A research team at Kyushu University has proposed a new solution: letting the packaging itself indicate food safety conditions. In their research published in the *Journal of Chemical Engineering*, they developed a soft and elastic hydrogel film embedded with natural plant pigments that changes color as food spoils, providing consumers with a visual signal without opening the packaging. This material also has self-healing capabilities, preventing bacteria from entering through damage during transportation and handling.

Food freshness is characterized by specific chemical properties. Take meat as an example: fresh meat is typically slightly acidic. As bacteria multiply, proteins break down and release alkaline compounds, causing the meat's pH to gradually rise before visible signs of spoilage appear. Anthocyanins are a class of natural pigments sensitive to pH changes; the colors of purple sweet potatoes and red cabbage originate from them. When the pH rises, anthocyanins gradually change from purplish-red to yellowish-green, a change that is visually noticeable. Because anthocyanins come from natural plants, they are relatively safe for use in food contact. However, the problem is that light and high temperatures can easily interfere with their color stability, leading to inaccurate readings, making them unsuitable for direct use as a long-term monitoring indicator.

The research team extracted anthocyanins from purple sweet potatoes, which were then ground and freeze-dried into powder. To stabilize the pigment, they introduced a metal-organic framework material called UiO66-NH₂. This type of material is known for its thermal and chemical stability, and its porous structure provides the conditions for immobilizing anthocyanin molecules. In this design, anthocyanin molecules are adsorbed onto the surface of the metal-organic framework through various chemical interactions, reducing molecular mobility and thus protecting them from degradation by oxygen, light, and heat. At the same time, the anthocyanin's sensitivity to pH is retained.

In tests using pork as a sample, the anthocyanins treated in this way exhibited stable responsiveness. As the meat spoiled and alkaline gases accumulated, the material's color gradually changed from purplish-red to yellowish-green, providing readable visual signals at different stages. The research team then incorporated anthocyanin-loaded metal-organic framework materials into hydrogels to create soft, malleable, and primarily plant-derived biodegradable films. In addition to its use in freshness monitoring, this material also extended the shelf life of pork by approximately 12 hours.

In addition to its color-changing function, this hydrogel film also exhibits self-healing properties. Professor Fumihiko Tanaka of the Faculty of Agriculture at Kyushu University explained that when the film is cut and re-attached, the damage almost completely disappears within minutes, and its tensile strength recovers to 99% within two hours. Traditional packaging, once cracked, becomes a channel for bacterial invasion, but this material can self-adhere, restoring its ability to protect the food inside and improving its durability in practical use.

The corresponding author, Xirui Yan, a researcher at the Japan Society for the Promotion of Science in the Faculty of Agriculture, Kyushu University, said that the inspiration for the research came partly from an academic conference. The porous structure and multifunctionality of metal-organic frameworks (MOFs) led her to consider whether such materials could be applied to everyday scenarios such as food preservation. A report by Nobel laureate Professor Susumu Kitagawa on the sustainable use of functional porous materials prompted her to further explore the potential value of MOFs in food systems.

The research team is currently exploring the development of a companion smartphone application, hoping to provide manufacturers, logistics providers, and consumers with an objective and reliable way to assess food quality in real time. Associate Professor Tanaka Fumina added that this smart material, combining natural ingredients and nanotechnology, may have applications beyond food packaging and could potentially play a role in many unexplored fields in the future