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Researchers at Kyushu University have developed a flexible packaging film that changes colour as food spoils and can repair itself after being damaged.

Detailed in a study published in the Chemical Engineering Journal, the material contains anthocyanins – natural pigments found in plants such as purple sweet potatoes and red cabbage – which respond to changes in pH.

Meat is mildly acidic when fresh, but its pH rises as bacteria multiply and release alkaline compounds. The film reflects this process by gradually changing from purple-red to yellow-green, providing a visible indication of spoilage without requiring the packaging to be opened.

While anthocyanins are suitable for food-contact applications, their response can be affected by light and heat. To improve their stability, the researchers combined pigments extracted from purple sweet potatoes with UiO66-NH₂, a thermally and chemically stable metal-organic framework.

The framework holds the pigment molecules in place and protects them from oxygen, light and heat while preserving their sensitivity to pH. In tests involving pork, the material changed colour consistently as alkaline gases accumulated during spoilage.

The pigment-loaded framework was incorporated into a soft, shapeable hydrogel film that is largely plant-derived and biodegradable. Researchers said the material also extended the pork’s shelf life by approximately 12 hours compared with untreated samples.

Alongside monitoring freshness, the film can repair cuts sustained during shipping or handling. When damaged sections were pressed together, the cut became nearly invisible within minutes and the material recovered 99% of its tensile strength within two hours.

Fumihiko Tanaka, professor at Kyushu University’s faculty of agriculture, said: “Another interesting thing about this material is that it doesn’t just protect food – it heals itself".

“In conventional packaging, any crack is permanent and becomes an entry point for bacteria. This material bonds back together on its own. The wound heals, and so does its ability to protect what’s inside, which makes it more durable and reliable in practical use.”

The researchers are now exploring the development of a companion smartphone application that could allow manufacturers, logistics providers and consumers to assess food quality in real time.

Associate professor Fumina Tanaka added: “This doesn’t have to stop at food packaging. Smart materials built from natural ingredients and nanotechnology may have uses we haven’t imagined yet. If anyone sees a place where this could work, we'd love to hear about it.”