Scientists at Kyushu University in Japan have created a flexible hydrogel film that can indicate when food in a sealed pack is starting to go off, without the package being opened.
The film uses a natural pigment from plants that alters colour as chemical conditions linked to spoilage change.
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It is also self-healing, so minor damage during transport or handling can mend itself and help preserve the barrier function of the packaging.
The work addresses a common weakness in standard food packaging: the outside of a sealed pack does not always reflect the state of the product inside.
To detect deterioration, the researchers used anthocyanins, pigments present in foods including purple sweet potato and red cabbage.
When pH levels increase, the colour moves from purple-red towards yellow-green, creating a visible sign of change.
Because anthocyanins can break down under heat and light, colour indicators based on them may lose reliability.
The research team sought to overcome this by attaching the pigment to a metal-organic framework (MOF), a porous structure that helps protect the anthocyanin while preserving its sensitivity to pH changes.
The protected pigment was added to a hydrogel, producing a soft and mouldable film made largely from plant-based material and designed to biodegrade.
Trials using pork found that the film’s colour shifted steadily as the meat spoiled, in line with the release of alkaline gases.
The researchers said the material did more than indicate freshness. In testing, pork wrapped with the film remained usable for about 12 hours longer than untreated samples.
For food producers, the project points to packaging that could combine several functions in one material.
Instead of using separate systems for protection, shelf-life improvement and freshness indication, the hydrogel is being developed to handle all three.
Most of the hydrogel is plant-derived and intended to be biodegradable, while the anthocyanin comes from an agricultural source.
Kyushu University professor Fumihiko Tanaka 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.”
