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Color change of poly(vinyl alcohol)/chitosan/anthocyanin films in contact with raw pork belly slices exposed to ambient air for 12 h (left) and 24 h (right). After 12 h, the wrapping film becomes pink indicating an acidic condition near pH 5–6 on the surface of the pork slices. With further exposure for another 8 h, the pork meat turned dark-brown and softer, while the pH indicative film became yellowish with pale green, corresponding to a slightly alkaline range ([221]).
