Pyranoflavylium-cellulose acetate films and the glycerol effect towards the development of pH-freshness smart label for food packaging

A bio-inspired anthocyanin-type pigment, namely a pyranoflavylium salt previously developed was incorporated into cellulose acetate-based films to build up a colorimetric pH-indicator as a smart label for monitoring food freshness. The pigment (0.1% (w/w)) was immobilized in cellulose acetate matrix in the absence and presence of different percentages of glycerol (10–40% (w/w)). The films were obtained by the casting method and characterized by the thickness, morphology and barrier properties, thermogravimetric analysis, color, among others. The chromatic responsive properties of the films were tested in solutions at different pH values (pH 4 to 8) and in the headspace of biogenic amines and ammonia-containing solutions at different concentrations. The results showed that the films without glycerol are not color-responsive to different pH values. On the other hand, the incorporation of glycerol above 20% (w/w) demonstrated effective pH responsiveness after 20 min of immersion and after exposition to an amine-rich environment. The glycerol-containing films showed relevant and remarkable color change at the pH range of food spoilage indicating a great potential for application as a food freshness indicator.