Co-adjustment of chitosan-based film structure by pH and anthocyanin concentration for the development of reshness-indicating labels on chilled mutton.

In this study, the pH values (pH = 4.2, 5.2, 5.6) of chitosan-dialdehyde starch/polyvinyl alcohol (CDS/PVA) and the concentrations of purple sweet potato anthocyanins (PSPA; 0.5, 1.0, 1.5, 2.0 mg/mL) were adjusted to obtain a novel colourimetric film [(PSPA-CDS/PVA)].The infrared spectroscopy and X-ray diffraction showed that the film components were compatible with each other. Additionally, the structure of the films loosened as the pH increased because their tensile strength decreased from 58.850 MPa to 23.052 MPa and the water‑oxygen barrier was weakened (27.624 to 38.406 g·m·h and 0.144 to 0.188 cm·cm·m·day·atm; p < 0.05), promoting volatile ammonia penetration within the films. The films showed better ammonia-responsive colour development and colour stability at a PSPA concentration of 1.5 mg/mL. The colour of 1.5 mg/mL (PSPA-CDS/PVA) changed from purplish-red to bluish-green during mutton refrigeration, accurately indicating mutton freshness and spoilage. The colour signals could also be monitored successfully using the WeChat applet.