Preparation of pH-sensitive and antioxidant packaging films based on κ-carrageenan and mulberry polyphenolic extract

In this study, antioxidant and pH-sensitive κ-carrageenan based films were developed by incorporating different amounts of mulberry polyphenolic extract (MPE). Due to abundant anthocyanin content, MPE showed pH-sensitive property with solution color changed from red to purple and final gray when pH value increased from 2 to 13. When MPE was incorporated into κ-carrageenan film, the composite films presented blue color. Notably, the incorporation of 2 and 4 wt% of MPE could significantly increase the thickness, tensile strength, thermal stability, antioxidant activity and pH-sensitive property of κ-carrageenan film. However, the moisture content, water vapor permeability, UV–vis light transmittance and elongation at break were greatly reduced when 2 and 4 wt% of MPE were incorporated. In addition, κ-carrageenan-MPE films exhibited heterogeneous and rougher surfaces as compared with κ-carrageenan film. The existence of intermolecular interactions between MPE and κ-carrageenan matrix was confirmed by infrared spectroscopy and X-ray diffraction. When applied to monitor the freshness of milk, κ-carrageenan-MPE films showed evident color changes as milk deteriorated after storage at 40 °C for 8 h. Our results suggested that κ-carrageenan-MPE films could be used as novel active and intelligent food packaging materials.