Modulating the functionality of black wolfberry anthocyanins-based freshness indicators by adjusting the pH value of film-forming solution

In this study, freshness indicators were prepared through solvent casting method by anchoring black wolfberry anthocyanins into cassava starch/polyvinyl alcohol matrix and adjusting the pH value of film-forming solution. The aim of this study was to explore the impact of the pH value (3.0, 4.0, 5.0, 6.0 and 7.0) of film-forming solution on the functionality of anthocyanins-based freshness indicators. The indicators were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, micrometer, UV–vis spectrophotometer, water vapor permeability tester, water contact analyzer, gas permeability tester, texture analyzer, thermogravimetric analyzer, differential scanning calorimeter and colorimeter. Meanwhile, the pH-responsive and ammonia-responsive properties of the indicators were determined. The indicators were finally used to indicate the freshness of shrimps. Results showed pH adjustment greatly changed the cross-sectional morphology and intermolecular hydrogen bonding strength of the indicators. The indicator prepared at pH 3.0 had the highest visible light/oxygen barrier ability and antioxidant activity, while the indicator prepared at pH 4.0 exhibited the highest water vapor barrier ability. The indicators prepared at pH 3.0, 4.0 and 5.0 showed remarkable color changes when exposed to buffer solutions and ammonia gas. The indicator prepared at pH 3.0 could effectively indicate the freshness of chilled shrimp through color changes. This study reveals that the functionality of anthocyanins-based freshness indicators can be modulated by adjusting the pH value of film-forming solution.