Red radish anthocyanin-based intelligent starch/pectin films for shrimp sub-freshness monitoring: effect of adjusting pH value of the film-forming solution

In this study, intelligent films based on red radish anthocyanin as indicator and corn starch/citrus peel pectin as the matrix were developed by adjusting the pH value (3.0, 4.0, 5.0, 6.0 and 7.0) of the film-forming solution. The microstructure and infrared spectral characteristics of the films were elucidated, and their physical properties were assessed, including light barrier properties, hydrophobicity and mechanical strength. Furthermore, the films’ responsiveness to pH and ammonia was evaluated, and their application in monitoring the freshness of shrimp during storage was investigated. The results indicated that the cross-sectional distribution of the films prepared at pH 3.0 and pH 7.0 exhibited unevenness and poor compatibility, which adversely affected their mechanical and hydrophobic properties. All films demonstrated significant color changes in response to pH and ammonia, with the films fabricated at pH 3.0 and pH 4.0 displayed more pronounced color transitions. Notably, the film prepared at pH 4.0 exhibited the highest tensile strength (3.24 MPa) and could visually distinguish between fresh (red, 1 day), sub-fresh (pink, 2 days), and spoiled (purple, 3 days) shrimp during storage at 4 °C over a period of 4 days. Thus, this study presents a novel starch/pectin film based on red radish anthocyanin for shrimp sub-freshness monitoring by adjusting pH of the film-forming solution.