Development of Multilayered pH-Sensitive Chitosan–Gelatin-Agar Intelligent Film Incorporated with Roselle Anthocyanin Extract for Monitoring of the Freshness of Snapper Fish

Natural dyes that are pH-responsive, such as anthocyanin, have been incorporated into intelligent film packaging for food spoilage detection. However, the stability of anthocyanin in a single composite film remains a challenge. Hence, this study aimed to develop a multilayer chitosan-gelatin-agar intelligent film with roselle anthocyanin extract as a pH biosensor for the spoilage of red snapper fish. The chitosan-gelatin-agar bilayer film with different concentrations of roselle anthocyanin extract (2, 4, and 6% w/v) was developed. The selected roselle anthocyanin extract concentration (4% w/v) was further developed into a single composite, bilayer, and triple-layer intelligent films. The physicochemical, mechanical, and light barrier properties of the intelligent films were analysed. The triple-layer film with 4% w/v of roselle anthocyanin extract was stored with red snapper fish for 7 days at 4 °C. The triple-layer intelligent film with roselle anthocyanin extract showed the lowest thickness (0.15 mm) and water solubility (33.88%) than single composite and bilayer film. It also demonstrated good mechanical properties (13.42 MPa tensile strength and 25.53% elongation at break), a high UV-Vis light barrier, and distinct colour changes under different pH environments. Visible colour variation was also observed in the triple-layer film with roselle anthocyanin extract from dark red (day 0) to pale red with a hint of green (day 6) as the total volatile basic nitrogen levels of fish exceeded the threshold of 20 mg/100 g. This indicates the potential of the triple-layer chitosan-gelatin-agar film with roselle anthocyanin extract as an intelligent film for monitoring the freshness of fish during storage.