Solvent–free fabrication of pH–sensitive freshness indicators using pomegranate juice in starch–PVA matrix for real–time fish spoilage detection

Novel freshness indicators were developed using a starch–polyvinyl alcohol matrix incorporating pomegranate
juice as a natural source of anthocyanins. To ensure a sustainable production process, the anthocyanins were not
isolated from the juice, thus avoiding the use of organic solvents and energy–intensive extraction processes. The
resulting smart indicators were characterised in terms of their optical properties, microstructure, infrared
spectrum, water affinity, color response across a range of pH values, reactivity to volatile amines (ammonia,
trimethylamine and dimethylamine), and color reversibility. The smart indicators were tested in fish fillet
packaging systems under controlled temperature conditions (2.8 ◦C), where their performance was evaluated
over time. An artificial intelligence tool was employed to classify the visual appearance of the films during
storage. Microstructure analysis revealed satisfactory microstructural integrity and compatibility between the
pomegranate juice and the biopolymer carrier. The incorporation of pomegranate juice enhanced the hydro-
philicity of the membranes. The color sensitivity test demonstrated that the smart films responded effectively to
increases in pH and volatile amine concentrations. A perceptible color shift from red to purple–green was
observed from the early stages of spoilage (day 4–5), with intensity increasing as degradation advanced. The
indicator response aligned with changes in headspace gas composition and was consistent with microbiological
and chemical quality indicators of fish flesh, highlighting their potential as effective tools for real–time moni-
toring of fish freshness and spoilage.