Nanocellulose-reinforced κ-carrageenan/gluten films with Ardisia elliptica anthocyanins: structure-property balance for freshness-related colour response

ABSTRACT The increasing demand for sustainable alternatives to petroleum-based packaging has stimulated the development of intelligent materials capable of providing visual information on food quality during storage. In this study, soil-burial-disintegrable biopolymer films based on κ-carrageenan, wheat gluten, cellulose nanofibrils (CNF), and anthocyanin extract from Ardisia elliptica fruits were prepared by the solution-casting method and evaluated for their structural, optical, water-related, mechanical, and colourimetric properties. Anthocyanin incorporation conferred a clear pH-dependent colour response across the tested pH range, while CNF-containing formulations exhibited reduced UV transmittance, increased surface wettability (lower water contact angle), and modulated tensile performance and water-related properties in a formulation-dependent, non-monotonic manner. Among the CNF-containing formulations, the film containing κ-carrageenan, gluten, anthocyanin extract, and 20% CNF (κCG-GL-ANT-CNF20) exhibited the lowest water solubility and swelling, coupled with low UV transmittance, with water vapour permeability remaining a formulation-dependent trade-off rather than the primary selection criterion. Soil burial assays showed progressive disintegration, reaching more than 93% weight loss by day 35. Proof-of-concept applicability was assessed using rose apple, apple, and a sardine-based model system, where visible film colour changes were observed during storage. Within the tested series, κCG-GL-ANT-CNF20 provided the most balanced formulation, combining improved resistance to liquid-water-induced disintegration, low UV transmittance, lower short-term anthocyanin release, and qualitative food-model colour responsiveness. The developed films therefore show potential as colour-responsive hydrocolloid materials for qualitative freshness-related assessment.