pH-responsive κ-carrageenan/HPMC biodegradable films incorporated with roselle anthocyanin for freshness indication

Abstract

IntroductionBiodegradable HPMC/κ-carrageenan-based intelligent films incorporating berry-derived anthocyanins have been reported; however, the effect of anthocyanin source on polymer compatibility and functional performance remains underexplored. Roselle (Hibiscus sabdariffa L.) anthocyanins, rich in delphinidin- and cyanidin-based derivatives with high acidity and antioxidant capacity, may enhance pH responsiveness and bioactivity in polymer matrices.MethodsHPMC/κ-carrageenan films containing 0–2% (w/w) roselle anthocyanin (RA) were prepared using solvent casting. Films were evaluated for mechanical properties, water solubility, swelling index, antioxidant activity, thermal stability (TGA), polymer–anthocyanin interactions (FTIR), biodegradation (soil burial), and color-response behavior during storage of button mushrooms at 25 °C.ResultsFilm thickness ranged from 0.047 to 0.076 mm. Increasing RA content significantly reduced tensile strength (43.5 to 11.5 MPa) and elongation at break (18.2% to 6.06%), while decreasing water solubility and swelling index, thereby improving moisture barrier performance. Antioxidant capacity increased with RA concentration, reaching 80.63% DPPH radical scavenging at 2% RA. FTIR confirmed polymer–anthocyanin interactions, and TGA showed maintained thermal stability. Soil burial tests indicated substantial biodegradation within 21 days. Films applied to button mushrooms exhibited a color change from red to black within 32–36 hours, indicating spoilage.DiscussionThe incorporation of roselle anthocyanin into HPMC/κ-carrageenan films improved antioxidant and moisture barrier properties while providing a clear pH-sensitive color response.