Biodegradable Alginate-Sericin Biofilm: A Colorimetric Indicator Using Mangosteen-Peel Anthocyanins for Protein Food Freshness

Abstract

Smart indicator films provide an eco-friendly approach for real-time monitoring of food freshness through easily interpretable visual color changes. In this work, a fully biodegradable pH-responsive film was fabricated from sodium alginate and silk sericin, with anthocyanins extracted from discarded mangosteen peel (ASA) serving as the natural chromatic indicator. Structural analyses verified the formation of a stable Alg–SS network that enhanced swelling resistance, lowered solubility, and improved moisture-barrier properties. Among all formulations, the film containing 7% anthocyanin (ASA7) exhibited the most favorable balance of functional attributes, including strong and stable colorimetric response, adequate mechanical strength, and moderate hydrophilicity. The ASA7 film also demonstrated pronounced antioxidant activity, with scavenging efficiencies of 37.10 ± 1.71% (DPPH) and 90.37 ± 1.30% (H₂O₂). Distinct, reversible color transitions from pink-red to blue occurred as pH increased, enabling reliable visual sensing. When applied to pork and shrimp packaging, the film’s color evolution showed strong correlation with total volatile basic nitrogen (TVB-N), accurately signaling the spoilage threshold (~25 mg N/100 g). Overall, the ASA film presents a promising waste-derived indicator system for intelligent food packaging, offering real-time freshness assessment while supporting sustainability and circular-economy goals.