Cost-effective and Sensitive Anthocyanin-Based Purple Cabbage (Brassica Oleracea var. Capitata L) Extract Biosensor for Diagnostic Spoilage Fish Product

Purple cabbage (Brassica oleracea var. capitata f. rubra) contains anthocyanins that exhibit pH-dependent colour changes and can function as visual indicators of food spoilage through their response to volatile basic compounds. This study developed a simple colourimetric indicator label using purple cabbage extract and evaluated its suitability for monitoring milkfish (Chanos chanos) spoilage under different storage temperatures. Extracts were prepared at three levels (30 g, 50 g, and 70 g of cabbage per 100 mL of solvent) and used to fabricate indicator labels, which were characterised using UV–Vis spectroscopy and FTIR to confirm anthocyanin-related spectral features and functional groups. The labels were tested in sealed 100 mL bottles containing 10 g of milkfish, with the indicator positioned 4 cm above the sample to enable headspace exposure without direct contact. Storage was conducted at 15 °C, 25 °C, and 40 °C. Colour changes were recorded at 2, 10, 24, and 32 h and quantified using image analysis. The indicator labels exhibited progressive colour shifts over time, consistent with the accumulation of volatile bases associated with spoilage. Temperature strongly influenced the response rate, with slower changes at 15 °C and faster, more pronounced changes at 25 °C and 40 °C. These findings indicate that the proposed purple-cabbage-based label has potential as a low-cost, non-destructive tool for monitoring milkfish freshness across practical storage conditions.