Development and validation of a red cabbage anthocyanin-based colorimetric indicator for milk spoilage detection

Abstract Milk is a highly nutritious yet perishable food commodity, and its spoilage due to microbial activity poses significant quality and safety challenges throughout the dairy supply chain. This study aimed to develop and validate a simple, eco-friendly, and paper-based colorimetric freshness indicator for real-time monitoring of milk spoilage using anthocyanins extracted from red cabbage ( Brassica oleracea var. capitata f. rubra ). Fresh raw milk samples were stored under controlled conditions to simulate spoilage, and changes in physicochemical (pH, titratable acidity), microbiological (standard plate count), and biochemical (lactate accumulation) parameters were systematically evaluated. The red cabbage extract, characterized by a high anthocyanin content, exhibited pronounced pH-dependent colour transitions and distinct UV–Vis spectral changes across a wide pH range, corresponding to structural transformations of anthocyanins. Paper-based indicator strips fabricated using the extract showed clear and sequential colour changes from blue to pink as milk freshness declined. Quantitative colour analysis using CIE L a b* parameters and total colour difference (ΔE) demonstrated significant correlations between colour response, pH reduction, acid development, and microbial proliferation. The developed indicator strips exhibited excellent analytical performance, achieving 100% sensitivity and specificity, along with high repeatability, reproducibility, and intermediate precision over extended storage. Fourier transform infrared spectroscopy confirmed successful immobilization of anthocyanins on the paper substrate without loss of functional groups critical for pH responsiveness. Overall, the study demonstrates that red cabbage anthocyanin-based paper strips provide a reliable, cost-effective, and sustainable tool for milk freshness assessment. This approach offers strong potential for application in on-site dairy quality monitoring and smart food packaging systems, contributing to accessible and environmentally responsible food safety solutions.