Application of santalin dye extracted from Pterocarpus santalinus (red sandalwood) as a natural colorimetric indicator for real-time monitoring of spoilage in ready-to-cook idly batter

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The growing demand for natural and eco-friendly solutions in food quality monitoring has sparked significant interest in using natural dyes as sustainable alternatives to chemical dyes for detecting food freshness. In this study, a colorimetric indicator based on natural santalin dye was developed for real-time monitoring of spoilage in ready-to-cook (RTC) idly batter. Santalin, extracted from Pterocarpus santalinus (red sandalwood), is a stable red pigment renowned for its antioxidant, antimicrobial, and antiinflammatory properties. The santalin dye was immobilised in a polyvinyl alcohol-methylcellulose matrix and applied to cellulose paper, creating a low-cost pH-sensitive colorimetric indicator label. The developed indicator label displayed a visible colour change from dark brown to lighter brown hues in response to increased carbon dioxide concentrations, corresponding with a decrease in pH from 5.3 to 4.0 and a rise in titratable acidity (TA) from 0.5% to 0.9 % during the spoilage of RTC idly batter. The colour changes were captured and converted into grayscale images for pixel intensity analysis using MATLAB. The results revealed a consistent decline in mean pixel intensity as fermentation and spoilage progressed, demonstrating the effectiveness of the santalin-based indicator. This novel, natural santalin dye-based indicator offers a promising alternative to synthetic indicators, addressing safety concerns such as chemical migration into food. It can thus serve as an "on-package sticker" for real-time monitoring of spoilage in RTC idly batter.