Intelligent double-layer pads containing blueberry anthocyanins/citric acid/tricolor lake for chilled pork real-time freshness monitoring

In this paper, an intelligent pH-sensitive pad was well designed using blueberry anthocyanin (BA) mixed with citric acid (CA) and yellow lake (Y), as composite indicator with significant color change (ΔE) from pink to yellow to green at pH 6.0-7.0 under tricolor principle. The double-layer poly(vinyl alcohol)/glycerol/gelatin (PGG)-CA/PGG-CA-BA-Y pad exhibited an excellent colorimetric pH/NH₃-response, water-absorption ability, mechanical properties and preservation performance. Applied into the freshness monitor of chilled pork, the pad color change was detected from red (fresh) to orange (semi-fresh) and then to green (spoiled) by both naked eye visually and mobile APP rapidly, as the correlation coefficients (R²) of ΔE and S_RGB significantly correlated with pH (0.99 and 0.98) and TVB-N (0.96 and 0.96), respectively. In summary, intelligent pH-sensitive double-layer hydrogel pads could preciously monitor food freshness and prevent the spoilage, by improving anthocyanin stability and increase the sensitivity of color in response to a narrow pH range.