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/NH 3 -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 (R 2 ) 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. Scheme 1. The optimal citric acid addition and the most suitable double-layer pad preparation were selected by the pH response of anthocyanins; the structure schematic of the double-layer pad was elaborated using the PGG-CA layer as the protective layer and the PGG-CA-BA-Y layer as the indicator layer; the chilled pork spoilage can be detected via naked-eye visually and a mobile APP rapidly using the PGG-CA/PGG-CA-BA-Y intelligent pH-sensitive indicator pads. • The double-layer hydrogel pad for intelligent pH-sensitive indication was prepared. • Color sensitivity at pH 6.0–7.0 was improved by adding citric acid and yellow lake. • Hydrogen bond supplied by plasticizer and complex indicators increased pad stability. • The prepared pad visually distinguished the freshness of the meat from the spoiled. • Chilled pork spoilage could be quantified by pad color change using a mobile APP.