Preparation and Application of Alizarin-Laccaic Acid Smart Indicator Label for Monitoring Metapenaeus ensis Freshness

A smart indicator label for monitoring the freshness of Metapenaeus ensis was prepared using a mixture of carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA) as substrate with the natural colorants laccaic acid (LA) and alizarin (ALI) added. By taking advantage of the co-pigmentation between LA and ALI, good color development performance and stability were achieved, enabling freshness monitoring of M. ensis. To avoid the problems of the health harms caused by the overuse of traditional chemical indicators and the poor stability and minimal color changes of natural colorants when used singly, the feasibility of applying natural colorant-based smart indicator labels for real-time non-destructive testing of the freshness of M. ensis was evaluated by comparative analysis of the effects of incorporating LA, ALI and their mixture on the microscopic morphology, thermodynamic properties, and color development performance of labels. The results showed that with the joint use of ALI and LA, the internal structure of the label material was denser, the thermal stability was enhanced, and it presented different color variations well in the pH range of 4–13. Compared with single-pigment indicator labels, the CMC/PVA/ALI/LA indicator label could accurately discriminate among fresh, semi-fresh and spoiled M. ensis during storage at 4 and 25 ℃, turning from light yellow to purple; ΔE values increased to 22.20 ± 0.42 and 38.89 ± 0.72 when used to detect shrimp stored at 4 ℃ for 8 days and at 25 ℃ for 48 h, respectively, which demonstrated a significant positive correlation (P < 0.05) with pH and total volatile basic nitrogen (TVB-N) content. Therefore, the smart indicator label could effectively monitor the changes in freshness of M. ensis during storage at 4 and 25 ℃.