On-package Dual Indicating Label for Real-time Monitoring the Freshness of Cyprinus carpio

A dual indicating label based on Bromophenol red and Bromocresol purple was developed for monitoring fish freshness in real time. The amount of total volatile basic nitrogen (TVB-N) of fish sample was detected regularly, and the color changing pictures of indicating label were collected. Results showed that the amount of TVB-N was 14.3 mg/100 g when the fish samples were stored at room temperature for 14 h. The quality of fish sample was close to first-grade according to national standards. Correspondingly, the color of the outer part of the indicating label changed from bright orange to light brown while the inner part turned to dark yellow. TVB-N increased to 30.09 mg/100 g when storage time reach to 26 h. The sample spoiled. In the meantime, both the outer and inner part of the indicating label turned to purple and even darker with the increasing storage time. Hue values were calculated based on RGB values of the indicating label pictures, and were fitted with the levels of TVB-N. The fitting equation was Y=-53.9463+13.238x-0.48757x², and the correlation coefficient was 0.982, which showed a good fitting result. It would be feasible to use the dual indicating label prepared in this study for real-time monitoring of fish freshness.