Development of pH-sensitive colorimetric labels base on chitosan-agar films embedded with alizarin and cinnamaldehyde essential oil for monitoring the freshness of fresh-cut papayas and shiitake mushrooms.

Abstract

The pH-sensitive colorimetric labels (PSCLs) were developed to monitor the freshness of fresh-cut papayas and shiitake mushrooms. These PSCLs were prepared using the solution-casting films and thoroughly characterized in terms of their physicochemical and thermal properties, microstructure, and application performance. Results demonstrated that the composite films (CAGAC1, CAGAC2, and CAGAC3), incorporating alizarin (Al, 3%, w/w) and different contents of cinnamaldehyde essential oil emulsion (CEOE, 1:8, 1:7, and 1:6, v/v) into a chitosan (CS) and agar (Ag) matrix, exhibited optimal performance as PSCLs. The Al in three CAGAC films exhibited distinct pH responsiveness. The addition of CEOE elevated the thermal degradation and crystalline melting temperatures, reduced the swelling degree and water solubility, and maintained the structural integrity of CAGAC films under high-humidity conditions. Morphological and structural analysis further indicated that the incorporation of CEOE reduced the aggregates between Al and matrix, improving their compatibility in the CAGAC films. And Schiff base formation, hydrogen bonding, and electrostatic interactions were also enhanced by CEOE addition. The application performance showed that three CAGAC films exhibited accurate colorimetric responses during storage of selected fresh-cut produce. Pearson correlation analysis showed strong positive correlations between the color parameters (b* and ΔE) of the three films and key quality indicators of weight loss rate, spoilage rate, relative electrical conductivity, in-package CO₂ content, and water vapor pressure, and negative correlations were observed with hardness and DPPH radical scavenging activity. These findings could provide a promising basis for intelligent packaging labels to non-destructively monitor the freshness state of fresh-cut produce.