Fabrication of pH-adjusted CO2-sensitive films for monitoring fresh-cut lotus root freshness.

This study employed methyl red (MR) and bromothymol blue (BTB) as a mixed indicator system to fabricate CO2-sensitive freshness indicator films by adjusting the pH of the film-forming solution. The MR/BTB/pH 8 film, prepared at pH 8, exhibited the highest CO2 sensitivity, with the most pronounced color change and the greatest ΔE. FTIR and XRD analysis revealed that polyvinyl alcohol (PVA), sodium carboxymethyl cellulose (CMC), and indicators were bound via hydrogen bonding interactions. When the MR/BTB/pH 8 film was applied to vacuum packaging (VP) fresh-cut lotus roots, the surface color of the fresh-cut lotus roots did not undergo any significant change. However, based on sensory evaluation, it was observed that an unfavorable flavor had emerged by the 6th day. At this point, the color of MR/BTB/pH 8 film transitioned from green to light greenish yellow. The results suggest that the fabricated MR/BTB/pH 8 film provides a promising tool for freshness monitoring in fresh-cut produce.