Preparation and characterization of a poly(ether-block-amide) film-based CO2 indicator for monitoring kimchi quality

In this study, pH dyes containing a poly(ether-block-amide) (PEBA) film that changes color depending on the CO2 levels were prepared and characterized. The CO2 level inside a packaging container is an indicator of the freshness or quality of packaged foods. Any change in CO2 levels or production of metabolites during storage can alter the pH. Films of PEBA and polyethylene terephthalate (PET) were incorporated into two different pH-sensitive dyes (methyl red (MR) and bromothymol blue (BTB)) that respond through visible color changes depending on the CO2 levels and/or metabolites formed during fermentation. The prepared films were characterized using Fourier transform infrared, scanning electron microscope, and X-ray diffraction analyses, and their barrier properties measured through the oxygen transformation rate and water vapor transmission rate analyses. The film indicators are made up of three different dye mixtures containing 5% polyethyleneimine (PEI) and exhibit prominent color changes depending on the compositions. Among them, the highest total color difference value was observed for the PET/PEBA + dye (MR + BTB) (3:7) + PEI 5%/PEBA combination, which is considered the most effective kimchi indicator. Thus, as-prepared non-porous breathable films can prevent the migration of dye substances and be applied for monitoring the fermentation stage of kimchi.