Chitosan-based carbon dioxide indicator to communicate the onset of kimchi ripening

- Chitosan-based CO2 indicator is designed to communicate kimchi ripening process.
- CO2 produced during kimchi ripening process makes chitosan soluble.
- Opaque CO2 indicator develops transparency due to chitosan dissolution at lowered pH.
- Chitosan-based CO2 indicator can detect onset of well-ripened packaged kimchi.

The objectives of this study are to develop a chitosan-based CO2 indicator that can notify the onset of ripening stage of packaged kimchi, and to study the correlation between the ripening stages of kimchi and the transparency levels of the chitosan-based CO2 indicator during storage. The chitosan-based CO2 indicators were developed by adding chitosan (0.4 g) and 2-amino 2-methyl 1-propanol (AMP, 5 mL) to distilled water (100 mL) at pH 7.0, subsequently packed in LDPE film sachets and placed in the headspace of kimchi packages. When the kimchi packages were stored for 2 weeks, the pHs of the chitosan-based CO2 indicators decreased up to pH 5.8 at 10 °C and up to pH 5.9 at 20 °C. A remarkable change was observed in the degree of transparency of the indicators, i.e. from below 30% to nearly 100% at the end of the testing period. Onset point of optimal kimchi ripening (pH 4.2-4.4 and 0.4-0.8% of titratable acidity) was observed after 1 day storage at 20 °C and 5 day storage at 10 °C. The chitosan-based CO2 indicator system offer a great potential as a convenient, easy-to-use technique for recognition of unripen packaged kimchi and/or onset of optimal ripening period.