Development of CO2-responsive colorimetric films for visual indication of kimchi fermentation maturity

This study presents a novel CO?-responsive colorimetric sensing film designed to visually indicate the degree of kimchi fermentation through color changes correlated with CO? concentration. During the fermentation of kimchi—a representative Korean fermented food—CO? generation is strongly associated with acidity and microbial activity. However, consumers typically rely only on production dates to estimate fermentation level, leading to uncertainty in product selection and quality perception.

To address this, we developed a pH-indicator-based colorimetric ink capable of responding sensitively to CO? via acid–base equilibrium. The system uses triethylamine (TEA) as an alkaline sensitizer and a combination of three pH indicators—Thymol Blue, Bromothymol Blue, and Methyl Red—to achieve multi-step color transitions reflecting fermentation stages (immature, optimum, and over-fermented). CO? exposure tests were conducted under both dry and humid conditions using PET substrates coated with ethanol-based binder formulations.

The optimized films demonstrated distinct color shifts across three states, driven by the sequential protonation of the indicators. TEA provided the most stable and responsive base among tested candidates (DBU, TBAOH, TEAOH), with an optimal concentration of approximately 9 wt%, ensuring stable alkalinity and reproducible sensing performance. Quantitative color analysis using RGB and Euclidean distance methods revealed that the color transition follows an Asymptotic 1 model, enabling predictive correlation between exposure time, gas concentration, and film composition.

These findings establish a reliable visual CO? indicator platform for fermented food packaging. The developed film provides consumers with an intuitive method to assess fermentation status at a glance, contributing to improved product transparency and satisfaction. Beyond kimchi, the approach can be extended to other fermented foods and smart packaging systems for real-time freshness monitoring.

Dr. Ki-Seob Hwang is a senior researcher at the Korea Institute of Industrial Technology (KITECH), specializing in sustainable polymer systems and functional composites. His current research focuses on thermoresponsive and bio-based polyurethane materials, optical polymer films, and advanced insulation systems for energy and marine applications. He has authored numerous technical reports and patents in polymer recycling, optical control, and composite materials engineering.