Colorimetric Covalent Organic Framework Gel as Thermal History Indicators for Food Freshness Monitoring

Time and temperature are the crucial factors affecting the quality of perishable products. The device capable of recording temperature fluctuation and maintaining time has huge practical potential. In this research, a novel thermal history indicator (THI) was proposed based on covalent organic frameworks (COFs) with the irreversible color change performance. The color change and absorbance of the THI were investigated over a broad monitoring temperature range (− 22 ~ 55 ℃) and were closely correlated with its thermal accumulation over time and temperature. The activation energy (E_a) of the THI was calculated to be 55.31 kJ mol⁻¹, which can be used to monitor the perishable products with deterioration reaction E_a ranging from 30.31 to 80.31 kJ mol⁻¹. In addition, the THI also demonstrated the significant response to the environmental temperature fluctuations. Furthermore, the potential applications were evaluated by recording the changes of the THI gel and the total viable count (TVC) of food under the same conditions. The results indicated that the digital images of the colorimetric THI gel were correlated with TVC of food at 4 ℃ and 25 ℃, respectively. These results support that the proposed THI is potential for real-time, accurate, non-destructive, visually, and quantitatively monitoring food freshness during food supply chain distribution.