Development of Colorimetric Indicators for Intelligent Food Packaging
Intelligent packaging systems with enhanced communication functions are getting an increasing amount of attention in the food packaging field, because they can inform the consumer of the actual quality of the products. This thesis is an investigation of the development of colorimetric indicators intended for use in intelligent food packaging. Two colorimetric indicators have been successfully developed to detect CO2 and NH3. The first CO2 colorimetric indicator described in this thesis used pH dyes and certain phase transfer agent to prepare the sensing inks. Thermal inkjet printing technology and multiplexing method were used to fabricate indicators and modify the indicator performances respectively. The effect of different printing substrates and printing passes on the indicator performance were studied. It was found that 10 printing passes on paper has the optimal color response towards CO2. The influence of multiplexing ratios using inkjet printer on indicator performance was also investigated. Results showed that the indicators based on multiplexed inks exhibited higher sensitivity upon the changes of CO2, and a multiplexing ratio of 5:5 was the most sensitive to CO2 and presented the most pronounced color change. The second colorimetric indicator outlined in this thesis is a 4×4 array indicator, targeted at CO2 and NH3. These 16 sensing elements were prepared using pH dyes with or without dopants in binding material (ethylcellulose polymer). ∆RGB patterns were generated by subtracting the “before” exposure image from the “after” exposure image. Multivariate analysis, e.g. PCA, was used to interpret the ∆RGB patterns. The influence of temperature was also studied. The array was found to be more sensitive at 4 than 25℃. At both temperature, the array successfully discriminate CO2 from NH3, as well as their mixtures. Different ∆RGB patterns were obtained for NH3 and CO2 of different concentrations, as well as their mixtures with different blending ratios. pKa and chemical structures of the pH dyes were found to be crucial for indicator sensitivity upon detection of CO2 and NH3. The observed results clearly indicate that for these colorimetric indicators, there is a potential application in food packaging systems, especially modified atmosphere packaging systems for perishable foods.