# Freshness Monitoring of Packaged Vegetables
## 4 Intelligent Packaging (IP)
### Indicators
#### 4.2.2 Gas and Humidity Indicators  
Gas and humidity indicators are used to monitor the gas composition and water content inside the food package. These indicators are in the form of printable labels, films, or tablets that are placed inside the package to give information about the changes in gas mixture inside the headspace of packaged vegetables, and other ready-to-eat packaged food such as salad. Changes in the gas composition are a result of respiration of vegetables, leakage, or generated by the microorganisms present that will change the indicator color. Oxygen, carbon dioxide and water vapor indicators are the most popular ones used in the food industry. Most of these indicators depends on the binding reaction between the gas and the indicator, or on redox reactions which will change the dyes such as methylene blue that is used to give visual indications [107]. Gas indicators are used to monitor the variation of gases in the head space of packaged vegetables. At the time of packaging, the ratio of gases mixture is adjusted through a modified atmosphere packaging (MAP) technique to extend the shelf life of vegetables. An example of oxygen indicators, known as Tell-Tab [179], show color changes from pink to purple when the oxygen concentration inside the package is 0.1% or greater. This type requires no activation. Therefore, it must be placed in oxygen absorbing packets until being introduced to packages. In contrast, for the Ageless Eye(r) oxygen indicator [180], the color changes from pink to blue when the oxygen concentration is 0.5% or more inside the package. For carbon dioxide indicators, pH responsive dyes such as cresol red are used [181]. In this case, the diffusion of carbon dioxide through the permeable membrane causes a chemical reaction with the pH dye, leading to a color change. As the concentration of CO\({}_{2}\) increases, the color changes from red to yellow. This type of indicator is reversible and insensitive to moisture inside the package. For moisture indicators, humidity cards that give visual color indications of the relative humidity were developed by IMPAK. These cards are injected with cobalt chloride solution that changes from sky blue to purple or pink as the humidity increases. The cost of such cards ranges from $0.14-$0.65 with an RH range of 8 to 80% [179]. Another interesting example for humidity monitoring is with carbon-coated copper nanoparticles dispersed on a glass substrate, resulting in a film that swells in low humidity due to the increase of the separation between the NPs. As the humidity gets higher, the water vapor starts to condense on the strip film, leading to destructive interference at perpendicular incident angles. Hence, reflection and absorption of light will have different light colors depending on the amount of water vapor concentration inside. This film can measure RH between 45% and 100%, with the color changing from orange (around 50% RH) to blue (70% RH), pink then green (100% RH) as water vapor saturation increases [115; 182]. This metallic color film has a reversible color change which gives the possibility of reusing and recycling such indicators. Using it inside the package as a film \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**TTI** & & & & & **Temperature** \\
**Commercial Name** & & & & **Activation** & **Cost (S)** & **Oper-Ating** \\ \hline Fresh-Check® & Polymeric & Red to black & N & 0.02–0.04 & - \\ Monitor Mark & Diffusion & Colorless to & Y & 2–4 & \(-\)15 \({}^{\circ}\)C to +31 \\ TM & & blue & & & \\ OnVuTM & Photochemical & Dark Blue to & Y & 0.01–0.05 & 0 \({}^{\circ}\)C to +25 \\ eO® & Microbial & Green to Red & N & 0.05–0.17 & 2 to +12 \\ Timestrip® & Diffusion & Green to red & N & \(-\)0.6 & 2 to 8 \\ eTimestrip® & electronic & Green LED off & Y & - & \(-\)30 to 70 \\  & & Red LED ON & Y & 0.05–0.15 & \(-\)20 to 5 \\ Checkpoint® & Enzymatic & & & & \\  & & & & & \\ Tempick® & Diffusion & & Barcode lines & Y & \(<\)0.01 & \(-\)35 to +32 \\  & & & & & \\ \hline \hline \end{tabular}
\end{table}