# Freshness Monitoring of Packaged Vegetables
## 4 Intelligent Packaging (IP)
### Commercial Biosensors
#### 4.1.4 Humidity Sensors  
Freshness of packaged vegetables is highly affected by the water content, which creates a suitable environment for mold and bacterial growth. Thus, for water vapor monitoring, many humidity sensors were developed over the years for different applications such as storage environment monitoring, ventilating, and air condition systems (HVAC systems), and home heating systems. The fact that  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Commercial** & **Gas** & **Principle** & **Operating** & **Operating** & **Operating** & **Packaging** & **Cost (S)** & **Ref.** \\
**Name** & **Detected** & **of** & **Range** & **Accuracy** & **Temperature(\%)** & **Humidity** & **Form** & **Cost (S)** & **Ref.** \\ Examples & **Detected** & **Operation** & **(\%)** & & **(\%)** & & **(\%)** & **(\%)** & **(\%)** \\ \hline \hline \multirow{4}{*}{O2 xydotsTM} & \multirow{4}{*}{O2} & Fluorescence-based & \multirow{4}{*}{5\% of [Ru(dpp),]Cl2 dye} & \multirow{4}{*}{0–30} & \multirow{4}{*}{0–50} & \multirow{4}{*}{0–100\%} & Dots & \multirow{4}{*}{\(\sim\)4.3} & \multirow{4}{*}{[144]} \\  & & using & & & & with & with & \\  & & [Ru(dpp),] & & reading & & & silicon & & \\  & & & Cl2 dye & & & & glue & & \\  & & & Fluorescence-based & & & & & \\  & & using & & & & & & \\  & & PtTBP & & & & & & \\  & & dye & & & & & & \\  & & Fluorescence-based & & & & & Dots & \\  & & using & & & & & with & \\  & & PtTFP & & & & & & silicon & \\  & & dye & & & & glue & & \\  & & Galvanic & & & & & & \\  & & Type & & & & & & \\  & & Type & & & & & & \\  & Cubic & & & & & & & \\  & & & & & & & & \\  & SRH & CO2 & NDIR & 0–20 & \(\pm\)2.0\%FS & \(-\)20–70 & - & in RFID & Up to & & \\  & & & & & & & tags & 100 & & \\  & & Thermal & 0–100 & & & & & & \\  & Conductivity & & & mV/\%CO2 & \(-\)10–50 & \(\pm\)95\% & RFID & & \\  & & Conductivity & & & mV/\%CO2 & \(\pm\) RH & tag & & \\ \hline \hline \end{tabular}
\end{table}
Table 4: Commercial O2 and CO2 sensors used in vegetable package monitoring.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Oxygen** & **Max Absorption Wavelength** & **Max Emission** & **Luminescence** & **Ref.** \\
**Sensing Dye** & **(nm)** & **Wavelength (nm)** & **Lifetime \(\mu\)s** & **Ref.** \\ \hline
[Ru(dpp),]2 & 452 & 613, 627 & 5.3 & [137] \\ PtTFP & 390 (323), 504 (23.2), 538 (29.4) & 647, 710 & 60 & [138] \\ PtOEPK & 398 (86.2), 592 (55.1) & 758 & 60 & [139] \\ PtTTPBPF & 430 (212), 615 (146) & 773 & 50 & [140] \\ \hline \hline \end{tabular}
\end{table}
Table 3: Photophysical properties of commonly used oxygen dyes in oxygen sensing.  
humidity sensors can be made now as thin films or membranes enabled the possibility of their use as part of packaged vegetable monitoring, especially with their possibility of integrating with RFID tags. The thin films can be classified into three main categories depending on the sensing material used: polymer, ceramic (semiconductor), and hybrid (polymer/ceramic [149]. Each one of them can be further classified into capacitive or resistive based on the transduction. Although ceramic has more advantages compared to polymer such as mechanical strength and physical stability, polymers are more promising and cost-effective option with acceptable sensitivity suitable for humidity monitoring in packaged vegetables.  
The sensing functionality of thin porous films depends on the measurable changes of electrical properties due to water uptake. Water molecules interactions have different dominating mechanisms that affect humidity-sensitivity characteristics. At low RH, the interaction of water molecules is dominated by electronic conduction. At high RH, ionic conduction is dominant, meanwhile at medium RH both are contributing [149; 150]. A summary for different types of thin film humidity sensors that are used or can be used as an intelligent system for package characterization in food industry are shown in Table 5 with their specifications.