# Freshness Monitoring of Packaged Vegetables
## 4 Intelligent Packaging (IP)
### Indicators
#### 4.2.1 Time-Temperature Indicators (TTIs)  
These indicators are labels placed on each package from outside to provide information about the temperature history of each package. TTIs act as a very useful visual or electronic indicator for the freshness of the products by tracing its temperature. The indication occurs as a result of irreversible responses to temperature variation due to chemical, electronic, or even nanoparticle changes. These indicators can be classified into three main categories: (1) Critical temperature indicators that give limited information about the product where indication is provided as to whether a reference temperature was exceeded or not. (2) Partial history TTIs which provide accumulative temperature versus time changes with respect to a reference temperature. (3) Full history indicators that provides all the temperature changes throughout the whole storage period [4, 5, 77, 167]. The working principle of these indicators depends on color change as a response to enzymatic-polymer-, microbial-, photochemical-, or diffusion-based reactions [105]. For an enzymatic-based indicator, the mixture between pH dye, the lipid substrate, and lipolytic enzyme will stimulate reactions of free fatty acids, which in turn degrade the pH leading to visual change. For a polymer-based indicator, it relies on the fast color change of the active solid-state polymer as a result of high temperatures and at a slower rate at low temperatures. This type depends on changes of optical properties of the active polymer in response to temperature variation, therefore no activation is needed before use, but attention to its storage temperature before use is essential [105]. A microbial-based TTI relies on increased growth rate of micro-organisms inoculated into the label due to high temperature exposures. For photochromic reactions, the use of photosensitive compounds causes a reaction resulting in changes in their color in response to temperature fluctuations over time. In a diffusion-based indicator, the colored fatty acid ester causes a temperature-time dependent diffusion through an indicator track. Here, the low-cost, small size, reliability, and the ease-of-use made them more commercially viable than other intelligent systems. However, exposure to very low temperatures affect the spoilage of some vegetables, too; thus, the limitation of TTIs lies in the difficulty to give information whether the package was exposed to subminimum temperatures or not for example, if carrots were frozen at temperature below \(-1.8\)\({}^{\circ}\)C, freezing injury may cause sunken, pitted areas, or even internal browning [168]. Hence, the need for more complicated systems is essential, depending on the packaged produce such as the use of RFID tags to quantitatively monitor temperature, moisture and gas concentration quantitatively [94, 169]. Many TTIs were commercialized and are widely used for monitoring perishable goods. Fresh-Check(r) is an example of polymer-based self-adhesive chemical indicator in the form of labels that are produced by TEMPTIME Corporation, [13, 167, 170, 171], as shown in Figure 14a. This indicator is a full history type that is based on a solid-state polymerization reaction. The active center polymer, such as diacetylene monomer placed in the inner ring, will change the inner circle color from red to black color matching the outside reference marker as the package is exposed to high temperatures over time, giving a visual indication regarding the quality of packaged food. The higher the temperature the package was exposed to, the faster the color will change. Monitor MarkTM is a commercial example of a partial-history TTI [13; 167; 170; 172]. This easy-to-use indicator provides temperature versus time history of the product and remains active for months when kept below its threshold temperature. Figure 14b shows an illustration of this indicator. This is a one-time use label made of layers of paper and films connected to a chemical reservoir.