# Freshness Monitoring of Packaged Vegetables
## 4 Intelligent Packaging (IP)
### Indicators
#### 4.2.1 Time-Temperature Indicators (TTIs)
The working principle is based on the diffusion of blue-dyed fatty acid ester through a film as a result of melting due to its exposure to a higher temperature than the reference temperature. Thus, a blue color will appear on the left edge and gradually move to the right at a rate that depends on the temperature exposure. If temperature is below the reference, the dye will be in the solid-state phase and hence will not move through the porous paper. This type require activation prior to use by removing the material from the porous material. Figure 14: (**a**) Fresh-Check® indicator. (**b**) Monitor Mark indicator. (**c**) Time-temperature effect on OnVuTM indicator. (**d**) eO® indicator. (**e**) Timestrip® indicator. (**f**) eTimestrip® indicator. (**g**) Checkpoint® indicator. (**h**) Tempix® indicator. activation strip. A response timecard is then used to interpret the extent of color movement (run out time) in terms of time and temperature. OnVuTM indicator is a photochemical-based TTI that indicates whether the product was exposed to high temperatures over time. This device is fabricated with the use of a temperature sensitive printable ink that changes its color from colorless to blue upon activation with UV light [173]. A UV blocking filter is then thermally transferred on the top of charged UV photochromic ink with the use of a thermo-transfer ribbon (TTR), to prevent further exposure to UV light and hence guarantee the irreversible change of color [169; 173]. With time, the color starts to revert to a color matching the reference at a rate corresponding to time and temperature change as shown in Figure 14c. The intensity and the pulse modulation of the UV light used for activation will control the shelf life of this label which is directly related to the shelf life of packaged vegetables. This indicator is considered a very cheap and flexible option for such applications [174]. eO(r) is a microbial-based TTI [105; 175], shown in Figure 14d. The food-grade microorganism in the label mixed with the nutritive environment in the shape of flower, will grow differently depending on the temperature history of the package. As a result, lactic acid will be produced, and the pH degradation will cause the pH dye to change accordingly from green to red color indicating spoilage of the packaged vegetable. Timestrip(r) is a diffusion-based TTI and is illustrated in Figure 14e [176]. The blue fatty esters will show a line upon activation. Then, this dye will melt and migrate through the porous membrane when the temperature is higher than the reference temperature, and the indicator will show the accumulated time of temperature breach of the packaged food. This indicator is now upgraded into a new electronic smart TTI version known as eTimestrip(r)[176]. Figure 14f shows an illustration of this new indicator. This type is a full history indicator that uses LEDs to show the status of package. A green light indicates that the temperature is ok, a flashing red light when exposed to high temperature, and an orange light if the indicator is ready to use or if it is not working. The real-time data for the temperature variation will be recorded with time and can be retrieved when scanning the label using a smart phone application. This will allow for more accurate data regarding temperature versus time. This new upgrade may be very costly if used for each package of vegetable. If it is used for bulk monitoring of packaged vegetables, the cost will be reduced. Checkpoint(r) indicator, shown in Figure 14g, is an example of enzymatic-based TTI that turns from green to red as a result of enzymatic reaction [4; 5; 105; 117; 167; 170]. Before activation, the self-adhesive label has two separate compartments: One has pH indicating dye mixed with lipolytic enzyme, and the other is lipid substrate such as glycerine, tricapronate (tricaproin), or tripelargolin. Upon activation by hand pressure, the two compartments will react in a rate corresponding to time and temperature variations.