Investigating time-temperature sensors in smart packaging

The shelf life of many food products is very sensitive to temperature changes, which is one of the main causes of deterioration and economic losses in these perishable products during transportation, handling, distribution, storage and consumption.
To limit the growth of pathogenic microorganisms or the formation of toxins for most perishable products in our diet, including a wide variety of natural, processed, raw and cooked foods of animal and vegetable origin, the FDA has defined these foods as TCS (time and temperature controlled for safety). Foods that require time and temperature control to ensure their quality and safety (FDA, 2013).
However, uncontrolled temperature fluctuations and shocks are inevitable for almost all commodities throughout the supply chain, and such fluctuations may lead consumers to misjudge the sell-by or use-by dates of products based on the ideal expiration date label on the product.
Packaging based on temperature monitoring and timing is critical to provide consumers with the necessary information about food quality and safety throughout the food circulation process.
To address this issue, time-temperature sensors have been developed to monitor time- and temperature-dependent changes in product quality or safety. Time-temperature sensors are usually indirect indicators and are commonly used in the food industry because they are relatively small, affordable, and user-friendly compared to other temperature monitoring devices.

 

Classification of time-temperature sensors in smart packaging

Time-temperature sensors are generally attached to individual consumer packages or shipping containers and can be classified into three types based on their capabilities:
(1) Critical temperature indicators (these only indicate whether a product has been exposed to a temperature above or sometimes below the reference temperature).
(2) critical time-temperature sensors (these indicate the cumulative effect of time-temperature changes on product quality or safety when a product is exposed to a temperature above a reference temperature).
(3) complete history indicators (continuous monitoring of how temperature changes with time throughout the product’s history);

Basic working principles of time-temperature indicators

The basic working principles of time-temperature sensors are to identify irreversible responses in the form of enzymatic, electronic, chemical, nanoparticle or biological changes after the product is exposed to higher temperatures.

Time-temperature sensors based on electronics

Electronic time-temperature sensors are defined as an electronic device that can provide an alert about the quality of a product using a thermal sensor that converts temperature signals into electrical signals and it then converts the electrical signals into a final visual output.
Because readout devices for time-temperature sensors are complex and specialized electronics-based, these types of time-temperature sensors are generally expensive and inconvenient or may require consumer training, leading to reduced market acceptance (by the product manufacturer, consumer and retailer) and limits the range of commercial applications.
However, compared to other types of time-temperature sensors, electronics-based sensors have relatively high accuracy and are generally superior technologies for monitoring and recording the thermal history of a product. In addition, most types of electronic time-temperature sensors are environmentally friendly and can be recycled.
With the development of electronics-based sensors, some new electronic time-temperature sensors have been invented that do not require professional reading devices or trained personnel to perform the test. This technology provides more convenience for consumers and increases the market demand for smart packaging.
However, to facilitate the use of electronics-based time-temperature sensors in the global market, without compromising accuracy and safety, time-temperature indicators should be made smaller, less expensive, and made of recyclable electronics.

Other types of time-temperature sensors (biological and chemical sensors)

For other types of time-temperature sensors (such as nanoparticle-based, enzyme-based, chemistry-based, and biological-based),irreversible color change is the main way to determine the thermal history of the product.
Color change can indicate time and temperature dependent changes in product quality or safety. These types of indicators are usually glued on or printed or coated on the product packaging and are less expensive, easier to read, and smaller than electronic time-temperature sensors.
The size, shape and surface morphology of metal nanoparticles change based on the time-temperature scenario they are exposed to. Nanoparticles exhibit an irreversible color change when exposed to a certain temperature for a certain period of time, and this feature makes them very useful for time-temperature sensors.

 

Gelatin/AuNP (Gold Nanoparticles) Based Thermal History Marker (THI)

developed a gelatin/AuNP (gold nanoparticle) (THI)-based thermal history marker that exhibits a clear color signal after 6 h of exposure to 30 °C. The intensity of the color signal was proportional to the duration of exposure to the desired temperature. In addition, the color intensity of AuNPs was maximum at 2% gelatin concentration.
However, gelatin/AuNP-based time-temperature indicators are specifically designed for low-temperature storage and have several advantages compared to alginate/AuNP-based THIs, including less sensitivity to color change, narrower range of temperature monitoring, and inability to prepare solid matrices such as THI.
Due to these features, a plasmonic THI was devised that takes advantage of the surface plasmon resonance of AuNPs synthesized in situ in alginate, which can be converted into a solid hydrogel by adding divalent calcium ions, and is more suitable and practical for end use.

Enzyme-based time-temperature markers

In enzyme-based time-temperature sensors, the hydrolysis reaction of an enzyme with a substrate causes varying degrees of color change depending on the actual time history and temperature. The observed color of a TTI can indicate the cumulative effect of time and temperature, and this information can be used to perform a dynamic assessment of product shelf life.
For example, the figure below shows a TTI (Vitsab Checkpoint) tag, which is a typical example of enzyme-based sensors. The TTI tag can be activated by applying gentle pressure to the “window” to initiate an enzymatic reaction between enzyme and substrate. The TTI window in the center of the words “Check Point” changes color from green to orange to red to indicate different stages of heat exposure.
A homogeneous green color in the “window” indicates mixing of enzyme and substrate mini-bags, which in turn indicates excellent transport and storage conditions for packaged foods. If the “check point” is yellow to bright orange, it indicates that the TTI tag has reached its preset temperature response time and the product is no longer acceptable.

 

Chemical time-temperature indicators

Chemical time-temperature sensors are based on many different chemical reactions (such as polymerization, photochromic, and oxidation reactions) and provide a characteristic color change due to the accumulation of changes in time and temperature.
Currently, some examples of chemistry-based TTIs include HEATmarker (N.J., U.S.A.) and Evigence Sensors. The principles of operation and their performance characteristics can be obtained by referring to the official websites of the product manufacturers.

 

Biologically based time-temperature markers

The working principles of biologically based time-temperature sensors are generally based on the change of pH under certain conditions, especially at a certain temperature, which results in a color change that shows the cumulative effect of time and temperature.

 

Conclusion

According to the active research on indirect indicators, these indicators have many problems, such as increasing the cost of the entire supply chain, introducing safety issues due to possible undesirable migration of chemical components, having accuracy and reliability under uncontrolled conditions (such as impact, compression and vibration) is subject to certain legal restrictions in Europe.
Therefore, to expand the range of applications of indirect indicators in the global market, future developments should be directed towards improving the stability and sensitivity of indicators to real temperature history, such as the use of non-toxic and even edible biopolymers to indicate thermal history.
A product with irreversible discoloration can also make amendments to the legal regulations governing smart packaging. According to the research and the new way that thesmart label of Vira is designed based on it, these labels are unique in their kind by using the simulation method and not needing to communicate with the internal environment of the packaging and solve all the problems in the field of expiration dates and it also does not have the problems of time-temperature labels (based on biology or chemical markers) that have been mentioned.