A warning sticker that indicates if cold-chain food products — such as fish, meat, and fruits and vegetables — are spoiled has been developed by the Korea Research Institute of Chemical Technology (KRICT), which is part of the National Research Council of Science & Technology.
The sticker is opaque when refrigerated due to nanofibre‐induced light scattering, but becomes irreversibly transparent at room temperature through self-healing-induced interfibrillar fusion, which leads to the appearance of a previously hidden warning sign.
The cold-chain safety sticker is designed to help prevent the occurrence of food poisoning resulting from the malfunction of refrigerator or freezer trucks.
In general, when refrigerated or frozen foods are exposed to room temperature, bacteria begin to grow and reproduce. However, it is difficult to discern with the unaided eye if they have spoilt the food. This is because certain bacteria do not affect the taste and smell of foods where they live, and frozen foods have almost the same appearance even after a cycle of melting and refreezing.
The low-cost nanofibre sticker allows us to see with the naked eye if any cold-chain food products have been exposed to room temperature (10°C or higher). Room temperature exposure history and time throughout the cold chain delivery process are indicated but cannot be manually edited.
A research team from the Research Center for Bio-based Chemistry of the Korea Research Institute of Chemical Technology (KRICT, Dr Dongyeop Oh, Dr Sung Yeon Hwang, Dr Jeyoung Park, Dr Sejin Choi) developed this technology and published the relevant results in the journal Advanced Materials online, IF:25.809 under the title ‘A Self-Healing Nanofiber-Based Self-Responsive Time-Temperature Indicator for Securing a Cold-Supply Chain’.
The core technology of the cold-chain safety sticker is its nanofibre film. At low temperatures, the film has a stable structure where thin threads intersect each other and thus appears opaque because the light is scattered. However, when exposed to room temperature for a certain period of time, the nanofibre structure collapses. More specifically, these thin threads start to melt and become entangled with each other. This allows light to transmit through the film, thereby making it appear transparent.
Based on this mechanism, when the film on the front surface of the sticker becomes transparent after being exposed to room temperature, the image produced on the typical film on the back becomes visible from the front. This change allows users to determine if their food products have spoiled.
Furthermore, the researchers found a way to control the time that is required for this nanofibre film to become transparent when exposed to room temperature. This was attempted based on the reasoning that the time needed before each food goes bad would vary.
Thus, each sticker was designed to become transparent after a minimum of 30 minutes and a maximum of 24 hours of exposure to room temperature. This works in the same way as a timer does. This technique was realised by controlling the composition and thickness of the used nanofibre.
Dr Dongyeop Oh from the KRICT said: “This sticker, once exposed to room temperature, cannot be restored to its original state even if one attempts to refrigerate or freeze it again. Also, room-temperature exposure time cannot be manually adjusted. This means that there is virtually no room for any manipulation.”
Designed for food product applications, the sticker is thin and flexible and its manufacturing cost is low, so the technology is expected to have high potential for applications in the rapidly growing fresh food delivery market. It could also have applications for the cold-chain distribution of expensive medicine and medical supplies.