Brazilian researchers have developed a biodegradable, starch-based sensor that changes color when moisture enters food packaging. With the addition of cobalt chloride, the sensor changes from blue to pink as moisture levels rise and back to blue as they fall. Consumers and manufacturers could use it to monitor the quality and crispness of foods such as cookies with the naked eye or a cellphone camera, without complex equipment.
The research was led by scientists at the São Carlos Institute of Chemistry at the University of São Paulo (IQSC-USP), and the results were published in June in the journal Food Chemistry.
The researchers began testing with cream crackers, which are highly sensitive to moisture and quickly lose their crispness when stored under inappropriate conditions.
The results showed that the color change corresponded to an increase in moisture, which was strongly correlated with a loss of texture in the crackers. This indicates that the sensor reflects changes in storage conditions.
In just a few seconds, the small, starch-based cylinder placed near the crackers changes color, shifting among shades of blue and pink. When the cobalt chloride is dry, it turns blue. When it absorbs water vapor, it turns pink. This happens because hydration changes the chemical structure of cobalt chloride and, consequently, its color.
"What set this study apart was having a larger surface area for the sensor, making it highly sensitive and allowing us to observe different humidity conditions inside the packaging. If I place it in a very dry environment, it responds. Conversely, if I place it in a very humid environment, it also responds," explains Bianca Maniglia, a professor at IQSC-USP and the research adviser.
The researchers expect that the industry and consumers will eventually be able to use the technology to determine the crispness and moisture content of various products.
Inspiration from childhood memories
The idea to use cobalt chloride to create the sensor came from a childhood memory. While developing the project, Maniglia recalled the "weather rooster," a kitchen ornament inspired by the "Rooster of Barcelos," one of Portugal's national symbols. Common in Brazilian and Portuguese homes in the 1980s, the ornament changed color to indicate whether it would rain or be sunny.
After conducting several tests, the researchers decided to coat the sensor with the same material used to coat the roosters. They adapted it for another technology the research group is developing: cryogel, a highly porous material made by freezing polymers (hence the name "cryo").
"The starch gel is frozen and subjected to freeze-drying [dehydration]. During that process, the frozen water is removed by sublimation, passing directly from the solid state to vapor. The removal of the aqueous phase preserves the porous structure formed during freezing, resulting in a material with a high capacity for interacting with the environment," Maniglia explains.
Further studies are needed. "For commercial use, conventional packaging would need to be redesigned because, for now, the presence of cobalt chloride makes the sensor toxic, and therefore it can't come into direct contact with food," Maniglia notes.
The research group is evaluating other techniques and materials, including bioactive compounds derived from fruits such as jabuticaba peel. "These natural pigments are sensitive to pH variations, exhibiting color changes that can be used as visual indicators, in addition to possessing antioxidant activity. In this case, the goal is to develop a system that can monitor the spoilage process of meat, for example. Incorporating these natural compounds is also important because it helps add value to them," Maniglia says.