Detecting decay in meat is often challenging. Fresh-looking meat inside a sealed package can conceal harmful microorganisms. Annually, food poisoning impacts millions globally, with 200 diseases linked to unsafe food consumption.

Consumers unknowingly ingest spoiled meat containing biogenic amines (BAs). Food inspectors traditionally detect these compounds through direct sampling and extensive lab analysis. However, once meat is packaged for retail, such testing becomes time-consuming and impractical, making spoilage hard to identify.

Researchers from the China Institute of Food Science and Technology have devised a novel approach for visually detecting spoilage inside sealed food packages. They utilized a tiny carbon-based material known as carbon dots, which are mere thousandths of a human hair in width. These nanoscale dots possess a unique ability to absorb ultraviolet light and emit visible fluorescence, with color variations contingent on their chemical environment. Although most carbon dots emit blue-green light, researchers are striving to shift this fluorescence to a noticeable red hue for easier identification.

The team synthesized these carbon dots using ethanol, which dissolves citric acid and a nitrogen-rich compound, o-phenyldiamine (OPD) known for enhancing red fluorescence. By heating this mixture at 220 °C (428 °F) for six hours and subsequently purifying it via centrifuge and filtration, researchers incorporated various elements to fine-tune the fluorescence properties of the carbon dots, developing OPD variants containing fluorine, chlorine, bromine, and iodine.

For sensitivity testing, researchers added up to 50 milligrams per liter (mg/L) of BAs to each carbon dot solution. They noted distinct fluorescence color changes after mixing for five minutes, with the chlorinated variant displaying the most pronounced transformation from orange-red to yellow. This reaction is attributed to BAs interacting with chlorinated carbon dots, altering their surface properties and resulting in color changes. Consequently, chlorinated carbon dots were identified as optimal indicators for visual BA detection. The biosensor was created by soaking filter paper in a 5 mg/mL chlorinated carbon dot solution for 30 minutes, followed by a 15-minute drying process at 37 °C (99 °F).

To evaluate real-world effectiveness, the researchers placed pork, beef, and mutton in separate plastic trays, attaching the biosensor underneath the lid. They sealed the trays and stored them at 25 °C (77 °F) under ultraviolet light. As a control, a similar tray was prepared containing only a moist sponge and the biosensor, without meat. Results indicated that the biosensors in pork and lamb trays turned bright yellow after 24 hours, while beef biosensors showed a color change after 36 hours. The control biosensor exhibited no noticeable changes.

Additionally, the team developed a smartphone app for color analysis, allowing for image processing and reporting of color values. This app computes numerical ratios between red, green, and blue color components, facilitating objective assessments of color changes linked to spoilage. They further compared these values with the globally acknowledged meat spoilage index, Total volatile basic nitrogen (TVB-N), a commonly used indicator for meat freshness. The researchers found a strong linear correlation between TVB-N values and their data, confirming that biosensor color changes reliably indicated spoilage.

In conclusion, the research team successfully created an efficient process to produce color-changing carbon dots functioning as visual spoilage sensors. Integrating these into food packaging enables real-time freshness assessment of meat, simply using ultraviolet light and a smartphone. This innovative technology holds potential to enhance food safety, better supply chain management, and reduce food waste.

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