Modified LEDs reveal invisible rot in food

18 Jul 2023

India project could help monitor the quality of fruit and vegetables.

The problem of commercially grown fruit and vegetables going rotten before making it to the customer is a constant issue for producers and vendors.

Low cost instruments to detect the presence of rot or other damage in produce have always been in demand, with optics-based techniques and advances in suitable light sources playing their part.

Examples have included the Canadian project developing a hand-held probe with LEDs emitting at selected wavelengths, and reflected light from the fruit detected with a photodetector and analyzed in a microcontroller. Changes in the reflectivity of the fruit can then be correlated with its freshness.

Another is the EU MAX-FRESH consortium, launched in 2020 and aiming to create a device for real-time monitoring of gasses released during ripening, fermentation, damage and rotting. This project created its first industrial prototype in August 2022, with research scheduled to conclude later in 2023.

A project at the Indian Institute of Science Education and Research, Pune (IISER) has now developed LEDs intended specifically to suitable as light sources for the detection of rot in fruit and vegetables, and reported the work in Angewandte Chemie.

The team's principle involved modifying the LEDs with perovskite materials, so as to make them emit in both the near-infrared range and the visible range. Utilizing perovskite's optical bandgap in LEDs with customized emission properties has been a topic of research for some time, and the Pune project has exploited the material to create dual-emission sources.

A phosphor-converted LED (pc-LED), in which a phosphor applied to the LED chip allows it to emit both white light and broad near-IR radiation, could simultaneously provide both visual inspection and early signs of rotting of food products, according to the project's paper.

"Food contains water, which absorbs the broad near-infrared emission at around 1000 nanometers," commented IISER's Angshuman Nag. "The more water that is present due to rotting, the greater the absorption of near-infrared radiation, yielding darker contrast in an image taken under near-infrared radiation.

Scalability and long service life

To make its dual-emission phosphor-converted LEDs, the IISER team applied a double perovskite doped with bismuth and chromium to its LED chip. Part of the bismuth component emits warm white light and another part transfers energy to the chromium component, de-exciting it and causing an additional emission in the NIR range.

In trials using the modified pc-LEDs to examine apples and strawberries, the team observed dark spots that were not visible in standard camera images. Illuminating the food with both white and NIR light revealed normal coloring that could be seen by the naked eye, as well as those parts which were starting to rot but where the evidence was not yet visible.

IISER envisages a compact device using these LEDs for simultaneous visual and NIR food inspection, although the project noted that the need for two detectors to go with the dual emitter could have an impact on the cost of such an instrument. However the pc-LEDs can be produced without chemical waste or solvents, and short-term costs could be more than recovered by a long service life and scalability of this device, according to the project.

"This easy, non-invasive imaging process can estimate the water content in different parts of food, assessing its freshness," said Angshuman Nag.