Project „Blue Apple“

The use of the disinfecting effect of blue light in the blue wavelength range (400-480 nm) has been intensively investigated since the availability of powerful blue LEDs in the late 1990s. Its advantages over other disinfection methods, such as the ability to kill many relevant microorganisms without the use of chemicals, with minimal exposure to human tissue and technical materials, and without the risk of resistance development, were first used in the medical field. Since then, new applications have been sought, including in the food industry. The goal of the project was to utilize blue light irradiation for the surface disinfection of stored apples to prevent storage rot, with the aim of improving storage life, particularly in organic fruit production.
A laboratory-scale irradiation station for blue light disinfection was set up after the selection and characterization of suitable blue LEDs. Furthermore, suitable bacteria were selected to determine the optimal irradiation wavelength and the necessary blue light dose for a safe LD90 reduction, and the corresponding tests were conducted.
Studies on the effects of blue light irradiation on apples have shown that continuous irradiation with a light output greater than 300 W/m² leads to excessive heating of the apple surface and thus to heat damage. Therefore, irradiation must be carried out at lower power over a longer period.
Furthermore, sensors were developed to monitor and control the irradiation process. Based on the preliminary studies, an irradiation system was constructed and installed at the fruit sorting facility of Bioobst Görnitz. Irradiation tests and ultrasound-assisted irradiation tests were conducted with this system on the ELSTAR and PINOVA apple varieties.
The blue light irradiation process in this project was designed to eliminate bacteria. This may have little impact on the storability of the apples, as is evident from the results of the assessments. What was striking about all apple batches used was that the bacterial load on the apple surface was very low. Most bacteria are found in the flesh and seeds, while the skin is less colonized. The problem of storage rot is caused by fungi. To reliably inactivate fungi, more than ten times the blue light dose would be required compared to bacteria. This means that the apples would have to be irradiated at the set power for at least three hours to avoid excessive heating. A problem with bacterial spoilage only arises when the apples are damaged or cut open for further processing, thus making fructose and nutrients available to aerobic bacteria in the environment. It would be worth considering whether the developed technology, e.g., by irradiating in a cold environment, can be adapted and used in this direction.

AIF Projekt GmbH, Central Innovation Program for SMEs (ZIM), BMWK,
Cooperation Project (KF); FKZ: KK 5069908AP1