Background:

Microbial contamination of food can lead to faster spoilage, but also to infection of the consumer. This is why disinfection processes that are efficient but have as little detrimental effect as possible on the nutritional content are required. In terms of disinfection, two spectral ranges have become increasingly important in recent years. The first is the so-called Far-UVC spectral range, with a wavelength below 230 nm, which only damages bacteria but barely any eukaryotic cells due to favorable DNA and protein absorption properties. The second range is visible violet light around 405 nm, which can also inactivate microorganisms such as bacteria and fungi in high irradiation doses but does not pose a great danger, at least to humans and human cells. In this study, leaf spinach was used to investigate the extent to which these radiations have an inactivating effect on E. coli as a typical microbial contaminant, but also to determine if the vitamin C content of the spinate leaves was reduced following these treatments.

Methods:

Spinach (Spinacia oleracea) leaves were contaminated with E. coli x pGLO, which is easy to detect by means of GFP fluorescence, and irradiated with both a 222 nm krypton chloride lamp and 405 nm LEDs. The achieved bacterial reduction was determined at different time points by plating the material on agar plates and through subsequent counting. The vitamin C concentration was determined by means of redox titration, and the concentrations of chlorophyll a and chlorophyll b were determined using spectrometry.

Results:

Both irradiations exhibited a strong antimicrobial impact on E. colis x pGLO, and a reduction of approximately 99% was achieved under the selected irradiation conditions. The average doses were 19 mJ/cm² (222 nm) and 87 J/cm² (405 nm). The vitamin C concentration decreased by 30% (222 nm) or 20% (405 nm), and the chlorophyll concentrations decreased by 22 and 33%.

Conclusion:

Both irradiation approaches are able to substantially reduce the number of microorganisms on spinach leaves, but this is associated with a reduction in nutrient content.