The present study investigated the growth inhibition effects of red (642 nm), green (521 nm), and blue (461 nm) LEDs on Gram-positive bacteria (Bacillus cereus, Listeria monocytogenes, and methicillin resistant staphylococcus aureus) and Gram-negative bacteria (Escherichia coli O157:H7, Salmonella Typhimurium, and Vibrio parahaemolyticus), which are foodborne bacteria, as well as the growth inhibition effects of co-treatment with acid stress. Growth inhibition effects and physicochemical characteristics were also investigated in apple juice irradiated with blue LED. The findings of the study were as follows:

First, all bacterial strains that were inoculated with TSB were irradiated with each LED for 10 h at 15°C. The results showed no growth inhibition effects following red LED irradiation and low inhibition effects following green LED irradiation. In contrast, all strains showed high inhibition effects from blue LED irradiation, even at low doses, indicating that blue LED is effective for bacterial growth inhibition.

Second, when irradiated with LED in for 10 h at 15°C in TSB co-treated with acid at pH 7.2, 5.0, 4.0, and 3.5, inhibition effects appeared in the order of red < green < blue, indicating that the blue LED was the most efficient inhibitor. Moreover, the growth inhibition effects on the bacteria were greater at lower pH, while co-treatment with LED irradiation had synergistic effects on growth inhibition. V. parahaemolyticus and MRSA showed the highest growth inhibition for all LED irradiation conditions, indicating that these species are the most sensitive to LED irradiation and acid stress.

Third, there were no distinct differences in growth inhibition effects between Gram-positive and Gram-negative strains irradiated with LED in TSB and pH-adjusted TSB. However, the sensitivity of the bacterial strains differed according to pH. Therefore, regardless of the Gram nature, LED irradiation can alter the sensitivity of bacterial strains based on the external conditions, such as wavelength, irradiance, irradiation time, temperature, and pH.

Fourth, in all bacterial strains, except V. parahaemolyticus, growth inhibition effects under blue LED irradiation at temperatures of 5 and 15°C after inoculation with apple juice showed higher growth inhibition than that seen in the control group treated under dark conditions (p<0.05). Moreover, L. monocytogenes and E. coli O157:H7 showed significantly high D-values at 15 and 5°C, respectively (p<0.05). Thus, growth inhibition effects differed between bacterial strains and at different temperatures.

Fifth, analysis of the changes in physicochemical characteristics of apple juice in response to blue LED irradiation at 5 and 15°C showed that the L and a-values were increased, whereas the b-value was decreased; the a-value increased as temperature and irradiation time increased (p<0.05). The sugar content increased slightly with no differences based on temperature or time, while pH and titratable acidity showed no differences compared to that seen in the control group. Additionally, the non-enzymatic browning index generally increased as temperature and irradiation time increased, confirming that non-enzymatic browning occurred, while vitamin C content generally decreased, which may have facilitated non-enzymatic browning.

The results of the present study showed that blue LED had stronger growth inhibition effects compared to red and green LED, and growth inhibition effects were increased upon co-treatment under acidic conditions. Moreover, blue LED, which showed the highest growth inhibition effects, was found to be effective when applied to processed food, such as apple juice, demonstrating its potential in food processing and storage. Consequently, by considering the wavelength and radiance of LED to select a wavelength with high bactericidal power while also considering external conditions, such as temperature and pH, nutritional content, and physicochemical characteristics of food, optimal bactericidal effects can be achieved for the production and storage of high-quality food products.

Although LEDs have advantages such as high efficiency, low-energy consumption, environmental friendliness, and safety, studies and reported cases of their application outside of the general lighting sector, such as for the inactivation of bacteria and food applications, are limited. Moreover, when LEDs are applied to food, their bactericidal effects may differ depending on various factors, and thus a database of various factors should be established for their application to specific food products. In addition to laboratory experiments, additional studies of the application of LED to large quantities of food are needed.