The blue LEDs are most effective when operating in cold temperatures (between 4°C and 15°C) and mildly acidic conditions of around pH 4.5.
Acidic foods such as fresh-cut fruits and ready-to-eat meat can be preserved under blue LEDs in combination with chilling temperatures without requiring further chemical treatments that are commonly needed for food preservation.
The findings have been published in the Food Microbiology journal.
Bacterial cells contain light sensitive compounds that adsorb light in the visible region of the electromagnetic spectrum (400-430 nm), which is mainly blue LED light. Exposure to illumination from blue LED light can start off a process within the cells that ultimately causes the cells to die.
Existing studies on the antibacterial effect of LED illumination evaluated its efficacy by adding photosensitisers to the food samples, or by using a close distance of less than 2 cm between the bacterial suspension and LED light source. These conditions would not be viable for application on food preservation.
The NUS team, led by Assistant Professor Yuk Hyun-Gyun, from the Food Science and Technology Programme at the NUS Faculty of Science, is the first so far to show that factors such as temperature and pH levels, which are typically related to food products, can affect the antibacterial effect of LEDs.
In the study, the team placed three major foodborne pathogens – Listeria monocytogenes, Escherichia coli O157:H7 and Salmonella Typhimurium – under blue LED illumination, and varied the pH conditions from acidic to alkaline. The team found that higher bacterial inactivation was achieved at acidic and alkaline pH conditions than when neutral. In particular, acidic conditions were more detrimental than alkaline conditions for L. monocytogenes. For E. coli O157:H7 and S. Typhimurium, alkaline conditions were most detrimental although acidic conditions were also sufficiently effective in deactivating them.
A previous study in 2013 by the same team had also looked at the effect of temperature on blue LED’s ability to deactivate bacterial cells and found the antibacterial effect to be most enhanced in chilling temperatures.