Blue light may be the key to keeping chemical preservatives out of certain food products. A team of scientists from the National University of Singapore has found that blue LEDs have strong antibacterial effect on major foodborne pathogens, and are most effective in cold temperatures (between 4 and 15 °C) and mildly acidic conditions around pH 4.5. "This could meet the increasing demand for natural or minimally processed foods without relying on chemicals such as acidulants and artificial preservatives to preserve food products," said professor Yuk Hyun-Gyun. The experimental setup used for testing the antibacterial effect of blue LEDs. Courtesy of the National University of Singapore. The research suggests acidic foods, such as fresh-cut fruits and ready-to-eat fish and meat, would be good candidates for blue-LED-based preservation techniques. Earlier studies on the antibacterial effect of LED illumination mostly evaluated its efficacy by adding photosensitizers to the food samples, or by using very close distance (less than 2 cm) between the bacterial suspension and LED light source. These conditions would not be viable for application in food preservation, the National University researchers said. The group instead has focused on the effects of LED illumination under temperature and pH levels associated with storage of food products. In one recent study, the team placed three major foodborne pathogens — Listeria monocytogenes, E. coli and Salmonella typhimrium — under blue LED illumination and varied the pH conditions from acidic to alkaline. They used LEDs emitting at 461 and 521 nm. The team found that higher bacterial inactivation was achieved at acidic and alkaline pH conditions than when the samples were neutral. In particular, acidic conditions were more detrimental than alkaline conditions for L. monocytogenes. For E. coli and S. typhimurium, alkaline conditions were most detrimental, although acidic conditions were also sufficiently effective in deactivating them. In 2013 the same group found that blue LEDs had greater antibacterial effect at colder temperatures. "The next step for us is to apply this LED technology to real food samples such as fresh-cut fruits, as well as ready-to-eat or raw sea foods and meats products, to investigate whether LED illumination can effectively kill pathogenic bacteria without deterioration of food products," Yuk said. The research was carried out in collaboration with the Singapore Agri-Food & Veterinary Authority (AVA). As part of this three-year study, AVA has been studying the effects of LEDs on some key quality parameters of vegetables (e.g., vitamin C, chlorophyll and beta-carotene) to investigate if the quality of the vegetables under LED treatment can be maintained. "AVA hopes that the research will affirm the potential of LED as a food preservation technique that can help reduce food loss in Singapore's fresh produce industry," said Khoo Gek Hoon, director of the agency's Post-Harvest Technology Department. The most recent pH research was published in Food Microbiology (doi: 10.1016/j.fm.2014.10.014). For more information, visit www.nus.edu.sg.
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