Researchers said they are most effective when 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 with chilling temperatures without further chemical treatments commonly needed for food preservation, said the scientists.
The team's findings could be applied to food chillers or cold supply chain to preserve fresh-cut fruits, ready-to-eat seafood such as sushi and smoked salmon, as well as chilled meat products.
Light sensitive compounds
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 a process within the cells that ultimately causes them to die.
The team placed Listeria monocytogenes, E.coli O157:H7 and Salmonella Typhimurium under blue LED illumination, and varied the pH conditions from acidic to alkaline.
They found that higher bacterial inactivation was achieved at acidic and alkaline pH conditions than when neutral.
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 study in 2013 by the same team 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.
Food preservation without chemical treatments
Yuk Hyun-Gyun, assistant professor, from the Food Science and Technology Programme at the NUS Faculty of Science, said the two studies point to potential for preserving acidic foods with chilling temperatures without chemical treatments.
“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,” he said.
"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.”
The team have been working with the Singapore Agri-Food & Veterinary Authority (AVA) since November 2012 to apply the technology to fresh-cut vegetables to see if it helps preserve or improve the nutritional quality of vegetables during storage.
AVA has been studying the effects of LED on quality parameters of vegetables (e.g. vitamin C, chlorophyll and beta-carotene) to investigate if they will be maintained.
IFT panel discussion
Meanwhile, light-based technologies were the subject of a panel discussion at the Institute of Food Technologists (IFT) trade show in Chicago.
The panel said while they are emerging as tools to enhance shelf life and guard against contaminants more research needs to be done.
The use of ultraviolet light, pulsed light and LED lights are being studied as a way to improve food longevity and assist in eliminating bacteria from such products as milk and juices.
However, scientists warn they need to learn more about how these light rays penetrate foods at varying degrees to ensure food safety.
“Light-based technologies can assist in breaking down bacterial cells in food products and are effective for surface sterilization,” said Dr Kathiravan Krishnamurthy, an assistant professor in the Department of Food Science and Nutrition at the Illinois Institute of Technology.
“But the main issue with light-based technology is the penetration depth. We need to make sure every part of the food product sees the light.”
Krishnamurthy added they’ve mainly been used in non-food applications.
Source: Food Microbiology volume 48, June 2015 pages 49-57
Online ahead of print DOI: doi:10.1016/j.fm.2014.10.014
“Enhancing the antibacterial effect of 461 and 521 nm light emitting diodes on selected foodborne pathogens in trypticase soy broth by acidic and alkaline pH conditions”
Authors: Vinayak Ghate, Ai Ling Leong, Amit Kumar, Woo Suk Bang, Weibiao Zhou, Hyun-Gyun Yuk