Recent outbreaks of illnesses due to foodborne pathogens such as Escherichia coli O157:H7, Salmonella spp., and Listeria monocytogenes continue to draw public attention to food safety.
In this study, scientists working at the US Agriculture Department’s Food Safety Intervention Technologies Research Unit evaluated the antimicrobial activity of zinc oxide quantum dots (ZnO QDs), nanoparticles of purified powdered ZnO, against these pathogens.
They claim that this research was triggered by the fact that, currently, there are very few studies related to the application of nanoparticles in food safety.
Method
The team said that ZnO QDs were utilized as a powder; bound in a polystyrene film (ZnO-PS); and suspended in a polyvinylprolidone gel (ZnO-PVP).
Bacteria cultures, they added, were inoculated into a growth media of tryptic soy broth (TSB) or brain heart infusion broth (BHIB) or liquid egg white (LEW) and incubated at 22°C:
“Room temperature was selected for the incubation tests after considering the worst case scenario in which food was left at room temperature, at which the pathogens grow much faster, rather than refrigerated,” explained the researchers.
LEW was selected for this study, added the authors, as both the physical and functional properties of liquid egg products are sensitive to thermal treatments and because occurrences of egg related Salmonellosis have heightened the concern for the safety of egg-related products.
“The high thermal sensitivity of egg components prevents the application of more intense heat treatments. Therefore, it would be desirable to use new antimicrobial agents or other non-thermal processing techniques to ensure the safety of liquid egg products while not altering their desired qualities,” argues the team.
Results
The inhibitory efficacies of ZnO QDs against three pathogens were concentration dependent and also related to type of application, found the research team.
“ZnO powder and ZnO-PVP showed significant antimicrobial activities against all three pathogens in growth media and LEW. However, the ZnO-PVP coating had less inhibitory effect than the direct addition of ZnO-PVP, while no antimicrobial activities of ZnO-PS film were observed,” found the authors.
They said the ZnO-PVP (3.2 mg ZnO/mL) treatment resulted in 5.3 log reduction of L. monocytogenes and 6.0 log reduction of E. coli O157:H7 in growth media after 48 hours incubation, as compared to the controls.
According to the findings, Listeria cells in the LEW control increased from 3.8 to 7.2 log CFU/mL during eight days incubation, while the cells in the samples treated with 1.12 and 0.28 mg ZnO/mL were reduced to 1.4 and 3.0 log CFU/mL, respectively.
The cell populations of Salmonella in LEW in the presence of 1.12 and 0.28 mg ZnO/mL were reduced by 6.1 and 4.1 log CFU/mL respectively, in comparision to the reduction levels in the controls.
Further work
The scientists claim that the study shows that ZnO nanoparticles possess antimicrobial activities against L. monocytogenes and Salmonella in liquid egg white and growth media, as well as against E. coli O157:H7 in growth media.
They maintain, however, that additional research is required to determine the full potential of the use of ZnO nanoparticles in food safety:
“Parameters such as concentrations, times, temperatures, and combination with other bacteriocins will be the focus of further study,” said the authors.
Source: Journal of Food Science Vol. 74, Nr. 1, 2009Published online ahead of print doi: 10.1111/j.1750-3841.2008.01013.xTitle: Antimicrobial Efficacy of Zinc Oxide Quantum Dots against Listeria monocytogenes, Salmonella Enteritidis, and Escherichia coli O157:H7Authors: T. Jin; D. Sun; J.Y. Su; H. Zhang; H.J. Sue.