Blueberries inoculated with E.coli O157:H7 or Salmonella were treated with PL directly (dry PL treatment) or immersed in agitated water during the PL treatment (wet PL treatment) for 5–60 seconds.
Both pathogens were effectively inactivated by dry PL treatments but the appearance of the blueberries was adversely affected and a maximum temperature of 64.8 °C on the surface was recorded.
Pulsed light challenges
Challenges when using pulsed light include the heating of samples which could damage produce quality and the shallow penetration depth of the technology means microorganisms on an opaque food surface must directly face the PL-strobe to be inactivated.
“After a 60-s wet PL treatment, the populations of E. coli O157:H7 inoculated on calyx and skin of blueberries were reduced by 3 and >5.8 log CFU/g, respectively. Salmonella on blueberry calyx and skin was reduced by 3.6 and >5.9 log CFU/g, respectively,” wrote the authors in Food Microbiology.
Pulsed light is a nonthermal technology that utilizes short, intense pulses of broad spectrum light (wavelength = 180–1100nm) to inactivate microorganisms and was adopted by the US FDA in 1996.
The mechanism of microbial inactivation is the photochemical dimerization of nucleic acids in microorganisms, caused by UV-C part of the PL spectrum, according to Gomez-Lopez et al (2007).
Both treatments resulted in higher log reductions of the pathogens inoculated on the blueberry skin rather than on the calyx.
Wet PL treatment
The visual appearance remained unchanged after wet PL treatments and sample heating was significantly reduced.
The wet PL treatments were more effective than chlorine washing on inactivating both pathogens, said the researchers.
Sample heating and shadowing effect have limited the application of pulsed light technology for decontamination of fresh produce.
However, they used water-assisted PL processing to tackle the problems.
“To overcome these two limitations, we developed a water-assisted PL system in which blueberry samples were immersed in agitated water during PL treatment,” said the researchers.
“With this new system, the temperature increases of water and the samples were minimized due to the large specific heat of water. Moreover, the blueberry samples could randomly move and rotate in the agitated water, thus allowing more uniform PL exposure of all the blueberry surfaces.”
For dry PL treatments, three blueberries were placed on a sterile petri-dish with the inoculation site facing the PL lamp. The samples was placed at the center of the PL chamber and directly illuminated by PL for 5, 15, 30 and 60 seconds.
The distance between the lamp and the quartz window was 5.8 cm and between the top of blueberries and the quartz window it was ∼15 cm
For wet PL treatments, three blueberries were immersed in 150-mL agitated tap water during the PL treatment in a 1-L glass beaker containing a 2.5-cm stirring bar.
An ultra-thin magnetic stirrer was placed under the PL chamber to agitate the water in the beaker so that random rotation and movement of blueberries could be achieved. The treatment durations were 5, 15, 30 and 60 seconds.
The distance between the top of blueberries and the quartz window was also ∼15 cm and the distance from the top of blueberries to the water surface was about 1cm.
Source: Food Microbiology Volume 40, June 2014, Pages 1–8
Online ahead of print, DOI: 10.1016/j.fm.2013.11.017
“A novel water-assisted pulsed light processing for decontamination of blueberries”
Authors: Yaoxin Huang, Haiqiang Chen