Bactericidal mechanisms of intense pulsed light against Salmonella Enteritidis on green Sichuan pepper

Intense pulsed light (IPL) is an emerging surface antimicrobial technology characterized by prominent efficiency but the performance in the decontamination of granular foods is yet to be improved. Using S. Enteritidis as a model bacterium, this article attempted to resolve the confusion on bactericidal mechanism of IPL treatment on spice products. The bactericidal effects of IPL treatment were compared on spices with different surfaces, and the result indicated that "shadow effect" from the shorter UV wavelengths of IPL was one of the reasons for decrease in antimicrobial efficiency. Then the critical bactericidal mechanism of IPL has been investigated by the evaluation of the viable but non-culturable (VBNC) state of the bacteria, apoptosis under IPL stress, cytomembrane destruction and DNA damage. The results revealed that "photochemical effect" of UV light and "photothermal effect" of IR light could have played different roles. After IPL treatment of low dosage (fluence <3.12 J/cm²), the UV light damaged the intracellular DNA and turned the bacteria into their VBNC state, which has caused an overestimate of bactericidal effect. On the other hand, the IR light inhibited bacterial resuscitation by destruction of the cytomembrane when IPL treatment of high dosage (fluence >6.24 J/cm²) was applied on green Sichuan pepper (GSP) with uneven surface. As a consequence, "photothermal effect" and diffraction of IR light improved the decontamination effect of IPL treatment against S. enteritidis, and higher dosage irritation could be a preferred selection for the shelf-life extension of spice products.