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The ability of four different bacteria to synthesize new ATP upon exposure to different doses of pulsed-light (PL) irradiation was investigated. The bacterial cells were PL treated on a gel surface, resuspended in phosphate buffered saline (PBS) and subsequently incubated in Tryptic Soy Broth (TSB) at 37 °C. Cellular ATP levels were monitored during a 2 h incubation period and compared to the respective colony count data. Although PL affected ATP production in a dose dependent manner, the results showed that bacteria, which had rendered unculturable after PL exposure, are still capable of generating significant quantities of ATP. Escherichia coli and Listeria innocua proved to be more resistant to PL than Salmonella enterica and Staphylococcus aureus , which was supported by the colony count data and the ATP synthesis capacity. These findings underline that bacteria undetectable by culture-based methods may still show cellular activity and synthesize new ATP. [ABSTRACT FROM AUTHOR]

The objective of this study was the determination of the efficiency of pulsed light (PL) treatments for the decontamination of endive salad and mung bean sprouts, as well as the assessment of quality changes in relation to discoloration and alteration of respiration activity. Produce samples were artificially inoculated with two bacterial test strains Escherichia coli (DSM 498) and Listeria innocua (DSM 20649) and exposed to PL at different energy doses. The inactivation efficiency with regard to the naturally occurring microbiota was also investigated. Besides microbiological investigations, color changes were determined as well as the produce respiration during chilled storage. The results indicated that inactivation of more than 2 log was possible with one flash in the case of fresh-cut salad, while the reduction on mung bean sprouts was limited to approximately 1.6 log with one flash, irrespective if the natural flora or inoculated E. coli or L. innocua were considered. The UV part of the PL proved to be exclusively responsible for the inactivation of microorganisms. Significant lower levels of microbial counts of treated compared with untreated samples were maintained for up to 6 days. In the case of endive salad, a dose-dependent progressive discoloration and increase in respiration was diminished by applying optical bandpass filters, which only slightly affected the inactivation efficiency. In contrast, PL treatments showed a positive effect on color and general appearance of mung bean sprouts, while the respiration was almost unaffected. However, care must be taken with regard to efficiency-limiting matrix effects and impact on food quality. These aspects have to be assessed for each treated product. The integration of PL in industrial food processing plants could be an alternative way to improve the microbial quality of fresh produce, and therefore have a positive impact on public health by reducing the risk of contaminations with pathogenic bacteria. [ABSTRACT FROM AUTHOR]

The impact of the fluence regulation on the inactivation efficiency of pulsed light (PL) surface disinfection treatments was investigated. E. coli and L. innocua were exposed to PL on a gel surface under variation of the applied voltage, the number of light flashes as well as the distance between the flash lamp and the sample surface. The results revealed deviations from the reciprocity law when the total fluence striking the sample surface was not applied at once, but subdivided into several successive light flashes. No differences were found when the fluence was delivered with only single light flashes, irrespective of the applied voltage. The pulse frequency did not have an impact on the microbial reduction within 1–5 Hz. Furthermore, the sensitivities of various bacterial strains, endospores and conidiospores were compared. Differences occurred for vegetative bacteria without a clear pattern, while bacterial endospores were more resistant. Dark pigmented mold spores were slightly more resistant than bacterial endospores. All dose-response curves exhibited a downward concavity, except for P. aeruginosa . Industrial relevance This study shows that the inactivation of bacteria on e.g. food surfaces by pulsed light systems depends on the way of fluence dosage. While it is irrelevant whether the fluence is regulated by the discharge voltage or the distance between the flash lamp and the treated surface, it is more effective to apply only single light flashes of high fluence instead of several consecutive light flashes. There is furthermore no distinct trend regarding the sensitivity of bacteria to PL, variations occur on species and strain level. Bacterial spores are in general more resistant while pigmented conidiospores show a slightly higher resistance than bacterial endospores. [ABSTRACT FROM AUTHOR]

This study evaluated the Pulsed Light technology (PL) for its efficiency to reduce the microbial loads on mung bean sprouts and endive salad during a simulated wash process for up to 60 s. The microbicidal action of PL proved to be more efficient than the sanitizers electrolyzed water (40 ppm free chlorine) and chlorine dioxide (15 ppm) within the same treatment time. Reductions of the microbial count up to 2.5 log were found in case of PL treated endive salad while both sanitizers only caused maximum reductions by about 1.5 log. A combination of PL and sanitizers did not have pronounced additional effect for both endive salad and mung bean sprouts. Overall, the decontamination treatments were more effective in case of endive salad compared to mung bean sprouts. It was also shown that the microbial loads in the washing water may be kept on a low level during PL treatments as count reductions of approx. 3 log were found in relation to simply washing in tap water. This study reveals that PL may be a suitable approach to reduce microbial hazards of industrially manufactured fresh produce. The application during the washing process not only reduces microbial loads on the product surface but also efficiently inactivates suspended microorganisms in the wash water and may therefore be an alternative to sanitizers for the prevention of cross contaminations. [ABSTRACT FROM AUTHOR]

Contamination of cold cuts with Listeria monocytogenes pose a risk to human health and frequently cause recalls of affected products. In order to avoid chemical additives for the preservation of ready-to-eat meat products, plant based antimicrobials are currently in demand. Beta acids from the hop plant are known for their strong antimicrobial effects especially against gram-positive bacteria like L. monocytogenes, making them a promising candidate as a functional ingredient. In this study, the antimicrobial activity of a beta acid rich hop extract in combination with sodium nitrite was characterized in vitro, at which the different hurdles of packaged cured boiled sausages were simulated. A complete growth inhibition of L. monocytogenes was found at a minimum inhibitory concentration of 0.2 μg/ml (MIC) when the hop extract was applied in combination with a reduced water activity (a w 0.970), a slightly acidic environment (pH 6) and anaerobic conditions. Under optimal growth conditions (a w 0.995, pH 7, aerobic incubation), the MIC was 16-fold higher (3.2 μg/ml). Sodium nitrite showed a comparably low antimicrobial activity against L. monocytogenes in vitro (MIC > 100 μg/ml). The presence of 1% sunflower oil negatively affected the antimicrobial activity of the hop extract, leading at least to a 32-fold increase of the MIC. The hop extract was subsequently added to a standard recipe of pork bologna at two concentrations. The pork bologna was produced by use of two different cutter processes and then sliced, inoculated on the surface with a two strain mixture of L. monocytogenes and stored at 7 °C under modified gas atmosphere for 28 days. At 0.4 g/kg of hop extract, L. monocytogenes populations were reduced up to 3 orders of magnitude in relation to bologna without hop extract while 0.8 g/kg of hop extract either prevented the growth of L. monocytogenes or resulted in at least 4 orders of magnitude lower final populations. There was no significant sensory deviation of hop bologna according to performed triangle tests (α = 0.05) and the product color remained unaffected. Based on the presented results, the use of beta acid rich hop extracts can be proposed as an additional hurdle to inhibit the propagation of L. monocytogenes on packaged ready-to-eat cold cuts like bologna. • Hurdle concept increases antimicrobial effect of hop extract. • Hop extract in bologna prevents growth of Listeria monocytogenes. • Hop extract could be used as functional ingredient in bologna. [ABSTRACT FROM AUTHOR]