Development and evaluation of a water-assisted decontamination system of pulsed light or ultraviolet for inactivation of Salmonella on fresh produce
Date
2018
Authors
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Publisher
University of Delaware
Abstract
Along with the increase of fresh produce production and the growing awareness of the health benefits of fresh fruits and vegetables, the number of foodborne illness outbreaks associated with fresh produce also increased. There is a need for methods to mitigate microbial contamination on fresh produce. As non-thermal processing technologies, ultraviolet (UV) and pulsed light (PL) possess potential for application on fresh produce without negatively damage to the quality and sensory characteristics. The aims of this study were: 1) to evaluate a water-assisted UV or PL system (fresh produce exposed to UV or PL while being agitated in water) for inactivating Salmonella on fresh produce in wash water and 2) to evaluate the decontamination ability of the water-assisted UV or PL system in combination with antimicrobial chemicals and develop strategies to improve the decontamination ability. ☐ In the first study, the Salmonella inactivation effects of a water-assisted UV system (WUV) and a water-assisted PL system (WPL) were compared on blueberries, grape tomatoes, and iceberg lettuce shreds. Two intensity levels of PL (~ 0.15 and 0.3 J/cm2 per pulse) and UV (~ 13 and 28 mW/cm2) and were tested for 1 and 2 min in combination with tap water washing (~ 1 ppm free chlorine). Overall, WPL and WUV treatments showed similar Salmonella inactivation on these three fresh produce items, and demonstrated advantages over tap water washing, ~ 1 – 2 log higher reductions. The intensity and treatment time did not significantly affect the Salmonella inactivation effect of WPL and WUV treatments. Even though, WPL and WUV treatments could not eliminate Salmonella in wash water, both of them helped reduce residual Salmonella in wash water by ~ 2 – 3 log comparing to tap water washing. ☐ In the second study, the effects of a WPL system on inactivating Salmonella on grape tomato were evaluated in combination with chlorine and hydrogen peroxide. WPL treatments combined with chlorine or hydrogen peroxide were more effective, or as effective, as chlorine washing for inactivating Salmonella on grape tomatoes. In the small-scale study (50 g), these two treatments could achieve > 5 and > 3 log reduction of spot- and dip-inoculated Salmonella, respectively. When I further tested the WPL treatment combined with hydrogen peroxide in larger sample sizes (300 g to 2000 g), comparable results obtained in small scale were found. Additionally, only a few residual Salmonella (< 5 CFU/mL) could be detected in the turbid wash water after WPL treatment combined with hydrogen peroxide. ☐ In the third study, strategies to mitigate Salmonella contamination on iceberg lettuce shreds were investigated using combined methods of pulsed light, ultrasound, washing, and chlorine. I found that in the small-scale study, combining WPL with chlorine was significantly more effective than chlorine wash alone. Treatments involving chlorine reduced Salmonella in wash water to < 2 CFU/mL for most replicates. Unlike grape tomatoes in the previous project, when the scale of the system and the sample size of lettuce shreds increased, the WPL Salmonella inactivation effect on lettuce was diminished. All the treatments adopted in this study could reduce ~1.5 and ~ 1.0 log reduction of Salmonella on lettuce for spot- and dip-inoculation, respectively. ☐ In the final study, a water-assisted UV system to inactivate Salmonella on five produce items in combination with chlorine and peroxyacetic acid was tested.Additionally, the effect of chemical oxygen demand, turbidity, and type of produce juice on UV inactivating Salmonella in turbid wash water were evaluated. Furthermore, a prototype of immersed WUV system (UV lamp installed under water) for fresh produce decontamination, especially leafy vegetables was developed and evaluated. Overall, the WUV treatment showed the best Salmonella inactivation on tomato, followed by carrot, lettuce, blueberry, and spinach. By incorporating chlorine, peroxyacetic acid, and hydrogen peroxide, the combination of WUV treatment and peroxyacetic acid was more effective than other treatments adopted in this study, and it was able to keep residual Salmonella < 2 CFU/mL for most replicates. As for the effect of water quality on UV inactivation of Salmonella, higher levels of chemical oxygen demand diminished the UV inactivation effect and turbidity levels < 100 NTU would not interfere with UV inactivation of Salmonella. The immersed WUV system was able to achieve similar Salmonella inactivation effects on tomato and blueberries (~ 2 – 3 logs) with a lower intensity. ☐ Overall, our studies demonstrated that WPL and WUV treatment can serve as potential microbial intervention methods for fresh produce with smooth surfaces. For produce with rough surfaces, such as lettuce and spinach, with the addition of antimicrobial chemicals, WUV and WPL treatment could help reduce microbial contamination on fresh produce and eliminate pathogens in wash water.
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Keywords
Biological sciences, Fresh produce, Pulsed light, Salmonella, Ultraviolet, Washing