Foodborne pathogenic decontamination and quality enhancement of fresh produce using pulsed light (PL) and sanitizer washing

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University of Delaware
Imported green onions have been associated with three large hepatitis A outbreaks in the United States. Contamination has been found on both domestic and imported green onions. Raspberries have been associated with outbreaks caused by C. cayetanensis , Calicivirus, hepatitis A and norovirus. C. cayetanensis , hepatitis A as well as norovirus contamination happened through a fecal-oral route which indicates that foodborne pathogens that share this transfer route, such as Escherichia coli O157:H7 and Salmonella spp. may also contaminate green onions or raspberries leading to outbreak. The objective of our study was to evaluate the potential of using pulsed light (PL) technology alone or in combination with other hurdles to decontaminate green onions and raspberries from Escherichia coli O157:H7 and/or Salmonella spp. The first study on green onions was conducted to evaluate decontamination ofSalmonella Typhimurium using new formula sanitizer washing, PL, as well as synergy between the sanitizer wash and PL. The results showed that for spot inoculated green onions, 0.4 mg/mL thymol individually and the five new formula sanitizers all achieved higher log reduction of Salmonella (4.5-5.3 log 10 CFU/g reduction) than 200ppm chlorine washing. The 5 second dry PL (4.6 log 10 CFU/g) or 60 second wet PL treatment (3.6 log 10 CFU/g) was better or comparable to chlorine washing. For dip inoculated green onions, none of our treatments provided >0.8 log 10 CFU/g (0.6-0.8 log 10 CFU/g) reduction of Salmonella . In the second study, green onions were cut into two segments, stems and leaves, to represent two different matrixes. Stems were more difficult to be decontaminated. Spot and dip inoculation method were compared as well. Similar to the first study, dipped inoculated green onions were found to be more difficult to be decontaminated. To further increase the degree of microbial inactivation, combined treatments were applied. PL combined with surfactant (Sodium dodecyl sulfate, SDS) was found to be more effective. The third study we investigated whether PL-surfactant combination would provide similar inactivation efficacy of Salmonella on green onions. Different surfactants (SDS and Tween 80) as well as different concentrations (10, 100 and 1000ppm) of each surfactant combined with PL were tested. Survival populations of Salmonella and quality of green onions (color and texture) were evaluated after treatments as well as during storage (15 days) at 4°C . The results showed that PL-SDS and PL-Tween 80 combinations at various surfactant concentrations provided similar additive inactivation efficacy on green onion leaves, but not on the stems. PL-1000ppm SDS combination negatively impacted color and texture of green onions during 15 day storage at 4°C. To consider both safety and quality of green onions, PL combined with lower concentration of surfactant is recommended. For fresh raspberries, we investigatedSalmonella and E. coli O157:H7 inactivation efficacy of dry PL. Raspberries were spot inoculated with pathogens and treated by PL for 5, 15 or 30s. PL 30s provided 4.5 and 3.9 log 10 CFU/g onSalmonella and E. coli O157:H7, respectively. All the PL treatments maintained lower pathogen survival population during 10 day storage at 4 °C compared with the control. We also evaluated the quality of raspberries after PL treatment and during 10 days storage. The results showed that color and texture of raspberries treated by PL 30s changed significantly in 10 days storage. PL 30s did not have a negative impact on total phenolic content (TPC) but increased total anthocyanin content (TAC) significantly. PL 30s provided the lowest total bacterial count (TBC) and total yeast and mold counts (TYMC) at day 0, but failed to maintain its advantage during storage. To consider both safety and quality of fresh raspberries, 5 or 15s PL treatment is recommended. In the end we observed the bacterial attachment and inactivation results under microscope using scanning electron microscopy (SEM). The results showed that dry PL, wet PL and PL-SDS treatments led to different cell morphology and viability of E. coli O157:H7. Different attachment behaviors have been observed on different food matrixes. Taken together, we can ascertain that PL is an intervention strategy that can potentially deliver > 3 log reduction and elimination of E. coli O157:H7 andSalmonella on green onions and raspberries. PL can be applied in both dry and wet conditions which make it readily to adapt into the green onion or raspberry industries. PL-surfactant combinations provided significantly better decontamination efficacy on green onions. Dry PL for <15s provided a decent decontamination efficacy on raspberries without compromising the quality during storage.