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After studies with powdered infant formula indicated that the enhancement of thermal inactivation of Cronobacter sakazakiiby butyl para-hydroxybenzoate (BPB) was blocked by high protein concentrations, we hypothesized that BPB would retain its synergistic activity in foods with limited protein and lipid concentrations. This hypothesis was tested by examining the ability of BPB to enhance the thermal inactivation of C. sakazakii607 at 58°C in commercial apple juice, including examining the effects of pH and possible synergistic effects with malic acid. Apple juice was adjusted to designated pH values of 3.2 to 9.0, supplemented with selected concentrations of BPB (≤125 ppm), inoculated with early-stationary-phase C. sakazakii607, and thermally treated (58°C) for 15 min with a submerged coil apparatus. The same methods were used to study the enhancement of thermal inactivation by malic acid. Samples were plated on tryptic soy agar for recovery and enumeration. Survival curves were plotted, and D-values were calculated by linear regression and compared using the Tukey honestly significant difference test. BPB significantly enhanced thermal inactivation in a concentration dependent manner, with D-values of a few seconds at the original pH (3.8). The enhancement of thermal inactivation was pH dependent over the pH range of 3.4 to 9.0. Malic acid enhanced thermal inactivation; the pH was decreased from 3.8 to 3.2. These results support the hypothesis that BPB can enhance the thermal inactivation of C. sakazakiiin low-protein and low-lipid foods.

In previous studies, parabens in model systems enhanced the thermal inactivation of foodborne pathogens, including Cronobacter sakazakii, Salmonella entericaserotype Typhimurium, Escherichia coliO157:H7, and Listeria monocytogenes. However, few studies have been conducted to evaluate this phenomenon in actual food systems. In the present study, the potential enhancement of thermal inactivation of C. sakazakiiby butyl para-hydroxybenzoate (BPB) was evaluated in powdered infant formula (PIF) and nonfat dry milk (NFDM) in dry and rehydrated forms. When PIF was rehydrated with water at designated temperatures (65 to 80°C) in baby bottles, BPB did not enhance thermal inactivation. When rehydrated NFDM and lactose solutions with BPB were inoculated and heated at 58°C, BPB enhancement of thermal inactivation of C. sakazakiiwas negatively associated with the concentration of NFDM solutions in a dose-dependent manner, whereas thermal inactivation was enhanced in the presence of lactose regardless of its concentration, suggesting an interaction between proteins and BPB. Fluorescence testing further indicated an interaction between BPB and the proteins in PIF and NFDM. In inoculated dry NFDM with and without BPB stored at 24 and 55°C for 14 days, BPB did not substantially enhance bacterial inactivation. This study suggests that BPB is not likely to enhance mild thermal bacterial inactivation treatments in foods that have appreciable amounts of protein.

Mung bean sprouts, typically consumed raw or minimally cooked, are often contaminated with pathogens. Internalized pathogens pose a high risk because conventional sanitization methods are ineffective for their inactivation. The studies were performed (i) to understand the potential of internalization of Salmonellain mung bean sprouts under conditions where the irrigation water was contaminated and (ii) to determine if pre- and postharvest intervention methods are effective in inactivating the internalized pathogen. Mung bean sprouts were grown hydroponically and exposed to green fluorescence protein–tagged SalmonellaTyphimurium through maturity. One experimental set received contaminated water daily, while other sets received contaminated water on a single day at different times. For preharvest intervention, irrigation water was exposed to UV, and for postharvest intervention–contaminated sprouts were subjected to a chlorine wash and UV light. Harvested samples were disinfected with ethanol and AgNO3to differentiate surface-associate pathogens from the internalized ones. The internalized SalmonellaTyphimurium in each set was quantified using the plate count method. Internalized SalmonellaTyphimurium was detected at levels of 2.0 to 5.1 log CFU/g under all conditions. Continuous exposure to contaminated water during the entire period generated significantly higher levels of SalmonellaTyphimurium internalization than sets receiving contaminated water for only a single day (P<0.05). Preintervention methods lowered the level of internalized Salmonellaby 1.84 log CFU/g (P<0.05), whereas postintervention methods were ineffective in eliminating internalized pathogens. Preintervention did not completely inactivate bacteria in sprouts and demonstrated that the remaining SalmonellaTyphimurium in water became more resistant to UV. Because postharvest intervention methods are ineffective, proper procedures for maintaining clean irrigation water must be followed throughout production in a hydroponic system.

Internalization of pathogens poses a tremendous health risk in the consumption of raw fresh produce, because conventional washing cannot remove pathogens effectively after internalization occurs. We investigated (i) the pattern of Salmonellainternalization in different parts of green onions when it was contaminated on their surfaces, and (ii) whether environmental factors (extreme weather) affect the extent of Salmonellainternalization. Green onions were surface contaminated with three different levels of SalmonellaTyphimurium (1, 3, and 5 log CFU per green onion). Each contamination group was irrigated with three different water volumes to mimic water stress and to determine if SalmonellaTyphimurium internalization was localized in different parts of the plant. The plants were collected 2 days after contamination, and surface bacteria were inactivated with ethanol and silver nitrate. The plants were then cut into two parts, upper and lower. The internalized SalmonellaTyphimurium in each part was visualized and confirmed with a laser scanning confocal microscope and was quantified with the plate count method and real-time quantitative PCR (qPCR). The results indicate that SalmonellaTyphimurium can be taken up through the plant surface and transported from the upper to the lower part of the plant. The level of viable internalized SalmonellaTyphimurium (plate count) was higher in the lower part than the level in the upper leafy part, especially when the leaves were contaminated with a high concentration of Salmonella(5 log CFU, P< 0.05), whereas the total internalized SalmonellaTyphimurium (by qPCR) was higher in the upper part (P< 0.05) at the same contamination level. The discrepancy between these results suggests that most internalized Salmonellalost viability in the upper part but survived in the lower part. Water stress did not significantly change the extent of internalization in either location of green onion, whether detected via plate count or qPCR when the contamination occurred on the surface.