Inactivation of bacterial and viral pathogens in berry products by high pressure and pulsed light
University of Delaware
Foodborne pathogens such as Salmonella, Escherichia coli O157:H7 and human norovirus are major causative agents of foodborne illnesses and have been frequently associated with foodborne outbreaks of berry products in recent years. The objectives of the present study were to evaluate the application of high pressure processing (HPP) alone or in combination with frozen storage to inactivate bacterial pathogens in strawberry puree and to develop a feasible pulsed light (PL) treatment strategy to decontaminate berries from bacterial and viral pathogens. In the first project, strawberry puree with different levels of E. coli O157:H7 or Salmonella was stored at -18 °C. Both pathogens survived in frozen puree for at least 4 and 12 weeks depending on inoculation levels. A complete elimination of both pathogens could be achieved by pressure treatment at 450 MPa for 2 min at 21 °C or pressure treatment at 250-300 MPa for 2 min followed by 4-8 days of frozen storage. HPP did not adversely impact the physical properties of strawberry puree such as color, soluble solids content, pH and viscosity. The second project demonstrated a prototype water-assisted PL (WPL) treatment. This new strategy solved two main challenges in the application of PL, including sample heating and shadowing effects. After a 60-s WPL treatment, the populations of E. coli O157:H7 and Salmonella inoculated on calyx and skin of blueberries were reduced by ≥3.0 and >5.8 log CFU/g, respectively. In the third project, the combinations of WPL treatment with 1% H2 O2 or 100 ppm SDS were evaluated for the inactivation of E. coli O157:H7, Salmonella and murine norovirus (MNV), on strawberries and raspberries. The combination of WPL with 1% H2 O2 (WPL-H2 O2 ) was the most effective treatment in reducing bacterial pathogens. However, no significant difference (P>0.05) was observed between the effectiveness of WPL and WPL-H2 O2 for MNV inactivation. In the final study, a small scaled-up WPL system was developed. WPL-H2 O 2 was the most effective treatment which reduced Salmonella on raspberries and blueberries by 4.0 and > 5.6 log CFU/g, respectively, in clear water. The efficacy of the WPL-H2 O2 treatment was not significantly impacted (P>0.05) by the organic load, water turbidity, and PL intensity. No viable bacterial cells were recovered from the wash water, which showed that WPL-H2 O2 could effectively prevent the risk of cross-contamination during treatment.