Purpose: The objective of this study was to evaluate the effectiveness of oregano essential oil on four different types of organic leafy greens (organic iceberg and romaine lettuces and mature and baby spinach) inoculated with Salmonella Newport. The difference in exposure times of treatments as well as storage temperatures was also investigated.
Methods: Each leaf sample was washed, dip inoculated with S. Newport (106 CFU/ml) and dried. Oregano oil was prepared at 0.1, 0.3 and 0.5% concentrations (v/v) in sterile phosphate buffered saline. Inoculated leaves were immersed in the treatment solution for 1 or 2 min, and individually incubated at 4 or 8 °C. Samples were taken at day 0, 1 and 3 for enumeration of survivors.
Results: Our results showed that oregano oil was effective against S. Newport at all concentrations. Romaine lettuce, iceberg lettuce, mature spinach and baby spinach samples showed between 0.7-4.8 log, 0.8-4.8 log, 0.8-4.9 log, and 0.5-4.7 log CFU/g reductions in S. Newport, respectively, when compared to the PBS control over the course of the experiment. The antibacterial activity also increased with exposure time. Leaf samples treated for 2 min generally showed greater reductions (by 1.4-3.2 log CFU/g) than those samples treated for 1 min; however, there was minimal difference in the activity among the samples stored under refrigeration and abuse temperatures.
Significance: This study demonstrates the potential of oregano oil to inactivate S. Newport on organic leafy greens.