Purpose: This study aimed to determine Shiga-toxigenic Escherichia coli (STEC) (serotypes O26, O45, O1O3, O111, O121, O145 and O157) capacity to survive in commercial cold-pressed juice and to assess the antimicrobial activities of 4 pure essential oils (EOs) against 7 STEC.
Methods: The antimicrobial activity of EOs and STEC survival was measured using macro and microdilution assays at various pH (4 and 7), temperatures (4 and 25°C), and storage times in-vitro and on fresh juice. Fresh cold-pressed juice was purchased in the local market. The pH of the juice samples ranged from 3.6 to 3.8. The effect of pH and temperature on STEC was tested in vitro in the following treatments: T1, pH 4 at 4°C; T2, pH 4 at 25°C; T3, pH 7 at 4°C; and T4, pH 7 at 25°C and on the juice, at 4 and 25°C, over a 5 day incubation period by quadruplicates (n=1,334).
Results: Five of 7 serotypes survived well in cold-press raw juice for at least 4 days at 4°C and pH 3.5 with no significant (P>0.05) reduction in viability. Similar results were observed in-vitro. All tested EOs showed antimicrobial activity against the 7 STEC. Thyme showed the lowest MIC (2 µl/L), while for salvia (SO) the MIC was higher (12-25 µl/L). The data showed that EOs, especially thyme and rosemary can reduce the 7 STEC serotypes to undetectable levels in an acidic mixed vegetable/fruit juice at refrigerated temperatures. It has been shown also that the top even STEC can survive in cold-pressed juices under refrigerated conditions and low pH.
Significance: The tested EOs was effective in reducing STEC numbers in-vitro conditions and in the commercial cold-pressed juice. These results are promising for future application of EOs as natural preservatives in foods.