LAUSR

The inactivation of a five-strain cocktail of Escherichia coli by high-pressure processing (HPP) and Melissa officinalis leaf extracts (MoEOs) or their similar chemical constituents (CCs; citral, geraniol, β-caryophyllene or a mixture) in fresh ground beef was investigated. The pathogenic bacteria post-process growth and survival were further determined at 4 °C, 7 days storage. Ground beef pressurized at 300, 350, and 400 MPa for 15 min in combination with 0.5 and 1.0% of MoEOs or CCs was investigated. A 5-log CFU/g reduction was achieved with properly selected pressure and MoEOs, CCs, or MIX [a reconstituted mixture of 1/3 citral, 1/3 geraniol, and 1/3 β-caryophyllene (weight basis)] concentration. Without the HPP, the inactivation potential of citral, geraniol, MoEOs, or MIX was similar and negligible, while under pressure, the inactivation potential increased significantly as the pressure and concentration increased. The Escherichia coli strains tested include Shiga toxin-producing E. coli (STEC) O157, O111, O121, O128, and O145, which are involved in many food-borne pathogen outbreaks worldwide. For 24 h under refrigeration, 1.0% citral, 1.0% geraniol, 1.0% MIX, and 1.0% MoEOs with 350 and 400 MPa could reduce ca. 3–6 log CFU/g of E. coli. The inactivation potential continued to show effectiveness during the low-temperature storage test (e.g., 4 °C, 7 days). In conclusion, the combination treatment of HPP and M. officinalis extracts was found to be significantly effective in the inactivation of E. coli. Transmission electron microscope (TEM) images further demonstrated the cell structure damaged under HPP and antimicrobial compound stresses.