Antimicrobial activity of gaseous Citrus limon var pompia leaf essential oil against Listeria monocytogenes on ricotta salata cheese

Contamination by Listeria monocytogenes is a particularly challenging problem in the food industry due to the
ability of the bacterium to develop under conditions normally used for food preservation. Here, we show that the
gaseous phase of Citrus limon var pompia leaf essential oil (hereafter PLEO) exerts specific anti-Listeria activity on
ricotta salata cheese stored at 5 °C. The synergic effect of gaseous PLEO treatment and refrigeration was first
confirmed in vitro on L. monocytogenes strains treated for 3 h with gaseous PLEO and then stored at 5 °C. Ricotta
cheese was then inoculated with L. monocytogenes strains and subjected to hurdle technology with different
concentrations of gaseous PLEO. Cell counts revealed gaseous PLEO to exert a bactericidal effect on L. monocytogenes
20600 DSMZ and a bacteriostatic effect on a mix of L. monocytogenes strains. Scanning and transmission
electron microscopy analyses of L. monocytogenes cells suggested that gaseous PLEO targets the bacterial cell wall
and plasma membrane. Chemical analyses of the liquid and vapor phases of PLEO indicated linalyl acetate to be
the predominant compound, followed by limonene and the two isomers of citral, whereas EO composition
analysis, although generally in line with previous findings, showed the presence of linalyl acetate for the first
time. Solid-phase microextraction coupled with gas chromatography confirmed the presence of all crude oil
components in the headspace of the box.