Purpose: To investigate the impact of oregano essential oil (OEO), free or encapsulated in liposomes, and High Pressure Processing (HPP) on: (i) the survival of Escherichia coli O157:H7 during refrigerated storage of ground beef patties, (ii) the heat tolerance of the pathogen and (iii) the organoleptic characteristics of the patties.
Methods: Ground beef patties were formulated with (a) no antimicrobials, (b) 0.5% v/w OEO, (c) 0.5% v/w OEO encapsulated in liposomes (bilayer capsules). All samples were inoculated (105 CFU/g) with E. coli O157:H7, vacuum packaged, received or not HP treatment at 250 and 400 MPa and stored at 5oC for 30 days. On day 0, 15 and 30, samples were cooked in a preheated oven to internal temperature of 65οC.
Results: Treatments reduced E. coli O157:H7 from minimum 1 log CFU/g (OEO only) to below the enumeration limit of 10 CFU/g (OEO and HPP) during refrigerated storage. OEO addition caused an initial reduction of 1 log CFU/g and additional decrease by 2.5 log CFU/g till the end of storage. OEO-loaded liposomes showed no remarkable reduction until the first 15 days; however inactivation by 1.5-2.0 log CFU/g was observed on day 30. HP treatment (400 MPa) combined with free OEO showed the highest synergistic effect, although affected the redness of ground meat negatively. OEO alone or in combination with HPP increased susceptibility of E. coli O157:H7 to heat resulting in additional reductions of 2.5 log CFU/g, compared to heated control (without OEO and HPP) samples.
Significance: Combination of EOs with HP treatment may be promising interventions for ground beef safety. Liposomes, as encapsulation systems, confer controlled release of the EOs while markedly improving the sensory properties (juiciness) of ground beef.