Purpose: To investigate the impact of nisin, lysozyme, oregano essential oil and EDTA, singly or in combinations, on (i) the survival of Escherichia coli O157:H7 during refrigerated storage of ground pork and (ii) the subsequent thermotolerance of the pathogen.
Methods: Ground pork samples were formulated with (A) no antimicrobials, (B) 0.1% or 0.2%v/w oregano essential oil (O.E.O.), (C) 50mM EDTA, (D) nisin (500 or 5000 IU/g) + 0.1% or 0.2%v/w O.E.O, (E) lysozyme (500 or 5000 IU/g) + 0.1% or 0.2%v/w O.E.O (F) nisin (5000 IU/g) + 0.2%v/w O.E.O. + 50mM EDTA, (G) lysozyme (5000 IU/g) + 0.2%v/w O.E.O. + 50mM EDTA, (H) nisin (5000 IU/g) + lysozyme (5000 IU/g) + 0.2%v/w O.E.O, and (I) nisin (5000 IU/g) + lysozyme (5000 IU/g) + 0.2%v/w O.E.O +50mM EDTA. All samples were inoculated (106 CFU/g) with a 3-strain composite of Escherichia coli O157:H7, vacuum packed and stored at 4°C for 12 days. On day 0, 6 and 12, samples (A), (H) and (I) were heat treated at 64°C in a water bath (n = 3 per sampling point), in order to calculate the D-values of the pathogen.
Results: All antimicrobial combinations (B to I) reduced (P < 0.05) the population of the pathogen by 1.1 to 2 log CFU/g during refrigerated storage for 6 days, while no further decrease was observed until day 12. The addition of O.E.O. or EDTA increased the antimicrobial effect of nisin or lysozyme during storage. Following heat treatment, the pathogen showed lower D-values (0.22-0.33 min) in samples with all antimicrobials (I) compared with (H) and control samples (0.47-0.54 min). Treatment (H) also showed shorter shoulder period (0 – 1 min) than control (1.25 – 1.5 min), suggesting that these treatments sensitized the pathogen during storage and reduced its subsequent thermotolerance.
Significance: Our findings may assist in determining the proper antimicrobial formulations in ground pork products in order to maximize their safety against E. coli O157:H7 in case of accidental undercooking.