Inhibitory Effects of Mentha piperita L. Essential Oil against Escherichia coli O157:H7 and Salmonella Enteritidis PT4 in Fruit Juices

Introduction: Outbreaks of foodborne disease have been associated with raw juices. Regulations in the United States require the application of treatments that result in a five-log reduction of the pertinent pathogens in juices. Mentha piperita L. essential oil (MPEO) has been cited as a safe antimicrobial for drinks; however, its inhibitory effects against pathogens in juices remains unknown.

Purpose: This study was conducted to evaluate the inhibitory effects of MPEO against Escherichia coli O157:H7 and Salmonella Enteritidis PT4 in juices of caja, guava, and mango.

Methods: Samples of juice (3600 μL) containing MPEO at 5 μL/mL (minimal inhibitory concentration) or 2.5 μL/mL were inoculated with 400 μL of bacterial suspension (10⁷ CFU/mL) and incubated at 8-10°C. The number of viable cells (log CFU/mL) was counted at 0, 1, 2, 4, 8, 12 and 24 h by serial dilution and plating on Eosine-Methylene-Blue agar (E. coli O157:H7) or Xylose-Lysine-Deoxycholate agar (S. Enteritidis PT4). Juices without MPEO were assayed similarly (controls). Statistical analysis was performed in Sigma Stat 4.0 (p<0.05).

Results: MPEO at 5 μL/mL caused a 5-log reduction of both strains after 4 h in caja juice and after 12 h in guava juice. In mango juice, at this concentration, MPEO reduced 5 log units of E. coli O157:H7 and S. Enteritidis PT4 after 12 h and 24 h, respectively. MPEO at 2.5 μL/mL caused 5-log reduction of E. coli O157:H7 and S. Enteritidis PT4 after 8 h and 12 h, respectively in caja juice. In guava and mango juices, MPEO at 2.5 μL/mL reduced approximately 4.5 log units of both strains after 24 h. No reductions in viable counts of test strains were observed in controls over 24 h.

Significance: MPEO may inhibit pertinent pathogens in raw juices; however, the concentration used for inactivation should be carefully evaluated to guarantee the microbiological safety in these products.