Utilization of Emulsified Clove Bud Oil and Thyme Oil to Inactivate Salmonella on Cherry Tomatoes and Escherichia coli O157:H7 on Baby Spinach

Introduction: In the produce industry, post-harvest rinses to remove debris are frequently used. This step has also been shown to be a primary point of contamination if antimicrobials are not utilized.  While chlorine and peroxyacetic acid are approved and effective, many organic producers prefer to use alternatives. The effectiveness of essential oils, including clove bud (CBO) and thyme oils (TO), have been recognized, but their hydrophobic nature makes implementation in aqueous systems difficult.

Purpose: Evaluate the efficacy of CBO emulsified with whey protein and TO emulsified with gum arabic against Salmonella on cherry tomatoes and E. coli O157:H7 on baby spinach.

Methods: Tomatoes and spinach were inoculated with Salmonella or E. coli O157:H7, respectively, and rinsed in: 1) water, 2) 200 ppm free chlorine, 3) emulsion with 0.5% CBO, and 4) emulsion with 0.5% TO.  Tests were conducted in clean water and water with 1% organic load.  Post-treatment, surviving populations on produce were enumerated on TSA with ammonium ferric citrate and sodium thiosulfate (Salmonella) or CT-SMAC (E. coli O157:H7). Two samples were analyzed for each treatment and all experiments were replicated three times (n = 6).  Analysis of variance was conducted to determine significant differences.

Results: A 2-log reduction of Salmonella was observed with CBO emulsion and was more effective than chlorine in the clean water system (P < 0.05).  TO emulsion also lowered the population of Salmonella by nearly 1 log CFU/g and performed similarly to chlorine.  Both treatments performed better than chlorine when water carried an organic load.  E. coli O157:H7 populations significantly decreased in essential oil emulsions with and without organic load.  TO emulsion performed better than chlorine in the systems with organic load with a more than 2-log reduction (P < 0.05).  

Significance: These data demonstrate potential for organic oil emulsions to be used as alternative antimicrobials for produce wash water systems.