Power Ultrasound and Organic Acid-Based Hurdle Technology to Reduce Listeria monocytogenes and Salmonella enterica on Fresh Produce

The increasing demand for fresh fruits and vegetables has been accompanied by a rise in foodborne illness outbreaks linked to fresh produce. Traditional antimicrobial washing treatments, such as chlorine and peroxyacetic acid, have limitations in efficacy and pose environmental and worker health concerns. This study evaluated the effectiveness of organic acids (citric, malic, and lactic acid) and power ultrasound, individually and in combination, for the reduction in Salmonella enterica and Listeria monocytogenes on four fresh produce types: romaine lettuce, cucumber, tomato, and strawberry. Produce samples were inoculated with bacterial cocktails at 8–9 log CFU/unit and treated with organic acids at 2 or 5% for 2 or 5 min, with or without power ultrasound (40 kHz). Results showed that pathogen reductions varied based on the produce matrix with smoother surfaces such as tomato, exhibiting greater reductions than rougher surfaces (e.g., romaine lettuce and strawberry). Lactic and malic acids were the most effective treatments, with 5% lactic acid achieving a reduction of >5 log CFU/unit for S. enterica and 4.53 ± 0.71 log CFU/unit for L. monocytogenes on tomatoes. The combination of organic acids and power ultrasound demonstrated synergistic effects, further enhancing pathogen reduction by <1.87 log CFU/unit. For example, S. enterica on cucumbers was reduced by an additional 1.87 log CFU/unit when treated with 2% malic acid and power ultrasound for 2 min compared to malic acid alone. Similarly, L. monocytogenes on strawberries was further reduced by 1.84 log CFU/unit when treated with 5% malic acid and power ultrasound for 2 min. These findings suggest that organic acids, particularly malic and lactic acids, combined with power ultrasound, may serve as an effective hurdle technology for enhancing the microbial safety of fresh produce. Future research can include validating these treatments in an industrial processing environment.