Cold Plasma and Pulsed Light Inactivates Escherichia coli O157: H7 in Romaine Lettuce and Preserves Produce Quality

Abstract

Fresh produce safety is important for consumer health. Intervention technologies that can lessen the pathogen threat and produce contamination is needed. In this research, cold plasma (CP), pulsed light (PL) and their combinations were assessed for inactivating Escherichia coli O157:H7 on Romaine lettuce. The effects of treatment on native microflora and sensory attributes of lettuce was also determined. An inoculum of multiple E. coli O157:H7 strains was employed for this study. Lettuce leaves were spot inoculated and then treated with PL (1–60 s), CP (15–60 s) or their optimized treatment combinations. A 30 s treatment with PL (fluence dose of 31.5 J/cm²), was optimum which provided 2.7 log CFU/g reduction of the pathogen, while 45 s treatment of CP was optimum, that delivered 2.1 log CFU/g log reduction. Combinations of PL and CP treatments were investigated for enhanced inactivation. For PL-CP combination, inoculated lettuce was treated with PL for 30 s followed by 45 s of CP exposure. While for CP-PL combination, treatments sequences were 45 s of CP treatment followed by 30 s PL treatment. Both combination treatments, PL-CP and CP-PL, resulted in synergistic inactivation of E. coli cells with > 5 log reductions of the pathogen. These combination treatments significantly (p < 0.05) reduced native microbiota and slowed their growth during storage. Additionally, treatment effects on lettuce quality was not adversely impacted. PL and CP are both non-aqueous, sustainable technologies. This study demonstrated that integration of PL and CP technology can enhance microbial safety and preserve quality of Romaine lettuce.