Purpose: The goal of this study was to determine the efficacy of potassium bisulfate in reducing foodborne pathogen surrogates on lettuce in the pre-harvest environment.
Methods: Pelleted lettuce (Lactuca sativa) was grown in a greenhouse until maturation and then inoculated with overnight cultures of either Escherichia coli or Listeria innocua by hand spraying onto the plant surface until the drip point. Inoculated lettuce was randomly allocated to the control (untreated lettuce), water control (plants sprayed with water until the drip point), or treatment (0.25% potassium bisulfate sprayed until the drip point) groups prior to treatment application at either two days or seven days prior to harvest. Immediately following harvest, samples were analyzed for Escherichia coli or Listeria innocua by plating onto MacConkey and Modified Oxford agar, respectively.
Results: When applied one week before harvest, potassium bisulfate significantly reduced Escherichia coli populations on inoculated lettuce by 1.3 log CFU/g (P=0.0215), 1.2 log CFU/g (P=0.0422) and 1.3 log CFU/g (P=0.0226) in comparison to the untreated control, 7-day water control and 2-day water control, respectively. No statistically significant differences were detected for all treatments applied to Listeria innocua inoculated lettuce.
Significance: This research suggests that potassium bisulfate may be a suitable pre-harvest intervention to reduce pathogenic Escherichia coli on lettuce. The modest reduction in Escherichia coli populations suggests that efforts to optimize the intervention (e.g., application with a surfactant to improve leaf surface coverage) may be warranted.