Purpose: Understand the uptake of bacterial cross-contamination during the washing step of fresh-cut lettuce by evaluating the accumulation of microorganisms in the water and their transfer from the water to the fresh-cut produce and, vice versa.
Methods: Twenty-five g of fresh-cut iceberg lettuce were washed in 1 L of tap water under stirring at 260 rpm for 5 min at room temperature, and repeated twice within 2 subsequent days with reused water. Bacterial concentration was quantified in water and in the produce before and after the washing treatment. Salmonella Enteritidis was also inoculated in water. Uncut lettuce was used to evaluate internalization in tissues. The influence of temperature, pesticides (20 ppb) and NaOCl (300 ppm), time (5-30 min) and speed of stirring (150-300 rpm) were evaluated.
Results: Similar bacterial concentration (105 CFU/g) is found in both types of lettuce. However, bacterial concentration in fresh-cut lettuce notably increases in subsequent days (107 CFU/g). Total coliforms are transferred from the lettuce to the water (5∙102 CFU/mL). Salmonella remaining in wash water reaches different batches of produce (from 105 up to 101 CFU/g depending on its concentration in water). No growth of bacteria in water occurs between cycles when using water at 4 and 25ºC but Salmonella remains in water at 4°C. NaOCl completely removes bacterial concentration in water but they still remain in the produce after washing.
Significance: Development of a deeper knowledge of the washing process is required to design an adequate water treatment to provide a barrier to cross-contamination, together with the minimization in water and energy consumption of the process, reducing costs for the vegetable processing industry.