Quantitative study of cross-contamination with Escherichia coli, E. coli O157, MS2 phage and murine norovirus in a simulated fresh-cut lettuce wash process

The water management in fresh-cut produce processing is an important factor affecting the microbial quality and safety of fresh-cut produce. For this study, a commercial lettuce washing process was simulated using two subsequent washing baths (WB). The worst case scenario, when no sanitizers are used and still common in some European countries, was investigated to fully understand the potential for cross-contamination and to obtain baseline data for further risk assessment. The two cross-contamination processes (from lettuce to water and from water to lettuce) were included in the simulation study and the transfer of Escherichia coli, E. coli O157, MS2 phage and murine norovirus was quantified. The mean reduction of initially contaminated lettuce through the washing in two successive WB was limited: 0.3 ± 0.1 log CFU E. coli/g after washing in WB1 and an additional reduction of 0.2 ± 0.1 log after WB2. The microbial load of the water in the washing baths, initially starting off with potable water, increased rapidly during the washing process of the contaminated lettuce. Furthermore to quantify the transfer of the four implicated micro-organisms from contaminated water to the lettuce, the first washing bath was inoculated with either approximately 3.0, 4.0 and 5.0 log CFU E. coli/100 ml or 4.8, 5.6 or 6.7 log CFU E. coli O157/100 ml, 4.0, 5.1 or 6.5 log PFU MS2 phages/100 ml or 6.5 log PFU/100 ml norovirus surrogate MNV-1. The contamination of the subsequently washed lettuce portions resulted in levels of ca. 1.0 up to 1.9 log CFU E. coli/g after passing the two washing steps. In addition, after a rapid initial increase, due to spill over of water from WB 1 to WB2, the contamination of WB2 further augmented during the washing process to approximately 1.0 to 0.5 log below the inoculation level of WB1. Transfer of E. coli O157, MS2 phages or MNV-1 from the water to the lettuce was respectively 0.9% ± 0.3%, 0.5% ± 0.2% and 0.5 ± 0.1% after WB1 and resulted in a contamination level for the highest inoculum level of WB1 of respectively ca. 2.9 ± 0.1 log CFU/g, 3.7 ± 0.1 log PFU/g and 4.4 ± 0.1 log PFU/g lettuce. The quantitative data of lettuce contamination and transfers established in this study further highlight the vulnerability of fresh produce to cross-contamination during the washing stage and stresses that notwithstanding the use of initial potable water and partial refreshment of water but without the use of sanitizers, microbial pathogens (or indicator organisms) may easily be introduced and reside for prolonged periods in washing baths enabling cross-contamination to the final fresh-cut product.