Impact of physiological transitions during forward processing on Shiga-toxin producing Escherichia coli risks in lettuce.

Cold stress during forward processing delays of lettuce can induce the formation of viable but nonculturable (VBNC) cells of Shiga toxin-producing Escherichia coli (STEC) O157:H7 and pose risks of foodborne disease outbreaks. This study investigated the effect of physiological changes during the forward processing cold chain on the risks of illness from consuming lettuce contaminated with STEC O157:H7. A probabilistic quantitative microbial risk assessment model was developed to quantify the risks associated with consuming field-bagged Romaine hearts and shredded and packaged lettuce contaminated with STEC O157:H7. The exposure assessment included the farm-to-consumer pathway, with probability distributions generated using 10⁵ Monte Carlo simulations. The risk of illness was calculated using a previously published beta-Poisson model. Scenario analysis was conducted to account for transition to VBNC over 5 days of cold storage. The median risk of consuming field-bagged Romaine hearts and shredded and packaged lettuce was 1.88×10^-8 (95 % CI = 1.59×10^-11; 4.97×10^-4) and 9.12×10^-7 (95 % CI = 2.41×10^-8, 3.90×10^-5), respectively. Convolution tests showed the distribution of risks of consuming Romaine hearts and shredded and packaged lettuce were not significantly (p > 0.05) different. Physiological changes due to cold stress during forward processing did not significantly increase the risk of illness for either field-bagged Romaine hearts or shredded and packaged lettuce (p > 0.05). While post-processing factors were the most important uncertainty factors influencing the risks from shredded and packaged lettuce, both pre- and postharvest factors most influenced the risks from field-bagged Romaine hearts. We concluded that cold stress along the lettuce distribution chain, despite inducing physiological changes in the cells, did not significantly increase the risks of illness.