Challenge test studies on Listeria monocytogenes in ready-to-eat iceberg lettuce

Shelf-life studies in ready-to-eat (RTE) modified atmosphere packaged (MAP) precut iceberg lettuce (minimally processed) were carried out in order to evaluate the natural microflora of the product and survival or multiplication of Listeria monocytogenes (L. monocytogenes), taking into consideration the impact of the production steps resulting in a reduction of the shelf life of the fresh-cut produce, due to the accelerated enzymatic activity, moisture loss, and microbial proliferation. The research first aimed to evaluate the characteristics of the natural microflora of the product, and then, L. monocytogenes dynamics were studied via challenge tests. L. monocytogenes concentration was studied at 8 and 12°C storage temperature for 10 days, 6 days longer than their shelf life. The number of L. monocytogenes in samples stored both at 8°C and 12°C increased gradually, more evidently in samples stored at 12°C. L. monocytogenes dynamics were studied to define maximum growth rate (μmax) at 8°C (0.0104 log₁₀CFU/g/h) and 12°C (0.0183 log₁₀CFU/g/h). Data obtained from the study were used to develop and validate a specific predictive model able to predict the behavior of L. monocytogenes in RTE MAP iceberg lettuce. According to the model, an increase in storage temperature of 6°C (e.g., from 8 to 14°C) would lead to an increase in L. monocytogenes concentration of more than 6 log₁₀CFU/g at the 10th day of the challenge test (12th days of shelf life). Storage at 4°C allowed to increase L. monocytogenes enumeration from 3.30 log₁₀CFU/g at D0 to 3.60 log₁₀CFU/g at D10. The model could be applied to microorganisms other than L. monocytogenes, modifying the coefficients of the polynomial equation on which it is based.