Challenge test studies on <i>Listeria monocytogenes</i> in ready‐to‐eat iceberg lettuce

Abstract

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 <i>Listeria monocytogenes (L. monocytogenes)</i>, 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, <i>L. monocytogenes</i> dynamics were studied via challenge tests. <i>L. monocytogenes</i> concentration was studied at 8 and 12°C storage temperature for 10 days, 6 days longer than their shelf life. The number of <i>L. monocytogenes</i> in samples stored both at 8°C and 12°C increased gradually, more evidently in samples stored at 12°C. <i>L. monocytogenes</i> 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 <i>L. monocytogenes</i> 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 <i>L. monocytogenes</i> 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 <i>L. monocytogenes</i> enumeration from 3.30 log₁₀CFU/g at D0 to 3.60 log₁₀CFU/g at D10. The model could be applied to microorganisms other than <i>L. monocytogenes</i>, modifying the coefficients of the polynomial equation on which it is based.