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 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.
Highlights
The demand for convenient foods has increased in the last years, as consumers are interested in buying quality products ready for use
A listeriosis outbreak occurring in United States, from July 2015 to January 2016, was linked to a L. monocytogenes isolate from packaged salad collected at retail, that was closely related by whole‐genome sequencing (WGS) isolated from ill people (FDA, 2017)
The possibility of carrying out challenge tests is expressly provided by EC Regulation 2073/2005, which requires the Food Business Operator (FBO) to ensure compliance with the microbiological criterion for L. monocytogenes of 100 cfu/g in foods that support the growth of this microorganism by carrying out studies "to evaluate the development or survival of the microorganisms in question that may be present in the product during production and the shelf life, under reasonably foreseeable conditions of distribution, storage" (Regulation (EC) No 2073/2005)
Summary
The demand for convenient foods has increased in the last years, as consumers are interested in buying quality products ready for use. The possibility of carrying out challenge tests is expressly provided by EC Regulation 2073/2005, which requires the FBO to ensure compliance with the microbiological criterion for L. monocytogenes of 100 cfu/g in foods that support the growth of this microorganism by carrying out studies "to evaluate the development or survival of the microorganisms in question that may be present in the product during production and the shelf life, under reasonably foreseeable conditions of distribution, storage" (Regulation (EC) No 2073/2005). The aim of this work was to carry out a challenge test in a RTE iceberg lettuce to study the kinetics of natural microflora and L. monocytogenes, that frequently contaminates vegetables and whose survival is strongly influenced by the product's storage temperature. Based on the data obtained from challenge tests, a mathematical model was developed and applied, useful to predict L. monocytogenes growth kinetics in RTE fresh vegetables during their shelf life, assuming different temperatures of storage as reported by Iannetti et al (2017) for food of animal origin
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