Prediction of Listeria monocytogenes Presence in a Frozen-vegetable Processing Plant Using Traditional Microbial Indicators and Listeria spp

Introduction: In food-processing environment, the prediction of Listeria monocytogenes presence through the use of microbial indicators is not always effective, because most of them do not share ecological characteristics with this pathogen. Particularly, in ready-to-eat products such as frozen vegetables, there isn’t information regarding relationship between microbial indicators (including Listeria spp.) with L. monocytogenes.

Purpose: The main goal of the present study was to determine the relationship between microbial indicators, Listeria spp. and L. monocytogenes in a frozen-vegetable processing plant.  

Methods: During one year, samples from contact-surfaces, non-contact surfaces and vegetables (broccoli, carrots, cauliflower, cob, pea, and champignon) were collected and analyzed by conventional bacteriological methods. In all the samples, the presence of Listeria spp. and L. monocytogenes was determined. Also, aerobic plate count (APC), total coliforms (TC) and lactic acid bacteria (LAB) were quantified exclusively in contact-surfaces samples. The relationship between Listeria spp. and L. monocytogenes was determined by Pearson test, and the correlation of microbial indicators with L. monocytogenes was calculated by logistic regression. 

Results: A total of 574 samples were analyzed. The incidence of Listeria spp. and L. monocytogenes in vegetables, contact-surfaces and non-contact surfaces was 10.04 and 7.53, 9.29 and 5.58, 28.78 and 27.27 %, respectively. In all samples, a positive correlation (P < 0.0001) was found between Listeria spp. and L. monocytogenes. In samples from contact-surfaces, the estimated median of APC, TC and BAL was 4.25, 2.62 and 2.92 log CFU/100 cm², respectively; only APC and TC showed a significant correlation with the pathogen presence (P < 0.05). 

Significance: The present study suggests that Listeria spp. could be an effective microbial predictor of the presence of L. monocytogenes in vegetables and surfaces in this industry. Additionally, APC and TC count worked as good indicators of the pathogen on contact-surfaces.