Prevalence and growth potential of Listeria monocytogenes in innovative, pre-packed, plant-based ready-to-eat food products on the Belgian market

Abstract

In recent years, pre-packed ready-to-eat (RTE) food products on the Belgian market have shifted to a more plant-based composition due to a variety of reasons, including consumer concerns about health, animal welfare, and sustainability. However, similar to animal-based RTE foods, plant-based RTE foods can be susceptible to the presence and outgrowth of Listeria monocytogenes (L. monocytogenes). Three innovative, pre-packed, plant-based RTE food product categories on the Belgian market were identified based upon data gaps regarding the prevalence and growth potential of this pathogen. These were vegetarian and vegan deli sandwich slices (category 1), fresh-cut (mixes of) leafy vegetables (category 2), and multi-ingredient salad bowls (category 3). Reports on associated listeriosis outbreaks and recalls were collected and a comprehensive literature review on the prevalence of L. monocytogenes (i.e. detection in 25 g food) was performed. In addition, the prevalence of L. monocytogenes was also determined through an exploratory retail survey of ca. 50 different RTE products of each category. A batch was considered positive if L. monocytogenes was detected in a food item, either on the day of purchase, at the end of shelf life, or both. During the retail survey, L. monocytogenes was not detected in category 2 (0 out of 51 batches), while 1 out of 51 and 6 out of 48 batches were found positive for respectively category 1 and 3. The observed L. monocytogenes concentration did not exceed 10 CFU/g at any point in time in any batch. Furthermore, challenge tests were performed to determine the growth potential of L. monocytogenes in nine pre-packed, plant-based RTE food products (two to four different products of each category, and three different batches per product). After inoculation, products were stored for half of their shelf life at 7 °C and half of their shelf life at 9 °C (simulation of resp. retail and consumer storage). In six of the nine challenge tests executed, growth of L. monocytogenes was supported (i.e. growth potential ≥0.50 log₁₀ CFU/g during shelf life). The highest growth potential was observed for fresh-cut iceberg lettuce (3.60 log₁₀ CFU/g in 9 days), but a large variation regarding the growth potential of L. monocytogenes was noted both between and within the three studied pre-packed, plant-based RTE food product categories. This variation was mainly caused by differences in product composition, physicochemical product characteristics, present (competitive) microbiota such as lactic acid bacteria, applied preservation techniques, and shelf life.

Keywords: Challenge tests; Microbiological food safety; Salads; Vegetables; Vegetarian and vegan foods.