Increasing consumer concerns about health, sustainability, and ethics have led to a surge in the demand and supply of plant-based food products on the European market in recent years (Bryant and Sanctorum, 2021). Besides, a protein shift is advocated by the European Green Deal (EC, 2022). This transition aims to alter the ratio between animal- and plant-based protein sources in the human diet (de Boer and Aiking, 2018). In Flanders, Belgium, the goal is to shift the animal/plant protein consumption ratio from 60/40, which is the current ratio, to 40/60 by 2030 (EP, 2021). In addition, an increasing share of consumers prefer foodstuffs produced by companies that pay attention to reducing greenhouse gas emissions and food waste, improving animal welfare, protecting biodiversity, applying sustainable packaging materials, and decreasing the amount of saturated fat, added sugars, and food additives in their products (Akaichi et al., 2020; Aschemann-Witzel et al., 2020; Asioli et al., 2017; Borrello et al., 2017; Horton, 2017; Stampa and Zander, 2022). These preferences may also explain the growing popularity of plant-based foods, defined in the present paper as foods for which the main ingredient, which constitutes at least 50 % (by mass) of the product, is not of animal origin, but belongs to one of the following categories: vegetables, fruits, (whole) grains, legumes, nuts, or seeds. As more and more consumers also lead a busy lifestyle and therefore increasingly choose for foodstuffs with short preparation times, these plant-based food products are often ready-to-eat (RTE), pre-packed, refrigerated foods with an extended shelf life, typically more than five days (Grasso et al., 2022). However, such food products can be susceptible to the presence and outgrowth of L. monocytogenes, as is already known for some animal-based RTE foods, such as cold-smoked fish, deli meats, and soft cheeses (EFSA, 2014a). For this reason, specific food safety criteria for L. monocytogenes in RTE foods were included in EU Regulation (EC) 2073/2005 (EU, 2005). For RTE foods able to support the growth of this pathogen, this regulation specifies that L. monocytogenes should not be detected in five samples of 25 g food before the product has left the immediate control of the manufacturer, and that the L. monocytogenes concentration should not exceed 100 CFU/g throughout the shelf life when these RTE foods are placed on the market. These criteria were set based on evidence-based decision making regarding the fact that RTE foods are considered as safe (enough) for consumption when the limit of 100 CFU L. monocytogenes/g is not exceeded throughout the entire shelf life, at least for non-pregnant adults younger than 65 years old without underlying medical conditions (EU, 1999; Farber et al., 2021; Voysey and Betts, 2017).

In 2017, quantitative modelling of L. monocytogenes baseline data (2008–2015) with regard to commonly consumed animal-based RTE foods in the EU suggested that >90 % of all invasive listeriosis cases are caused by the ingestion of RTE foods containing >2000 CFU L. monocytogenes/g, and that one-third of these cases are due to outgrowth in the consumer phase (EFSA, 2018). This supported the stipulation in EU Regulation (EC) 2073/2005 that for RTE foods able to support the growth of L. monocytogenes, the “not detected in 25 g food” criterion which normally applies before the product has left the immediate control of the manufacturer is not applicable if the manufacturer is able to demonstrate, to the satisfaction of the competent authority, that the food safety limit of 100 CFU/g will not be exceeded throughout the shelf life. However, implementing this stipulation can be difficult for manufacturers, as it does not specify which food safety limit or tolerance value for L. monocytogenes should then be applied during verification analysis immediately after production. In Belgium, this ambiguity was resolved by implementing intermediate tolerance values that are based on the worst case growth potential of L. monocytogenes, determined through challenge testing, and that can be used for each type of RTE food during verification analysis immediately after production if the manufacturer can demonstrate the applicability of this stipulation (FASFC, 2023). Still, in most other EU countries, these tolerance values do not apply. Besides, only few studies on L. monocytogenes growth in pre-packed, plant-based RTE foods have already been reported.

The current data gap regarding the presence and outgrowth of L. monocytogenes in plant-based RTE foods makes it difficult for food manufacturers and food safety authorities to assess the listeriosis risk when these innovative RTE products, which are often pre-packed, refrigerated, and have a variable but prolonged shelf life (>5 days), are set to the market. In this regard, the present study puts the focus on three pre-packed, plant-based RTE food product categories: vegetarian and vegan deli sandwich slices (category 1); fresh-cut (mixes of) leafy vegetables (category 2); and multi-ingredient salad bowls (category 3). The main objective of the present study is to assess L. monocytogenes food safety concerns related to these pre-packed, plant-based RTE food product categories based on quantitative and qualitative evidence. This was achieved by (i) gathering and evaluating data from recent (2017–2022) notifications, recalls, and outbreaks linking L. monocytogenes with these RTE foods; (ii) determining the prevalence of L. monocytogenes by performing a literature review, accessing monitoring results from the Belgian Federal Agency for the Safety of the Food Chain (FASFC), and performing an exploratory retail survey (sampling and analysis of ca. 50 samples for each product category); (iii) determining the growth potential (δ) of L. monocytogenes by executing challenge tests based on the protocol described in the technical guidance document ‘On challenge tests and durability studies for assessing the shelf life of RTE foods related to L. monocytogenes’ of the EU Reference Laboratory for L. monocytogenes (EURL Lm, 2021); and (iv) relating the challenge test outcomes to the food safety criteria for L. monocytogenes in EU Regulation (EC) 2073/2005 (EU, 2005).

Overall, the present study aims to provide more insight in the likelihood of encountering elevated L. monocytogenes levels (i.e. >100 CFU/g) in pre-packed, plant-based, refrigerated RTE foods with an extended shelf life (i.e. more than five days), and to support food manufacturers and competent food safety authorities in addressing this food safety issue effectively.