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

In recent years, pre-packed ready-to-eat (RTE) foods on the Belgian market have shifted to a more plant-based composition due to 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 L. monocytogenes. This can give rise to health risks for vulnerable consumers, which is substantiated by multiple recent listeriosis outbreaks reported for plant-based RTE foods in the EU and USA. In this regard, the prevalence and growth potential of L. monocytogenes were evaluated in this study for three pre-packed, plant-based RTE food types on the Belgian market, i.e. sliced vegetarian and vegan deli sandwich fillings, fresh-cut (mixes of) leafy vegetables, and multi-ingredient salad bowls. The prevalence of L. monocytogenes was determined through a retail survey on ca. 50 different RTE foods of each category, purchased in supermarkets and small retail shops in Flanders, Belgium. Besides, challenge tests were performed based on the protocol described by the EU Reference Laboratory for L. monocytogenes (2021) to determine the growth potential of this pathogen in ca. three different pre-packed, plant-based RTE foods of each category (three different batches per product). During the retail survey, L. monocytogenes was not detected in fresh-cut (mixes of) leafy vegetables (0 out of 51 batches), while 1 out of 51 and 6 out of 48 batches were found positive for respectively sliced vegan and vegetarian deli sandwich fillings and multi-ingredient salad bowls. In six out of 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 was noted both between and within the three studied pre-packed, plant-based RTE food categories. The variation was mainly caused by differences in product composition, physicochemical product characteristics, present (competitive) microbiota, applied preservation techniques, and shelf life. The results of the present study indicate the potential health risk of the investigated plant-based RTE foods for vulnerable consumers

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

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

Bacillus weihenstephanensis can readily evolve for increased endospore heat resistance without compromising its thermotype

Proper elimination of bacterial endospores in foods and food processing environment is challenging because of their extreme resistance to various stresses. Often, sporicidal treatments prove insufficient to eradicate the contaminating endospore population as a whole, and might therefore serve as a selection pressure for enhanced endospore resistance. In the sporeforming Bacillus cereus group, Bacillus weihenstephanensis is an important food spoilage organism and potential cereulide producing pathogen, due to its psychrotolerant growth ability at 7 degrees C. Although the endospores of B. weihenstephanensis are generally less heat resistant compared to their mesophilic or thermotolerant relatives, our data now show that non-emetic B. weihenstephanensis strain LMG 18989(T) can readily and reproducibly evolve to acquire much enhanced endospore heat resistance. In fact, one of the B. weihenstephanensis mutants from directed evolution by wet heat in this study yielded endospores displaying a > 4-fold increase in D-value at 91 degrees C compared to the parental strain. Moreover, these mutant endospores retained their superior heat resistance even when sporulation was performed at 10 degrees C. Interestingly, increased endospore heat resistance did not negatively affect the vegetative growth capacities of the evolved mutants at lower (7 degrees C) and upper (37 degrees C) growth temperature boundaries, indicating that the correlation between cardinal growth temperatures and endospore heat resistance which is observed among bacterial sporeformers is not necessarily causal

Developing a methodology for the validation of industrial heat inactivation for sustainable production of food

Aim: Different heat inactivation techniques are used in the food industry to inactivate pathogens. These heat inactivation techniques need to be validated to estimate their actual heat inactivation of pathogens in the specific food matrix during the industrial process. This can be performed by using a non-pathogenic, surrogate strain that has the same heat resistance as the targeted pathogen. This non-pathogenic strain will be inserted in the industrial process and the inactivation after processing will be determined to provide information about the efficacy of the industrial process. This methodology will help to prevent recalls, leading to less food waste but can also lead to a reduction of the intensity of the process with less energy consumption. Therefore, it will help to obtain more sustainable production on an industrial scale. Method: The flour was inoculated with the different micro-organisms: three vegetative food pathogens Listeria monocytogenes, Salmonella spp., Escherichia coli O157:H7, and two surrogate strains: Enterococcus faecium strains. The flour was plated out on different media: selective, non-selective, and combination of both. The dough was prepared, standardized cookies were made and the heat resistance of the four food pathogen cocktails was compared with that of the two candidate surrogate strains during a baking process for 1, 2, 3, 4, 5, 7.5, and 10 minutes at 205°C. Results: The inoculation of the flour was optimized and resulted in an inoculum of 7.4 ± 0.3 log CFU/g in the cookie dough. The plating medium was also optimized and the overlayer method was used consisting of a layer of non-selective and selective media. E. faecium and the L. monocytogenes cocktail showed a similar heat inactivation curve. E. coli O157:H7 cocktail was less heat resistant and absent after 5 minutes of baking at 205°C. The Salmonella spp. cocktail followed a similar inactivation curve as the E. faecium. Conclusion: The selected E. faecium strains showed to be a good surrogate strain for the Salmonella spp. cocktail (most heat resistant pathogen) for this setup