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Veterinary session

Occurence of Listeria monocytogenes in raw and microwave-cooked quick-frozen vegetables
Törőcsik Beatrix - year 5
University of Veterinary Medicine Budapest, Department of Food Hygiene
Supervisors: Dr. Dániel Pleva, Nádaskiné Dr. Katalin Szakmár


The use of frozen vegetables is becoming more and more common in households, saving time, money and energy. Proper heat treatment during preparation is of paramount importance, as even after careful factory preparation, microbes of production site origin may remain on the vegetables and facultative pathogens may multiply in the biofilm layer on the machinery, which may then contaminate the product.

In today's modern, fast-paced world, microwave ovens are a popular alternative to more time-consuming and equipment-intensive cooking. It is essentially a safe, practical appliance, but if not used with the correct cooking technique, food prepared in it can pose a food safety risk. Microwaves generate energy by stimulating the vibration of molecules, especially water, to cook food by boiling water, but this cooking is not uniform. Because of the varied structural make-up of different vegetables, hidden pathogens can more easily remain in the food.

The potentially food-borne pathogenic bacterium we are investigating, Listeria monocytogenes, may pose a risk to susceptible individuals. People with a healthy immune system, even if infected, can usually get through the infection as asymptomatic carriers, unlike breastfeeding mothers, elderly people or people with a compromised immune system. That is why, in response to many previous cases of infection, manufacturers are already highlighting the importance of proper heat treatment of vegetables on the packaging.

In order to assess the magnitude of the resulting risk, testing of raw or heat-treated vegetable samples for Listeria monocytogenes by redox potential measurement was not considered a suitable test method for our experiment. However, the results of our series of experiments with samples obtained by disruption of vegetable sample filtrates enriched with culture medium of Listeria monocytogenes culture on selective media support the efficacy of microwave heating against these pathogens.

To simulate home conditions, during the experiment, we used frozen vegetables in retail bag packs. Four types of vegetables (maize, spinach, broccoli, peas) were tested in parallel divided into two 10 g samples. No or only low levels of Listeria monocytogenes were detectable in our examination, so contamination was used to obtain more detectable results. For all four vegetable samples, we tested samples which were raw, contaminated with Listeria monocytogenes bacterial cultures, and also contaminated and additionally microwaved. The microbial counts of the 10 g artificially inoculated samples were reduced to levels undetectable in the laboratory after 25 seconds of heat treatment, demonstrating that microwave heating for sufficient time and power can kill microbes.

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