Prevalence of Listeria species and Listeria monocytogenes on raw produce arriving at frozen food manufacturing facilities

Authors: MAGDOVITZ, BRITTANY - University Of Georgia; GUMMALLA, SANJAY - American Frozen Food Institute; GARREN, DONNA - American Frozen Food Institute; THIPPAREDDI, HARSHAVARDHAN - University Of Georgia; Berrang, Mark; HARRISON, MARK - University Of Georgia

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/2021
Publication Date: 6/22/2021
Citation: Magdovitz, B., Gummalla, S., Garren, D., Thippareddi, H., Berrang, M.E., Harrison, M.A. 2021. Prevalence of Listeria species and Listeria monocytogenes on raw produce arriving at frozen food manufacturing facilities. Journal of Food Protection. https://doi.org/10.4315/JFP-21-064.
DOI: https://doi.org/10.4315/JFP-21-064

Interpretive Summary: Listeria monocytogenes is a human bacterial pathogen that is ubiquitous in nature and can be found on fresh produce. Since this organism can survive cool and freezing temperatures, it is particularly concerning for the frozen vegetable industry. In this study, we collected samples of raw produce upon delivery to commercial vegetable freezing plants to determine the prevalence of Listeria spp and Listeria monocytogenes on these commodities prior to washing, packaging and freezing. Samples of corn, carrots, green beans, peas and spinach were cultured for Listeria species and Listeria monocytogenes. Prevalence of Listeria species in each commodity was: spinach 67%, peas 50%, corn 32%, green beans 22% and carrots 13%. Listeria monocytogenes was detected on 14% of corn, 6% of peas and 4% of green beans. These data show that raw produce can be contaminated with human pathogens upon arrival at the frozen vegetable processing plant. These data are useful to vegetable processors as they design adequate measures to sanitize vegetables prior to and during the freezing process in order to provide a safe and wholesome product to the consumer.

Technical Abstract: Ubiquity of Listeria monocytogenes in the environment impacts the food industry and presents concerns for frozen food facilities. This study determined the prevalence and numbers of Listeria species and L. monocytogenes on raw produce arriving at frozen food facilities. Raw produce was collected using multi-level blinding protocols to ensure anonymity of participants and avoid traceback. Five raw vegetables were selected: corn, carrots, green beans, peas, and spinach. Raw products were collected after arrival at the facilities but before any cleaning or other pre-processing steps that are typically performed inside the facility. The Food and Drug Administration's Bacteriological Analytical Manual (BAM) method for detection of Listeria spp. and L. monocytogenes was followed, with PCR screening followed by selective plating methods. Listeria numbers were estimated from positive samples using the most probable number (MPN) methodology. A total of 290 samples were collected, with 96 and 17 samples positive for Listeria spp. (33.1%) and L. monocytogenes (5.9%), respectively. Enumeration data for the 96 Listeria spp. samples indicated 82 samples had greater than 100 MPN Listeria spp./g and 14 samples less than 100 MPN Listeria spp./g. The prevalence of Listeria spp. varied by commodity: spinach (66.7%), peas (50%), corn (32.2%), green beans (22.2%), and carrots (13%). L. monocytogenes prevalence was determined in corn (13.6%), peas (6.3%), and green beans (4.2%) arriving at processing facilities. U.S. regulators consider L. monocytogenes an adulterant and apply a zero tolerance regulatory action limit for the presence of this pathogen in ready-to-eat foods. Prevalence and pathogen concentration data from raw commodities found in this study can provide the industry information to conduct more accurate quantitative risk assessments and provide a baseline to model and target appropriate pathogen reduction steps during processing.