Growth and inactivation of Listeria monocytogenes in sterile extracts of fruits and vegetables: Impact of the intrinsic factors pH, sugar and organic acid content

Authors: LICHTENDWALD, MARINA - ORISE FELLOW; BOLTEN, SAMANTHA - ORISE FELLOW; Luo, Yaguang - Sunny; MICALLEF, SHIRLEY - UNIVERSITY OF MARYLAND; Millner, Patricia; Nou, Xiangwu

Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/26/2022
Publication Date: 12/2/2022
Citation: Lichtendwald, M., Bolten, S., Luo, Y., Micallef, S.A., Millner, P.D., Nou, X. 2022. Growth and inactivation of Listeria monocytogenes in sterile extracts of fruits and vegetables: Impact of the intrinsic factors pH, sugar and organic acid content. International Journal of Food Microbiology. https://doi.org/10.1016/j.ijfoodmicro.2022.110043.
DOI: https://doi.org/10.1016/j.ijfoodmicro.2022.110043

Interpretive Summary: In recent years, Listeriosis outbreaks have been more associated with fresh produce than meat and dairy as in the past. In order to prevent contamination, the fresh produce industry needs to know which commodities are more at risk for growth of Listeria monocytogenes (Lm). According to the FDA Food Code most fresh produce falls under product assessment for safety due to a water activity above 0.92 and pH greater than 4.2. The Food Code does not specifically define which foods support the growth of pathogens. In this study significant growth of Lm was detected in juices with pH =5.6 (cantaloupe, carrot, celery, green pepper, parsley, and romaine lettuce) putting them at higher risk for Lm contamination, whereas juices from acidic products with pH=4 did not support Lm growth (including apple, blueberry, grape, mango, peach, pineapple, and tomato). Additionally, cauliflower juice had antibacterial activity against Lm. This data specifically defines which foods support or do not support Lm growth. This information will help the fresh produce industry in assessing risks for Lm contamination during handling of such products, and in developing commodity specific Lm control practices.

Technical Abstract: The availability of nutrients and other intrinsic characteristics determine the survival and growth of Listeria monocytogenes (Lm) on fresh produce. In this study, sterile fresh produce juice has been used to analyze the growth potential of Lm among 14 different commodities and to identify physicochemical characteristics in those juices that affect Lm growth. Significant growth of Lm was detected in juices with pH =5.6 (cantaloupe, carrot, celery, green pepper, parsley, and romaine lettuce), slight reduction of Lm at pH 4 (mango and tomato) and no Lm cells were detected in juices with pH =3.8 (apple, blueberry, grape, peach, and pineapple). Although these acidic fruit juices possessed a high sugar content, the pH of produce juice seemed to be the primary determinant for Lm growth which favored near neutral pH. The pH neutralization of acidic juices (apple, blueberry, grape, mango, pineapple, and tomato) abolished Lm growth inhibition at 37 °C except for peach juice. Furthermore, Lm populations significantly decreased in cauliflower juice extract. However, successive dilutions of apple and peach juice increased Lm survival, and 25-50 % dilution of cauliflower juice enabled significant growth of Lm. Organic acids in fruit juices and glucosinolates in cauliflower juice might have contributed to Lm growth inhibition. This data will benefit the fresh produce industry in assessing risks for Lm contamination, and in developing commodity specific Lm control practices.