Growth Kinetics of Listeria monocytogenes in Chopped Produce Using Predictive Modeling

Introduction: The FDA Food Code defines potentially hazardous food as requiring time/temperature control for safety (TCS) to limit growth of pathogens, such as Listeria monocytogenes, and covers the storage and handling practices for fresh produce including leafy greens, tomatoes, and cut melons. However, no recommendations have been made for other chopped produce items.

Purpose: To study the growth kinetics of three L. monocytogenes foodborne outbreak strains on chopped produce using predictive modeling.

Methods: Mushroom, broccoli, onion, green and black olives, celery, cantaloupe, and avocado were chopped and inoculated with ca. 10³ - 10⁴ CFU/g of three antibiotic resistant L. monocytogenes strains (1/2a, 1/2b, 4b) and stored at 5°C for 14 days, 10°C for 12 days, or 25°C for 6 days. At different time intervals, samples were stomached and plated onto PCA with appropriate antibiotics for enumeration. Growth kinetics were analyzed using Prism GraphPad and modeled using DMFit. A P-value of less than 0.05 was considered significant.

Results: The growth rates ((log CFU/g)/h) of L. monocytogenes in all produce items at 25°C were significantly higher than at 5°C and 10°C. Using the Baranyi/Roberts growth model, the overall highest rates were seen for serotype LS-(1/2a) at 5°C (0.023 ± 0.007 in avocado), 10°C (0.067 ± 0.004 in cantaloupe), and 25°C (0.179 ± 0.008 in avocado). The longest lag phases (h) were observed at 5°C by LS-(1/2b) in celery (308.2 ± 65.7) and LS-(1/2a) in onion (311.6 ± 2.1). The highest overall maximum populations (log CFU/g) were attained by LS-(1/2a) in cantaloupe at 5°C (7.92 ± 0.011), 10°C (9.16 ± 0.09), and 25°C (8.79 ± 0.08).

Significance: The modeling results obtained can be used to provide guidance to the retail food industry, will help educate consumers on safe handling practices for cut produce, and will aid in determining whether TCS designation should be applied to other produce items.