Persistence of Listeria innocua on fresh apples during long-term controlled atmosphere cold storage with postharvest fungal decay

Authors: HAMILTON, ALEXIS - Washington State University; RUIZ-LLACSAHUANGA, BLANCA - Washington State University; MENDOZA, MANOELLA - Washington Tree Fruit Research Commission; Mattheis, James; HANRAHAN, INES - Washington Tree Fruit Research Commission; CRITZER, FAITH - Washington State University

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/3/2021
Publication Date: 9/9/2021
Citation: Hamilton, A.M., Ruiz-Llacsahuanga, B., Mendoza, M., Mattheis, J.P., Hanrahan, I., Critzer, F.J. 2021. Persistence of Listeria innocua on fresh apples during long-term controlled atmosphere cold storage with postharvest fungal decay. Journal of Food Protection. 85(1):133–141. https://doi.org/10.4315/JFP-21-232.
DOI: https://doi.org/10.4315/JFP-21-232

Interpretive Summary: Food safety of perishable fruit and vegetables is a critical issue impacting United States producers and consumers. While fresh apples are generally low risk for contamination with human pathogens, incidents of food borne illness from consuming fresh apple products have occurred. As apples can be stored for up to a year after harvest and food borne pathogens can exist in the apple postharvest environment prior to packing, this study was conducted to evaluate survival of a mock pathogen on the surface of or in wounds of apple fruit stored under typical industry conditions. Study results overall showed poor survivability of the mock pathogen during cold storage of apple fruit, but risk is also dependent on apple variety, storage duration, whether the fruit had been wounded and the presence of decay organisms also on the fruit.

Technical Abstract: Recent apple-related recall and outbreak events have exposed a need for better food safety controls along the supply chain. Fresh apples can be harvested and stored under a modified atmosphere for up to one year after harvest before packing and distribution, making the crop susceptible to many opportunities for contamination that increase the quantity of postharvest losses. Botrytis cinerea (BC) and Penicillium expansum (PE) cause significant rot-associated losses to the apple industry. These fungi can colonize and rot apple tissue as storage time increases, which many impact the growth of saprophytic foodborne pathogens like Listeria monocytogenes. Thus, the objective of this study was to observe population changes of Listeria innocua (LI) as a surrogate for L. monocytogenes on apples inoculated with BC or PE under long-term modified atmosphere cold storage conditions to identify the effect of postharvest mold growth with that of a food safety-relevant microorganism. Gala and Cosmic Crisp apples (n = 1,080) were harvested, treated with pyrimethanil, and inoculated with LI only, or LI and one of the mold species on unwounded and wounded portions of the apple equator. Apples were treated with 1-methylcyclopropene and stored at a modified atmosphere (1% O2, 2% CO2, 1°C) for 1 week and 1, 3, 6, 9 and 11 months before enumeration. After three months, LI consistently fell below the limit of detection (1.35 log CFU/g) and samples were enriched following a modified BAM method with PCR confirmation. Listeria persistence was dependent on the length of time in storage and fungal contamination, and surface wounding may impact these trends depending on the apple variety. Listeria innocua populations were higher on Cosmic Crisp apples; however, percent of LI detected on samples was greater on Gala apples. Future studies should more closely examine the interactions on the fruit surface that occur during the seemingly critical timeframe of three-to-six months in storage.