Listeria monocytogenes growth and survival on peaches and nectarines as influenced by stone fruit packing house operations, storage and transportation conditions.

The recent multi-state Listeria monocytogenes outbreak associated with stone fruit consumption highlights the potential for stone fruits to serve as a vehicle in Listeria transmission. Further, the outbreak also demonstrates the pathogen’s ability to persist and survive on stone fruits during handling, storage and transportation. While investigations on the persistence of Listeria have been performed on other produce, there is a general lack of knowledge on the behavior of pathogens associated with stone fruits. Additionally, each produce type has unique compositional and physical characteristics that require produce-specific management practices. Therefore, to develop stone fruit-specific risk reduction knowledge and preventive controls, this study will investigate the survival and growth of Listeria on peaches and nectarines under packing house environment, storage and transportation conditions. The study will be performed under conditions simulating stone fruit unloading and staging, waxing and fungicide application, storage and transportation from the packing facility. It is expected that results from this study will provide quantifiable data on the effect of current practices on Listeria survival on stone fruits. Furthermore, identification of food safety risks associated with different steps within the packing house continuum will help develop comprehensive preventive controls for foodborne pathogens including Listeria monocytogenes.

The recent recall of stone fruits due to potential Listeria monocytogenes contamination and the stone fruit associated multi-state outbreak establishes a novel link between human listeriosis and stone fruit consumption. The outbreak further highlights the pathogen’s ability to survive and persist on stone fruits through the handling, storage and transportation chain. This is of particular concern with fresh fruits like peaches and nectarines since there is no kill step that will effectively eliminate the pathogen prior to ingestion. Therefore, there is a critical need for quality control in stone fruits intended for human consumption. In order to develop effective food safety practices, it is essential to understand the influence of fruit processing, storage and transportation conditions on pathogen survival and growth. Although several studies have demonstrated the effect of processing and storage conditions on Listeria monocytogenes survival on produce, there is a general lack of knowledge on the behavior of pathogens associated with stone fruits. Moreover, each produce type has unique compositional and physical characteristics that require specific growth conditions, harvesting protocols, processing practices and storage conditions. Hence food safety practices and preventive controls must be highly produce-specific. Therefore, to develop risk reduction practices to eliminate potential foodborne hazards from stone fruits, this study will investigate the influence of stone fruit packing house, storage and transportation conditions on Listeria survival and growth on peaches and nectarines. In order to simulate commercial practices, Listeria survival will be evaluated under stone fruit unloading and staging, waxing and fungicide application and cooling, storage and transportation conditions. It is expected that results from this study will provide quantifiable data on the ability of Listeria to persist (survival/growth) on stone fruits under current processing conditions. Identification of food safety risks associated with a process or multiple processes within the packing house continuum will help develop comprehensive preventive controls for foodborne pathogens including Listeria monocytogenes.

To investigate the survival and growth of Listeria monocytogenes at high (5 log CFU/fruit) and low inoculation (3 log CFU/fruit) levels on yellow flesh peaches (var. Autumn Flame) and nectarines (var. August Fire) as influenced by stone fruit post-harvest processing, cooling, storage and transportation conditions: 

1. Evaluate the effect of unloading and staging conditions at the stone fruit packing facility (18– 20 or 28–30°C [ambient cool and warm season temperatures], 40–50% RH [ambient], 1–18 h storage). 

2. Evaluate the effect of fruit waxing (mineral oil– and vegetable oil–based fruit finish) and fungicide application (Fludioxonil and Propiconazole) at the stone fruit packing facility (18– 20 or 28–30°C [ambient cool and warm season temperatures], 40–50% RH [ambient], 1–6 h storage). 

3. Evaluate the effect of cooling, storage, and transportation conditions (1–2°C, 85–95% RH, 1–4 weeks storage) at the packing facility.

1. Currently employed stone fruit handling and storage conditions at the packinghouse did not favor or support Listeria growth. No increase in Listeria populations was observed under any of the three stages simulated in the study. 

2. Listeria can survive on peaches and nectarines in significant numbers during stone fruit staging, waxing and fungicide application, and cold storage for extended periods of time. 

3. Initial pathogen load (population during contamination) had no effect on pathogen survival. Significant Listeria populations were recovered from fruit surfaces at both high and low inoculum levels under all three objectives. 

4. The most commonly employed fungicide and wax treatments did not exert any inhibitory effect on Listeria survival on peaches and nectarines. 

5. Current stone fruit storage and transportation conditions did not deter pathogen survival or persistence (no reduction in Listeria population was observed).