Survival of Listeria monocytogenes and Salmonella spp. on the Epicarp of Avocados (Persea americana var. Hass) as Affected by Simulated Transport Conditions

Introduction: Mexico is the major exporter of avocado to the United States. Avocados may become contaminated with pathogens at the farm or the packing facility, before shipping to the US. Avocados are transported under refrigeration to delay ripening and spoilage. If transportation conditions deviate, pathogens attached to the surface of avocados may survive or multiply.

Purpose: To determine the survival of Listeria monocytogenes and Salmonella on the surface of avocados as affected by simulated transport conditions.

Methods: Unripe non-washed avocados (n = 432) were surface inoculated (steam scar and epicarp) with a bacterial cocktail containing six rifampicin-resistant (Rf+) strains of L. monocytogenes or six Rf+ Salmonella strains (≈9 log CFU/ml). The avocados were maintained at 25°C for 1h, randomly divided into groups and stored in a chamber at 5, 12 or 25°C. At times 0, 3 and 6h, and every 24h for 10 days, two avocados from each group were sampled. Ten cm² surface samples from each inoculated area were excised and combined to enumerate loosely and strongly attached cells. Each experiment was conducted in triplicate.

Results: After 10 days at refrigeration (5.2 ± 0.5ºC; 83.7 ± 5.4% RH), L. monocytogenes counts were reduced from 4.1 ± 1.1 and 3.3 ± 0.9 to 1.0 ± 0.7 and 0.3 ± 0.7 log CFU/cm² for loosely and strongly attached cells, respectively, while Salmonella loosely and strongly attached cells were reduced from 3.8 ± 0.5 and 3.6 ± 0.8 to 1.8 ± 1.2 and 2.0 ± 0.9 log CFU/cm², respectively (P < 0.05). Bacterial counts for both pathogens were also reduced (P < 0.05) after 10 days at 12.3 ± 0.8ºC (85.6 ± 2.9% RH). At 24.9 ± 0.4°C (89.2 ± 5.8% RH), only L. monocytogenes counts were reduced (P < 0.05).

Significance: L. monocytogenes and Salmonella survived for 10 days on the surface of avocados under refrigeration and temperature abuse conditions. If avocados become contaminated during harvesting or processing, these pathogens will survive during the time required for avocado transportation from Mexico to the US.