Survival of Salmonella enterica and Listeria monocytogenes on the surface of whole avocados and subsequent transfer to pulp after cutting.

Abstract

Avocados have emerged as a popular ready-to-eat fruit, but they may become contaminated with foodborne pathogens during harvesting and handling, and those bacteria may be transferred from the exocarp to the edible pulp during cutting. This study was undertaken to assess the survival of Salmonella enterica and Listeria monocytogenes (serotype 4b) on avocado exocarp and their subsequent transfer and growth in pulp after cutting. Five-strain cocktails of each pathogen were spot-inoculated on whole Hass avocado exocarp, and survival was monitored at 4 °C and 21 °C for 8 and 49 days, respectively. Transfer to pulp was evaluated by cutting the exocarp with either smooth or serrated knives and measuring viable bacterial counts in the pulp and on the exocarp during 21 days at 4 °C. Microbial enumeration of viable cells was performed on differential media. Statistical analysis was conducted using ANOVA, with a significance level of 0.05. The populations of S. enterica on avocado exocarp declined by approximately 2.5 log CFU/piece, whereas L. monocytogenes decreased by more than 5.0 log CFU/piece over 49 days at 4 °C. At 21 °C after 8 days, population reductions of more than 3.5 log CFU/piece were observed for both pathogens. Transfer rates from avocado exocarp to pulp immediately after cutting were 0.71% for S. enterica and 0.73% for L. monocytogenes with inocula of 7.5 log CFU/piece (high). The transfer rate was lower after inoculating 5.0 log CFU/piece (low), with approximately 0.57% of S. enterica and 0.41% of L. monocytogenes cells on the exocarp passing into the pulp. During 21-day storage at 4 °C, the viable counts of S. enterica declined by 1.9 log CFU/piece from pulp cut by a smooth knife with an initial exocarp inoculum of 5.0 log CFU/piece; however, the count of L. monocytogenes increased by more than 4.0 log CFU/piece from pulp cut by a serrated knife. The type of knife did not significantly impact the tr