The Influence of Ripeness and Bruising on the Survival of Escherichia coli O157:H7 and Salmonella on the Surface of Pears

Introduction: Pears are one of the few fruits that do not ripen successfully on the tree, they are harvested when they reach full maturity but before they are ripe. While often desirable, shipping ripe pears is problematic because they are prone to bruising.

Purpose: The purpose of this study was to determine the influence of ripeness, bruising and storage temperature on the survival of foodborne pathogens on the surface of Bartlett pears.

Methods: Bartlett pears at two stages of ripeness (8 kgf (hard) and 3 kgf (soft)) were bruised using a standard method. Both unblemished and bruised surfaces (3 cm²) were inoculated with five-strain cocktails of rifampicin-resistant Salmonella or Escherichia coli O157:H7 at a level of 4 log CFU/3 cm². Pears were stored at 1 and 20°C and 90% RH for up to 48 h. The inoculated area was excised to recover surviving populations, samples were diluted in 0.1% peptone and plated onto specific and non-specific media containing rifampicin or, when appropriate, enriched using standard methods.

Results: In all cases, population densities of Salmonella or E. coli O157:H7 decreased by 1 to 2 log CFU within the first 2 h and were near or below the limit of detection (1 CFU/3 cm²) by 24 h. The percent positive samples (of 12) at 48 h ranged from 0 to 100%; greater survival was associated with harder fruit. For Salmonella, the number of positive samples was identical for unblemished and bruised fruit except for soft pears stored at 20°C; at that temperature Salmonella was isolated more frequently from soft unblemished fruit. For E. coliO157:H7 greater survival was consistently noted for bruised pears (both hard and soft).

Significance: Shipping riper pears should not increase food safety risks associated with Salmonella or E. coli O157:H7.