Survival of Salmonella on Dried Fruits and in Aqueous Dried Fruit Homogenates as Affected by Temperature

Introduction: Dried fruits have not been considered as likely vehicles of foodborne pathogens. However, documented presence of pathogens in dried fruits and recent evidence showing that Salmonella can survive on dried fruits for at least 3 months have raised interest in learning more about survival during long-term storage.

Purpose: The purpose of this study was to (1) determine the ability of Salmonella to survive on four dried fruits stored at 4 and 25°C and (2) determine if Salmonella can grow in aqueous homogenates of dried fruits.

Methods: Dried cranberries (a_w 0.47), date paste (a_w 0.69), raisins (a_w 0.46), and strawberries (a_w 0.21) were mist- or dry-inoculated with a five-serotype mixture of Salmonella.  Inoculated fruits stored at 4 and 25°C were analyzed for Salmonella at 1-week to 2-month intervals up to 8 months.  Survival and growth of Salmonella in 10 and 50% aqueous homogenates of dried fruits were monitored over a 12-week period.

Results: Salmonella survived on cranberries, date paste, raisins, and strawberries stored at 25°C for 21, 84, 21, and 42 days, respectively.  The pathogen survived at 4°C on cranberries, date paste, and raisins for at least 242 days (8 months) and on strawberries for at least 182 days (6 months).  Compared to cells grown in broth, cells grown on an agar medium survived longer in date paste.  Salmonella did not grow in 10 and 50% fruit homogenates stored at 4°C but grew in 10% date paste and raisin homogenates at 25°C.

Significance: Results suggest the need to subject dried fruits that may be contaminated with Salmonella to a lethal process before eating out-of-hand or using as ingredients in ready-to-eat foods.  The ability of Salmonella to grow in aqueous homogenates of date paste and raisins emphasizes the importance of minimizing contact of these and perhaps other dried fruits with high-moisture environments during handling and storage.