Survival of Salmonella on Dried Fruits
Dried fruits have historically 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.
The purpose of this study was to (1) determine the ability of Salmonella to survive on dried fruits as affected by conditions used to culture cells before inoculation, method of inoculation, and storage temperature after inoculation and (2) determine if Salmonella can survive and grow in aqueous homogenates of dried fruits.
Five serotypes of Salmonella enterica were acid adapted by growing in tryptic soy broth supplemented with 1% glucose (TSBG). Dried cranberries (aw 0.47, pH 2.52), date paste (aw 0.69, pH 5.08), raisins (aw 0.46, pH 4.08), and strawberries (aw 0.21, pH 3.32) were mist- or dry-inoculated with the five-serotype mixture, followed by drying to the original aw. In addition, cells cultured on tryptic soy agar supplemented with 1% glucose (TSAG) were used to mist-inoculate date paste.
Inoculated fruits were stored at 4ºC and 25°C and analyzed for Salmonella at 1-week to 2-month intervals for up to 8 months. Changes in populations of Salmonella in 10% and 50% aqueous homogenates of dried fruits stored at 4ºC and 25ºC were monitored over a 12-week period.
Salmonella (initial population, 6.57 - 7.01 log CFU/g) survived on mist-inoculated 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 TSBG, cells grown on TSAG survived longer in date paste. This supports previous observations showing that Salmonella grown on solid media has increased resistance to subsequent stresses compared to cells grown in broth. At an initial population of 2.18 - 3.35 log CFU/g of dry-inoculated fruits, Salmonella survived on cranberries and date paste for at least 242 days and raisins and strawberries for at least 182 days at 4ºC. Inactivation was more rapid at 25ºC, regardless of the method of inoculation.
At an initial population of 2.76 log CFU/ml, Salmonella did not grow in 10% (aw 0.997±0.002) and 50% (aw 0.958±0.003), cranberry, date paste, raisin, and strawberry homogenates stored at 4°C. Inactivation was more rapid in cranberry and strawberry homogenates than in date paste and raisin homogenates. The pathogen decreased to 0.86 CFU/ml of 10% date paste homogenate held at 4ºC for 70 days and was not detected (<1 CFU/ml) at 84 days; in 10% raisin homogenate held at 4ºC, the pathogen decreased to 0.36 log CFU/ml at 49 days and was not detected at 70 days. Salmonella increased by 2.5 log CFU/g in 10% date paste and raisin homogenates held at 25°C for 2 and 6 days, respectively, followed by rapid inactivation. Growth did not occur in 10% cranberry and strawberry homogenates held at 25ºC.
Results of this study suggest the need to treat dried fruits that may be post-process contaminated with Salmonella with a lethal treatment before eating out-of-hand or using as ingredients in ready-to-eat foods. The ability of Salmonella to grow in dilute 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.