UC Davis

Dried fruits are one of the most economically valuable specialty crops in California. Dried fruits contain essential nutrients and health-promoting bioactive compounds such as antioxidative phenolic compounds and phytoestrogens. Unfortunately, there have been outbreaks associated with dried fruits that have sickened people worldwide. The limited literature about the behavior of common foodborne pathogens on various dried fruits and the intrinsic and extrinsic factors impacting their behavior has hindered the development of microbial food safety risk assessments of dried fruits. To better address the knowledge gaps associated with dried fruit safety, a survey was first designed and conducted to identify current common practices that are being used by different sizes of processors. Results showed that the majority of processors use dehydrators to dry their fruits while the rest of the processors use oven or sun-drying. Pre-drying treatments, including dipping or soaking fresh fruits in sulfur, lemon juice, or citric acid solutions, are being used by some processors. Unfortunately, most processors do not have a validated method for determining if their products are adequately dried or not, but rather go by what processors before them have said. To investigate the behavior of common foodborne pathogens on dried fruits, a challenge study was conducted, in which 5-strain cocktails of Salmonella spp., Escherichia coli O157:H7, and Listeria monocytogenes were artificially inoculated onto dried fruits and their survival was monitored for 6 months. Dried peaches, dried peaches processed with sulfur, dried pluots processed with sulfur, sundried tomatoes, high-moisture Medjool dates, and low-moisture Medjool dates were obtained from local farmers markets. Two inoculation carriers (sand and phosphate buffered saline) were first tested for their potential to be used for the inoculation of dried fruits. Based on the measurement of the chemical and physical properties of inoculated dried fruits, sand as a dry carrier was determined to be appropriate to use with dried peaches and Medjool dates and phosphate buffered saline (PBS) as a wet carrier was determined to be appropriated to use with dried peaches, dried pluots, and sundried tomatoes. The sand inoculation led to initial Salmonella levels of 6.43 ± 0.07 log CFU/g to 7.26 ± 0.14 log CFU/g while PBS inoculation lead to initial Salmonella levels of 9.39 ± 0.32 log CFU/g to 9.73 ± 0.14 CFU/g. Since the drying of the liquid inoculum happened on the dried fruits, the properties of the dried fruits impacted the initial inoculation level after drying. For example, in dried pluots the initial inoculation of Salmonella of 9.39 ± 0.31 log CFU/g dropped to 8.09 ± 0.07 log CFU/g after 48 h of drying. Sand inoculation led to lower initial inoculation level, as up to 3.38 log CFU/g of reduction was observed during the preparation of the Salmonella sand inoculum. Inoculated dried fruits were stored at refrigerated and ambient temperatures. Pathogens populations were determined 0, 5, and 15 days after inoculation, and every 30 days for 6 months. The limit of detection (LOD) by direct plating was 1.9 Log CFU/g; samples that fell under the limit of detection were enriched following FDA protocols. Salmonella survived longer than the other two pathogens. From high-moisture dates, Salmonella was recovered at 5.31 ± 0.06 log CFU/g after 180 days of storage at 5 °C. E. coli O157:H7 was recovered at 4.14 ± 0.31 log CFU/g after 150 days of storage and dropped below the LOD by 180 days. L. monocytogenes was recovered at 5.90 ± 0.07 log CFU/g after 120 days of storage and dropped below the LOD by 150 days. The three pathogens survived better in storage at refrigerated temperature than at ambient temperature. When stored at 5 °C, Salmonella on low-moisture dates was recovered at 5.30 ± 0.16 log CFU/g after 180 days. When stored at 20 °C, the recovery was 4.43 ± 0.09 log CFU/g after 60 days and dropped below the LOD by 90 days. Intrinsic factors influenced pathogen survival as well, with pathogens surviving longer in dried fruits with lower pH and higher water activities. Sulfur treatment also had an impact on pathogen survival. L. monocytogenes wet-inoculated onto dried unsulfured peaches was recovered at 4.26 ± 0.18 log CFU/g up to 120 days. In contrast, on dried peaches processed with sulfur, recovery of L. monocytogenes was 7.21 ± 0.46 log CFU/g up to 5 days and dropped below the LOD by 15 days. Primary linear models were built to describe the behavior of pathogens during storage. Among the three pathogens, Salmonella had the largest difference in rate of decline between the two storage temperatures. It declined at a rate of 242 days/log reduction when dry inoculated onto low-moisture dates in refrigerated storage and at a rate of 15 days/log reduction at ambient temperature. E. coli O157:H7 declined with a rate of 54 days/log reduction at refrigerated storage temperature and 22 days/log reduction at ambient temperature. L. monocytogenes declined at a rate of 35 days/log reduction at refrigerated temperature and 18 days/log reduction at ambient temperature. Pathogens declined more quickly on the sulfured dried fruits, peaches and pluots, followed by sundried tomatoes, non-sulfured peaches, and dates. Taking Salmonella as an example, the sulfured dried fruits had too rapid of die-off to calculate a D-value at ambient storage. The rates of decline in the sundried tomatoes, dried peaches, low-moisture dates, and high-moisture dates were 7.86, 10.89, 14.50, and 14.02 days/log reduction respectively. In summary, common foodborne pathogens can survive on a range of dried fruits. The behavior of pathogens is impacted by intrinsic factors associated with dried fruits (e.g. pH, water activity, sulfur, and available nutrients) and extrinsic factors (e.g. storage temperatures). In general, pathogens declined faster at ambient temperature than refrigerated temperature. Salmonella, a pathogen with well-known history of association with low moisture foods, survived the best amongst the three tested pathogens. Pre-drying treatments (e.g. sulfur treatment) can have long-lasting antimicrobial effects during storage. Additional research that can systematically illustrate the antimicrobial effects of various pre-drying treatments as well as post-drying strategies is still needed to better control the potential food safety risks associated with dried fruits.