The L. monocytogenes population on the wet-inoculated pluots and sundried tomatoes immediately after inoculation were 9.35 ± 1.07 and 9.59 ± 0.25 log CFU/g respectively . On Day 0, higher inoculation levels were seen from sundried tomatoes . Approximately 5.35 log CFU/g of L. monocytogenes were inoculated onto dried pluots. After inoculation and during storage, L. monocytogenes decreased rapidly on dried pluots. On Day 5, a greater than 2.84 log CFU/g of injured cells were observed at 5 °C . Similar significant difference between TSAR and MOXR was seen on Day 5 at 20 °C. Starting from Day 15, no L. monocytogenes could be detected from dried pluots regardless of the storage temperature. Sharp reductions of survival L. monocytogenes cells on sundried tomatoes were seen on Day 60 for both storage temperatures. The reductions were greater than 2.15 log and 4.83 log respectively for 5 and 20 °C. Since Day 60, no L. monocytogenes could be detected from sundried tomatoes even by enrichment. pH and water activity. The initial pH of low-moisture dates, high-moisture dates, dried pluots and sundried tomatoes were 5.04 ± 0.23, 4.65 ± 0.06, 3.17 ± 0.12, and 3.50 ± 0.05 respectively. The initial water activities of low-moisture dates, high-moisture dates, dried pluots and sundried tomatoes were 0.63, 0.65, 0.77, and 0.82 respectively. In general, package of blueberries the water activity and the pH did not change significantly during storage, except the water activity of sundried tomatoes.

Three types of dried fruits were selected in this study including Medjool dates, sundried tomatoes, and dried pluots processed with sulfur dioxide. The order of water activity was sundried tomatoes > dried pluots > high-moisture dates > low-moisture dates. The order of pH was high-moisture dates > low-moisture dates > sundried tomatoes > dried pluots. The selection of inoculation carriers was based on two criteria: it has the lowest impact on the visual and physiochemical properties of dried fruits, and it’s relevance to the commercial processing of dried fruits and ability mimic or represent contamination that happens during various processing or storage stages. Given wet inoculation caused detachment of date skin, dry inoculation was used to mimic the contamination during growing. In addition, dates are typically grown in sandy regions where sand storms are common and some dates are cleaned by air pressure without any contact with water before packaging . Although not big, dry inoculation reduced the pH of dried pluots and aw of sundried tomatoes. Additionally, due to seasonal production of fresh fruits, some dried fruits such as sundried tomatoes and dried pluots are processed in large quantities, stored at frozen temperatures, and then washed and dried before packaging . Therefore, wet inoculation was used for sundried tomatoes and dried pluots to mimic the contamination during washing. When comparing Figures 3.1-3.3 the survival of pathogens on dry-inoculated dates was determined by bacteria species, storage temperature, and date type. Among the three tested pathogens, Salmonella survived better than E. coli O157:H7 and L. monocytogenes regardless of the date type or storage temperature. Juneja et al. also found that Salmonella and E. coliO157:H7 wet-inoculated dates at ~8 log CFU/g increased by 0.2-0.4 log CFU/g after 31 days of storage , while L. monocytogenes population decreased by 1.32 log CFU/g. L. R. Beuchat and Mann evaluated the survival of Salmonella on dry -inoculated date paste and they found that at a low inoculation level of 3.18 log CFU/g, all samples remained positive of Salmonella by enrichment after 242 days of storage at 4 °C. Liu et al. found that when Salmonella and L. monocytogenes were dry-inoculated on dried apricots made without sulfur dioxide treatment at ~6 log CFU/g, Salmonella survived for the entire storage period at 22 °C with the final level of ~2.5 log CFU/g after 90 days, while L. monocytogenes fell below the limit of enumeration at the end of storage.

Salmonella is a microorganism that has been mostly commonly involved in outbreaks associated with low water activity food . The exact mechanisms used by Salmonella to survive in low moisture conditions remain to be fully elucidated. Finn et al summarized the potential responses of Salmonella upon transition into a low moisture environment; these responses include uptake of potassium ions, transportation of osmoprotectants, synthesis of glutamate and trehalose, up-regulation of sigma factors RpoE and RpoS, increased fatty acid catabolism, and filament formation. These responses facilitate the survival of Salmonella in low moisture conditions. The survival of L. monocytogenes, E. coli O157:H7, and Salmonella also significantly correlated with storage temperatures in low aw food. As the temperature increases microorganisms die more rapidly . In this study, a faster die-off was also observed at 20 °C than 5 °C for all three pathogens tested in both low- and high-moisture dates. Similarly, when Salmonella was inoculated in date paste at 6.57 log CFU/g, bacterial populations were reduced by 0.77 and 4.25 log CFU/g at 4 °C and 25 °C, respectively, after 21 days of storage . Populations of L. monocytogenes on dried raisins fell below the limit of detection after 14 days of storage at 23 °C while only decreased by 1.4 log after 336 days of storage at 4 °C . The overall survivability of all three pathogens was better on the low-moisture dates than the high-moisture dates. Although the initial water activity of the low-moisture dates was lower than that of the high-moisture dates , it increased to similar levels with that of the high-moisture dates within 5 days of storage at both temperatures . Since both dates were inoculated with the dry carrier and they have comparable pH and aw , the overall better survival of pathogens in low-moisture dates might be due to other intrinsic factors. For example, naturally dried Medjool dates at a water activity of 0.55 contained significantly higher levels of phenolic acids and sugars compared with sundried Medjool dates at a water activity of 0.49 . Phenolic acids have antimicrobial effects against foodborne pathogens while sugars such as sucrose might improve pathogen survival . PBS was used as the carrier for sundried tomatoes and dried plouts made with sulfur treatment. Although drying the inoculum on sand also led to significant reductions of each pathogen, once inoculated onto the final dried fruits, no more immediate change in bacterial counts was observed .

When using the liquid carrier, the drying took place on dried fruit surfaces. Various intrinsic factors associated with dried fruits may impact the reduction of pathogens during drying, such as the presence of free sulfur dioxide. This might be one of the reasons why there E. coli O157:H7 populations of 8.70 ± 0.71 log CFU/g survived on dried tomatoes after 48 h of drying while E. coli O157:H7 populations of 6.28 ± 0.15 log CFU/g were recovered on dried plouts in the same time. This observation indicates a drawback associated with the use of liquid carrier. Since the drying processing took place on dried fruits, such inoculation method can lead to uneven initial inoculation levels on various products, thus impacting pathogen behavior during storage. Once being stored at designated conditions, drastic decrease, and die-off of all three pathogens were immediately observed in dried plouts . After 15 days of storage at both temperatures, no E. coli O157:H7 or L. monocytogenes was recovered from dried plouts after enrichment. Salmonella was not recovered from inoculated dried plouts after 30 days of storage at 20 °C and 60 days at 5 °C. The presence of free sulfur should be one of the factors contributing to this rapid die-off during storage. In the study conducted by Liu et al. , Salmonella was dry-inoculated on dried apricots using sand at ~6.5 log CFU/g, square plant pots no cell was recovered from sulfur-treated apricots after enrichment during 90 days of storage at 22 °C, while ~2.5 log CFU/g of Salmonella was recovered from dried apricots made without sulfur dioxide . However, although sulfur dioxide treatment facilitates bacterial die-off, it has the potential to induce asthmatic reactions in some people . Besides inoculation method and the presence of sulfur dioxide, the rapid decrease of bacterial populations on sundried tomatoes might also be due to having the lowest pH and highest water activity of the dried fruits tested. In the study by Cuzzi et al., the die-off rate of L. monocytogenes in dried apples and dried strawberries was faster than that in raisins during storage at 4 °C and 23 °C. In a recent study in which the authors analyzed 67 publications about survival of foodborne pathogens in low water activity food held at temperatures less than 37 °C, the data showed that aw could significantly impact the survival of both pathogenic and generic E. coli . Another interesting observation made from the sundried tomato is that, unlike the faster die-off at 20 °C for all pathogens on dry-inoculated dates or wet-inoculated dried pluots, L. monocytogenes and Salmonella population decreased more rapidly at 5 °C than 20 °C on sundried tomatoes. This might be due to the increased aw of sundried tomatoes during the storage at 20 °C . Similar results were reported by Farakos et al. . In this study, when Salmonella was inoculated on hazelnuts and stored at 25 °C with different RH , corresponding to nut aw of 0.37 or 0.54 respectively, the time for the first log reduction of Salmonella was estimated at 24 and 9 weeks, respectively .

Salmonella died faster on higher water activity nuts. Overall, our results showed that common foodborne pathogens can survive on dried fruits, emphasizing the importance of implementing additional control strategies to improve their microbial safety from every aspect. Alp and Bulantekin reviewed methods to effectively inactivate microorganisms on dried foods at various stages of production including pre-drying treatments, novel drying methods, and post-drying treatments. For example, dipping Gala apple slices in 0.5% ascorbic acid, lactic acid, citric acid, and sodium bisulfate for 2 min following dehydration for 5 h at 60 °C caused an additional 2.29, 2.69, 2.75, and 5.58 log CFU/greduction of Salmonella compared with the untreated dehydrated control . Phungamngoen et al. found that vacuum drying and low-pressure superheated steam drying had superior antimicrobial effect against S. Anatum on cabbages compared with hot air drying. The drying time to obtain a 3-log reduction of Salmonella on cabbages was 270, 94, and 58 min for hot air drying, vacuum drying, and LPSSD, respectively . At the post-drying stage, advanced ozonation process, which simultaneously applied UV-C, ozone, and hydrogen peroxide, reduced Salmonella inoculated on raisins, dried strawberries, and dried apples at 5.55-6.59 log CFU/g to undetectable level even after enrichment . Active packaging that can generate gaseous chlorine dioxide reduced background fungi and bacterial populations of semi-dry longan pulp by ~3.4 and 2.0 log CFU/g compared with the untreated controls after 28 and 180 days of storage at ambient temperature , respectively . For the dried fruits that are stored at freezing temperature and rewashed and dried before packaging, sanitizers can be used to reduce potential contamination. For instance, 75 ppm peroxyacetic acid wash for 1 min reduced Salmonella, E. coli O157:H7, and L. monocytogenes inoculated on dried Deglet Noor dates at ~7 log CFU/g by 4.80, 4.08, and 4.96 log CFU/g, respectively .Microbial behavior is closely related to the different intrinsic and extrinsic factors associated with food . Being able to estimate and predict microbial behavior in various food is the foundation for risk assessments and a key in preventing foodborne illnesses . There are two main types of mathematical models for predictive microbiology as reviewed by Stavropoulou and Bezirtzogou . They are kinetic models and probability models. Kinetic models are used to calculate the rate of growth or death responses and predict the concentration or levels of a given microorganism in given conditions . Probability models are used to predict the production of microbial toxins. These models suggest the probability of bacterial growth and their toxins but not the growth or die-off rates . There are different levels of kinetic models that can be established based on the data obtained from challenge studies. Primary predictive models are typically developed to describe population dynamics of pathogenic and spoilage bacteria under different environmental conditions . Based on the primary models, the secondary predictive models are constructed to evaluate the effect of temperature on growth rates or inactivation rates of bacteria .