Taking dried peaches made without sulfur as an example, Salmonella survived the entire 180 days of storage with the final level of 4.38 ± 0.08 log CFU/g when the storage temperature was 5 °C. When the storage temperature was 20 °C, Salmonella fell below the limit of detection after 90 days of storage although they could still be recovered by enrichment from two of six samples that were tested. The presence of sand together with the presence of sulfur speeded up the die-off of Salmonella. The impact of dry inoculation might be due the low initial inoculation level or the limit nutrient available when Salmonella cells were dried on the surface of the sand. As shown in Figure 2.1D and Table 2.3, Salmonella fell below the limit of detection after 30 days of storage at 5 °C and could no longer be detected even by enrichment. When the storage temperature was at 20 °C, Salmonella fell below the limit of detection and could not be detected by enrichment after 15 days of storage. Injured cells were observed starting from Day 0. The difference between the counts obtained from TSAR and XLT-4R indicated the formation of injured cells. These differences were as big as 3.68 log CFU/g .Survival of E. coli O157:H7 on dried peaches. Figure 2.2 and Table 2.4 show the behavior of E. coli O157:H7 survival on the dried peaches. The E. coli O157:H7 population on the wet-inoculated peaches and peaches with sulfur immediately after inoculation were 9.40 ± 0.32 and 9.43 ± 0.45 log CFU/g respectively .
The initial E. coli population on the wet-inoculated dried peaches was 8.70 ± 0.26 log CFU/g . After 5 days of storage, blueberry packaging approximately 1 log reduction was observed from the inoculated dried peaches made without sulfur and stored at 5 and 20 °C. From Day 5 to Day 15, greater reduction was observed from peaches made without sulfur that were stored at 20 °C. A 3.53 log reduction was observed from 20 °C, while 0.97 log reduction was observed from samples stored at 5 °C . From Day 15 to 60, while E. coli O157:H7 further declined to 3.77 ± 0.40 log CFU/g on inoculated-dried peaches without sulfur stored at ambient temperature and maintained at similar levels from Day 15 to Day 60. The surviving E. coli O157:H7 on wet inoculated dried peaches made without sulfur fell below the limit of detection after 90 days of storage at both refrigerated and ambient temperatures and could no longer be detected by enrichment after 150 days of storage. The addition of sulfur speeded up the die-off of E. coli O157:H7. When the storage temperature was at 5 °C, E. coli O157:H7 on dried peaches made with sulfur decreased to 5.74 ± 0.12 log CFU/g after 15 days of storage. After additional 15 days , E. coli O157:H7 could not be detected by neither directly plating nor enrichment. When being stored at ambient temperature, E. coli O157:H7 fell below the limit of detection after 15 days of storage and could not be detected via enrichment after 30 days of storage. Similar to what was observed from Salmonella, lower initial inoculation levels were seen from sand-inoculated samples.
When the storage temperature was 5 °C, E. coli O157:H7presence on dry-inoculated peaches made without sulfur gradually decreased from 6.52 ± 0.45 log CFU/g to 3.27 ± 0.06 log CFU/g on Day 120. No E. coli O157:H7 could be detected via plating or enrichment on Day 180. When the storage temperature was 20 °C, E. coli O157:H7 only decreased by approximately 1.3 log by Day 60. After Day 60, a sharp decrease was seen on Day 90 as no E. coli O157:H7 can be detected by plating. The pathogen can be detected by enrichment until Day 150. No pathogen can be detected on Day 180. When looking at the dry-inoculated peach made with sulfur treatment, a 2.15 log reduction was observed in the first 5 days during the storage at 5 °C . While E. coli O157:H7 could still be detected by enrichment on Day 15, it could not be detected after Day 30. Storing at ambient temperature increased the reduction see on Day 5. Greater than 4.25 log reduction was observed in the first 5 days. E. coli O157:H7 could not be detected after 15 days of storage. The differences between counts obtained from TSAR and MACR were also observed due to the formation of injured cells during inoculation and storage. Survival of L. monocytogenes on dried peaches. Figure 2.3 and Table 2.5 show the behavior of L. monocytogenes survival on the dried peaches. The L. monocytogenes population on the wet-inoculated peaches and peaches with sulfur immediately after inoculation were 9.52 ± 0.70 and 9.54 ± 0.34 log CFU/g respectively . The initial L. monocytogenes population on the wet-inoculated dried peaches was 8.57 ± 0.04 and 8.20 ± 0.23 log CFU/g as determined on TSAR and MOXR respectively. When the storage temperature was 5 °C, L. monocytogenes decreased to 7.92 ± 0.06 and maintained at similar levels for the between Day 5 to Day 90. When the storage temperature was 20 °C, a 2-log reduction was observed during the first 5-day of storage. Another sharp decreasing of survival L.monocytogenes numbers was observed between Day 30 and Day 60.
An approximately 2.4 log reduction was seen from TSAR. Starting from Day 90, L. monocytogenes could not be recovered by either direct plating nor enrichment on peaches made without sulfur. The pre-drying sulfur treatment sped up the die-off of L. monocytogenes. Starting from Day 15, L. monocytogenes could not be recovered from any dried peaches made with sulfur treatment stored at both temperatures. Dry inoculation yielded lower initial inoculation levels compared with wet inoculation. There were 6.56 ± 0.09 log CFU/g of L. monocytogenes on peaches made without sulfur treatment and 5.81 ± 0.03 log CFU/g of L. monocytogenes on peaches made with sulfur treatment. This lower inoculation level together with the presence of sulfur led to a significant reduction of L. monocytogenes during the first 5 days of storage regardless of the storage temperatures. Starting from Day 5, L. monocytogenes could only be detected from dry-inoculated dried peaches made with sulfur treatment by enrichment. With dried peaches made without sulfur treatment, bluleberry packaging box the surviving L. monocytogenes gradually decreased from Day 0 to Day 120 when the storage temperature was at 5 °C. Starting from Day 150, L. monocytogenes could no longer be recovered from dry-inoculated dried peaches without sulfur treatment. When the storage temperature was at 20 °C, L. monocytogenes could not be detected by either plating nor enrichment starting from Day 120. Sulfur measurement. The amount of total sulfur dioxide and free SO2 was measured during the first 30 days of ambient storage . The measurement was suspended after Day 30 due to the “shelter-in-place” order placed in March 2020. As shown in Table 2.6, the initial level of free SO2 and total SO2 in the dried peaches made with sulfur treatment were 830± 32 mg/Kg and 2,108 ± 32 mg/Kg respectively. The wet inoculation led to a loss of approximately 122 mg/kg of free SO2 and approximately 73 mg/kg of total SO2. The 2-day drying after inoculation only impacted the total SO2 level and didn’t impact the free SO2 level . No significant change was observed in free SO2 from Day 0 to Day 5. A significant loss of free SO2 was seen from Day 5 to Day 15 . On Day 30, there were 393 ± 46 mg/kg of free SO2 and 1,544 ± 12 mg/kg of total SO2 present in these dried peaches.The survival of common bacterial pathogens, Salmonella, E. coli O157:H7, and L. monocytogenes, was monitored on dried peaches made without and with sulfur pre-drying treatment. Two inoculation carriers were applied, and the two storage temperatures were tested. This impact is applicable to all pathogens tested. For example, Salmonella could not be recovered by enrichment from wet-inoculated dried peaches made with sulfur treatment after 60 days of storage at 5 °C and 15 days of storage at 20 °C. When they were inoculated onto dried peaches made without sulfur treatment, there were 4.95 ± 0.07 log CFU/g of Salmonella survived on these samples by the end of 180 days of storage at 5 °C. Even when the storage temperature was at 20 °C, Salmonella was still detected via enrichment from dried peaches made without sulfur.
The impact of sulfur treatment on the survival of pathogens was also reported by Liu et al. . Similarly, no Salmonella cell was recovered from sulfurtreated apricots via enrichment after 90 days of storageat 22 °C, while ~2.5 log CFU/g of Salmonella was recovered from dried apricots made without sulfur dioxide . Although sulfur dioxide treatment facilitates bacterial die-off, it has the potential to induce asthmatic reactions in some people . The free SO2 levels detected in dried peaches made with sulfur used in this study were higher than previously reported numbers. Although the processors did label the packages with “made with sulfur treatment”, additional studies may be needed to gain insight into sulfur levels present in samples sold at the farmers markets and the changes of free and total SO2 during storage. Storage temperature is another factor that generates a significant impact on pathogen survival. In general, pathogens survived at a higher level for longer period of time at low temperatures than ambient temperature. For example, Salmonella survived on dried peaches made without sulfur at 5 °C for up to 180 days with a final level of 4.59 log CFU/g on wet-inoculated ones and 4.38 ± 0.08 log CFU/g on dry-inoculated ones. On the same samples stored at ambient temperature, Salmonella could only be detected via enrichment after 90 days of storage, indicating the surviving level was below 1.9 Log CFU/g. Cuzzi, et al. found similar results. In their study, L. monocytogenes was inoculated onto dried applies, strawberries and raisins with sand and stored at 4 °C and 23 °C . Since L. monocytogenes could not be recovered from inoculated dried apples at Day 0 , only the survival in dried strawberries and raisins were monitored in this study . When the storage temperature was at 23 °C, L. monocytogenes decreased rapidly by greater than 4 and 3.6 log CFU/g after 14 and 7 days of storage on raisins and dried strawberries. However, when the storage temperature was at 4 °C, L. monocytogenes only decreased by approximately 0.1 and 0.2 log CFU/g/month. After 336 days of storage, L.monocytogenes only decreased by 1.4 and 3.1 log CFU/g on raisins and strawberries, respectively. The impact of inoculation carriers on the survival of pathogen was completed by the fact that different carriers led to, sometimes, different initial inoculation levels before storage. For example, the wet inoculation brought 9.45 ± 0.06 log CFU/g of Salmonella on dried peaches made without sulfur on Day 0, while the dry inoculation had an initial inoculation level of 7.26 ± 0.14 Log CFU/g to dried peaches. Lower initial inoculation levels from sand inoculated samples were also seen for E. coli O157:H7. In this case, the impact of carriers can not be fully studied. In the study conducted by L. R. Beuchat and Mann , two inoculation methods were used to inoculate the dried fruits. One was misting dried fruits with an aqueous suspension of a 5-serotype cocktail of Salmonella, and the other was mixing the dried fruits with sand on which a 5-serotype cocktail had been dried. Authors found that the survival of Salmonella on dried cranberries, raisins, strawberries, and date paste inoculated using the dry carrier and wet carrier followed similar trends. In the study conducted by Feng et al. , plate-grown E. coli O157:H7 were inoculated onto in-shell hazelnuts via wet or dry carriers . After that, samples were stored at 24 ± 1 °C for 12 months. Their results showed that E. coli O157:H7 reduced rapidly on sand-inoculated hazelnut than wet-inoculated ones, although the initial inoculation levels before storage were similar . In the study conducted by Liu et al. , Salmonella was inoculated onto dried apricots made with or without sulfur. The liquid inoculum was diluted for wet inoculation so that the same initial inoculation levels were achieved for both wet- and dry-inoculated dried apricots. Based on their results, Salmonella survived at higher levels for longer period of timeon sand-inoculated dried apricots.