To evaluate the survival of Salmonella on raw almond surfaces, whole almond kernels were inoculated with Salmonella Enteritidis phage type (PT) 30 collected from a 24-h broth culture or by scraping cells from an agar lawn. Kernels inoculated with lawn-collected cells to 8, 5, 3, and 1 log CFU per almond after a 24-h drying period were stored for 161 days at 23 +/- 3 degrees C. Calculated rates of reduction were similar for the four inoculum levels (0.22, 0.28, 0.29, and 0.22 log CFU/month, respectively). Kernels inoculated to 7.1 or 8.0 log CFU per almond after drying were stored for 171 or 550 days, respectively, at selected temperatures, including -20 +/- 2 degrees C, 4 +/- 2 degrees C, 23 +/- 3 degrees C, and 35 +/- 2 degrees C. No significant reductions of Salmonella were observed during storage at -20 and 4 degrees C over 550 days. At 35 degrees C, a biphasic survival curve was observed, with calculated reductions of 1.1 log CFU/month from days 0 to 59 and no significant reduction from days 59 to 171. At 23 degrees C, reductions of 0.18 and 0.30 log CFU/month were calculated for 171 and 550 days of storage, respectively. When combined with data from the study of inoculum levels, an overall average calculated reduction at 23 degrees C was 0.25 +/- 0.05 log CFU/month. Significantly greater reductions were observed during the 24-h drying period when broth-collected cells were used as the inoculum, suggesting that cells collected from agar lawns were more resistant to drying. However, after initial drying, the rates of reduction at 23 degrees C did not differ significantly between the inoculum preparation methods. Salmonella Enteritidis PT 30 survives for long periods on almond kernels under a variety of commonstorage conditions.

In 2001, Salmonella Enteritidis phage type (PT) 30 was isolated from drag swabs of 17 61-ha almond orchards on three farms linked to an outbreak of salmonellosis associated with consumption of raw almonds. The objective of this study was to evaluate the long-term persistence of Salmonella Enteritidis PT 30 in one of the almond orchards associated with the outbreak. Swabs (gauze saturated with full-strength sterile evaporated skim milk and attached to string) were pulled along the orchard floor in a standardized manner for 55 m. At each sample time, two pooled samples (four swabs each) were collected from each orchard quadrant. Swabs were enriched for Salmonella using a delayed secondary enrichment procedure developed for isolation of Salmonella from poultry houses. Suspect Salmonella isolates were selected, confirmed, serotyped, and phage typed, and pulsed-field gel electrophoresis (PFGE) patterns were determined after cleavage with XbaI and BlnI. Salmonella was recovered infrequently from pooled samples collected from January through July (3 of 56 samples, 5.3%). In general, Salmonella isolation frequency per sample time increased during and immediately after the harvest, when large amounts of dust were generated in or near the orchard: August, 4 (20%) of 20 samples; September, 13 (20%) of 64 samples; October, 27 (42%) of 64 samples; November, 4 (25%) of 16 samples; and December, 2 (25%) of 8 samples. All 53 Salmonella isolates during the 5 years were identified as Salmonella Enteritidis PT 30, and two PFGE patterns that differed by the presence of an approximately 40-kb fragment were identified. These data demonstrate the potential for long-term environmental persistence of Salmonella in almond orchards.

Propylene oxide (PPO) is commonly used to reduce microbial populations in U.S. bulk raw almonds, but the process has not been validated for reduction of foodborne pathogens. The reduction of Salmonella Enteritidis phage type (PT) 30 inoculated onto almonds was evaluated after exposure to a standard commercial PPO treatment. Almonds were inoculated with Salmonella Enteritidis PT 30 to approximately 8.0 log CFU/g after drying. Inoculated almonds were placed in bags designed for gaseous sterilization and positioned in the center of 900-kg bins or 22.7-kg boxes of warmed almonds. Almonds were further warmed to an initial temperature of 23 to 34 degrees C, treated with PPO (0.5 kg/m3 for 4 h), and held for 0 or 2 days at 38 to 43 degrees C followed by storage for 2 to 5 days at 15 to 18 degrees C. Salmonella Enteritidis PT 30 was recovered by vigorously shaking 100 g of almonds in 100 ml of Butterfield's phosphate buffer, plating onto tryptic soy or bismuth sulfite agar, and incubating at 35 degrees C for 24 or 48 h, respectively. Populations of Salmonella Enteritidis were consistently reduced by > 5.0 log CFU/g (5.2 to > 8.6 log CFU/ g) when initial counts were compared with counts obtained 5 days after PPO treatment. Reductions of 1.2 to 4.4 log CFU/g occurred during post-PPO storage. Reductions were not significantly improved (P < 0.05) when almonds were held at 38 to 43 degrees C after PPO treatment. PPO residues were > 400 ppm immediately after removal from the PPO chamber and declined to < 300 ppm during post-PPO storage. PPO is an effective treatment for reducing populations of Salmonella Enteritidis PT 30 on bulk almonds.

Pulsed light (PL) treatment can effectively reduce microbial populations in clear substrates and on surfaces, but its effectiveness varies as a function of substrate or treatment-related factors. For PL to be successfully adopted by the food industry, all factors of influence, as well as the inactivation kinetics for the microorganisms of concern, must be elucidated. In this study, the inactivation kinetics of Listeria innocua and the effect of inoculum size on PL inactivation were investigated. Stainless steel coupons (50.8 by 101.6 mm) of defined surface properties and transparent glass chamber slides (25.4 by 50.8 by 10 mm) were each inoculated with 1 ml of aqueous suspensions of L. innocua containing inoculum populations of up to 10(9) CFU. The thickness of the liquid layer in the glass slides was 1.16 mm. The inoculated substrates were exposed to PL treatment of up to 17 J/cm2 in a static PL chamber equipped with a pulsed Xenon lamp. Survivors were recovered and enumerated by both standard plate counting and most-probable-number procedures. The data indicated that in clear liquids, PL resulted in more than a 6-log reduction of L. innocua after a 12-J/cm2 treatment, regardless of the initial inoculum size. For the stainless steel surfaces, less than a 4-log reduction after a 12-J/cm2 treatment and a noticeable effect of substrate characteristics and inoculum size on inactivation were observed. The survivor curves showed pronounced tailing for all substrates used in the study. The Weibull model accurately predicted the survivor ratios for the PL treatment of L. innocua in clear liquids, with a shape and scale parameter of 0.33 and 3.01, respectively. The Weibull model resulted in significant overestimation of PL effectiveness for the stainless steel substrates, where the influence of various substrate properties and inoculum level on inactivation was significant.

The risk of listeriosis associated with ready-to-eat foods is a major concern in the United States. Pulsed light (PL) treatment has been effective for killing Listeria. The possibility of enhancing the antilisterial capability of PL treatment by combining PL with an additional hurdle, the natural antimicrobial nisin, was explored in this study. First, the ability of Listeria innocua to mimic the response of Listeria monocytogenes to PL treatment was demonstrated. Subsequently, a series of inoculation studies was performed in which canned sausages were surface inoculated with L. innocua as a surrogate for L. monocytogenes and then treated with a commercial preparation of nisin (Nisaplin), PL, or a combination of the two treatments. The application of a Nisaplin dip alone resulted in an immediate reduction of L. innocua by 2.35 +/- 0.09 log CFU. PL reduced L. innocua by 1.37 +/- 0.30 log CFU after exposure to 9.4 J/cm2. A total reduction of 4.03 +/- 0.15 log CFU was recorded after the combined treatment of Nisaplin and PL for 48 h at 4 degrees C. The long-term survival of L. innocua was evaluated on sausages stored at 4 degrees C. Treatment with Nisaplin and PL resulted in a 4- to 5-log reduction for two replicate studies. The combination treatment resulted in no significant microbial growth during 28 and 48 days of refrigerated storage in the first and second replicates, respectively. These results suggest that this combination treatment can be used as an effective antilisterial step in the production of ready-to-eat foods.