Effect of Pulsed Light on Quality of Shelled Walnuts

Shelled walnuts are considered a microbiologically low-risk food but have been linked to some outbreaks, and a treatment aiming to decrease this risk is desirable. Pulsed light (PL) may be an alternative, providing it does not seriously impair their quality. This work assessed the impact of PL on some quality attributes of walnuts. To do this, measurements of rancidity, volatiles, total phenols, antioxidant activity, and descriptive sensory analysis were carried out on untreated and PL (43 J/cm2)-treated kernels. PL had no statistically significant (p > 0.05) effects on TBARS, peroxide value, total phenols, and antioxidant activity but significantly increased the concentration of volatiles related to green/herbaceous odors and decreased compounds related to fruity and citrus odors. The descriptors nut overall, walnut odor and flavor, and aftertaste were given statistically significantly (p < 0.05) higher scores, while descriptors woody odor and sweet received lower scores; 16 other traits such as all those related to color, texture, and rancidity were unaffected. No significant (p > 0.05) effects on total phenols and antioxidant activity in general were observed during the course of PL treatment. It can be concluded that PL technology may be used in shelled walnuts with only mild effects on their quality; a storage study must be carried out in order to determine the effect of PL treatment on its shelf-life.

Survival of Salmonella Enteritidis Phage Type 30 on Brazil Nut Kernels and Pumpkin Seeds Stored at 8, 23, and 37°C

ABSTRACT

Experiments were performed to assess the survival of Salmonella on whole Brazil nut kernels and pumpkin seeds stored at 8, 23, and 37°C. Brazil nut kernels and pumpkin seeds were inoculated with bacterial inoculum containing 10.4 log CFU/mL Salmonella Enteritidis phage type 30 and aseptically dried at room temperature for 24 h. After the drying step, levels of Salmonella recovered from Brazil nut kernels and pumpkin seeds were 8.67 ± 0.01 and 9.27 ± 0.03 log CFU/g, respectively. The survival of Salmonella and change in water activity was assessed over 413 days. Although Salmonella survived throughout the storage period, significant differences were recorded between the storage temperatures. Results showed that the survival of Salmonella Enteritidis phage type 30 was more enhanced at 8°C compared with storage at 23 and 37°C. Comparing the survival of Salmonella on the two products at different storage temperatures, there was no significant difference between the means of Salmonella counts for the two products. Results show that Salmonella survived longer on pumpkin seeds stored at 8°C (P = 0.53, compared with Brazil nut kernels), and at 23 and 37°C, Salmonella survived longer on Brazil nut kernels (P = 0.12, compared with pumpkin seeds). The highest and lowest survival of Salmonella was observed on pumpkin seeds with decay rates of -0.003 ± 0.001 and -0.015 ± 0.001 log CFU/g/day for pumpkin seeds stored at 8 and 37°C, respectively. The water activity values recorded on days 2 and 413 for Brazil nut kernels stored at 8, 23, and 37°C were 0.424 and 0.434, 0.383 and 0.385, and 0.372 and 0.256, respectively. For pumpkin seeds stored at 8, 23, and 37°C, water activity values recorded on days 2 and 413 were 0.754 and 0.412, 0.627 and 0.350, and 0.787 and 0.205, respectively. The data obtained in this study provide useful insight on the influence of temperature on the survival of Salmonella on the surface of Brazil nut kernels and pumpkin seeds.

HIGHLIGHTS

Attachment and Survival of Escherichia coli O157:H7 on In-Shell Hazelnuts

The multistate Escherichia coli (E. coli) O157:H7 outbreak associated with in-shell hazelnuts highlights the pathogen's ability to involve non-traditional vehicles in foodborne infections. Furthermore, it underscores significant gaps in our knowledge of pathogen survivability and persistence on nuts. Therefore, this study investigated the ability of E. coli O157:H7 to attach and survive on in-shell hazelnuts. In-shell hazelnuts were inoculated with a four-strain mixture of E. coli O157:H7 at 7.6 log colony forming units (CFU)/nut by wet or dry inoculation, stored at ambient conditions (24 ± 1 °C; 40% ± 3% relative humidity (RH) and sampled for twelve months. For the attachment assay, in-shell hazelnuts were inoculated and the adherent population was enumerated at 30 s^-1 h following inoculation. Irrespective of the inoculation method, ~5 log CFU of adherent E. coli O157:H7 was recovered from the hazelnuts as early as 30 s after inoculation. Conversely, pathogen survival was significantly reduced under dry inoculation with samples being enrichment negative after five months of storage (p < 0.05). On the other hand, wet inoculation led to a significantly longer persistence of the pathogen with ~3 log CFU being recovered from the in-shell nuts at 12 months of storage (p < 0.05). These results indicate that E. coli O157:H7 can survive in significant numbers on in-shell hazelnuts when stored under ambient conditions.

Prevalence of Escherichia coli O157:H7 and Salmonella on Inshell California Walnuts

Inshell walnuts collected from California walnut handlers over four harvests were evaluated for the presence of Escherichia coli O157:H7 and Salmonella. E. coli O157:H7 was not detected in any of 2,903 375-g samples evaluated in 2011, 2012, and 2013 (<0.034% prevalence; 95% confidence interval [CI], 0 to 0.13%). Salmonella was not isolated from any of the 935 samples in 2010 (100 g evaluated; <0.11% prevalence; 95% CI, 0 to 0.41%) but was isolated from 2 of 905 (375 g; 0.22% prevalence; 95% CI, 0.061 to 0.80%), 1 of 998 (375 g; 0.10% prevalence; 95% CI, 0.018 to 0.56%), and 1 of 1,000 (375 g; 0.10% prevalence; 95% CI, 0.018 to 0.56%) samples in 2011, 2012, and 2013, respectively, for an average annual prevalence of 0.14% (375 g; 95% CI, 0.054 to 0.35%). The levels of Salmonella in positive samples determined by a modified most-probable-number (MPN) method were estimated to be 0.32 to 0.42 MPN/100 g (95% CI, 0.045 to 3.6 MPN/100 g).

Growth and survival of Enterobacteriaceae and inoculated Salmonella on walnut hulls and maturing walnut fruit

Postharvest contamination of in-shell walnuts may occur when the fruit is dropped to or harvested from the orchard floor or as the outer hull is removed with mechanical abrasion and water. To evaluate the effect of maturity on the potential for microbial contamination, 'Howard' walnut fruits were collected weekly from the tree canopy, from 6 to 7 weeks before to 1 week after typical commercial harvest. The numbers of microorganisms able to form colonies on plate count agar, MacConkey agar (presumptive Enterobacteriaceae), or violet red bile lactose agar (presumptive coliforms) were compared on whole walnut fruits collected by hand directly from the tree or after exposure to the orchard floor for 10 min or 24 h. Salmonella Enteritidis PT 30 was inoculated at <1 to 8 log CFU/g onto 5-g hull pieces (from walnut fruit of different maturities) and stored at ambient temperature (23 to 26°C) in unsealed bags (38 to 90% relative humidity [RH] within bag) or in low humidity (20 to 45% RH) or high humidity (68 to 89% RH) for up to 14 days. Salmonella at 2 or 5 log CFU/ml was inoculated onto hulls before or up to 14 days after blending with water. As the walnut fruit matured, the indigenous bacterial levels on the surface increased, irrespective of whether fruit was collected from the tree or the ground. The RH influenced the growth of inoculated bacteria on hull pieces: Salmonella declined to <0.3 log CFU/g within 24 h at low RH but multiplied from 2 to 6 log CFU/g over 14 days of storage at >40% RH. Salmonella populations declined to <1 CFU/ml within 24 h in freshly blended green hulls but survived or multiplied in blended brown hulls or in blended green hulls that had been stored for 24 h or more before being inoculated.

Efficacy of sanitizers in reducing Salmonella on pecan nutmeats during cracking and shelling

Studies were done to evaluate the efficacy of chlorine (200 to 1,000 μg/ml), lactic acid (0.5 to 2%), levulinic acid (0.5 to 2%), sodium dodecyl sulfate (SDS, 0.05%), lactic acid plus SDS, levulinic acid plus SDS, and a mixed peroxyacid sanitizer (Tsunami 200, 40 and 80 μg/ml) in killing Salmonella on or in immersion- and on surface-inoculated pecan nutmeats (U.S. Department of Agriculture medium pieces and mammoth halves). The addition of SDS to treatment solutions containing lactic acid or levulinic acid resulted in generally higher reductions of Salmonella, but differences in these reductions were not always significant. Lactic and levulinic acids (2%) containing SDS (0.05%) were equivalent in killing Salmonella on immersion-inoculated nutmeats. Tsunami 200 (40 μg/ml) was less lethal or equivalent to 1 or 2% lactic and levulinic acids, with or without 0.05% SDS. Reductions did not exceed 1.1 log CFU/g of immersion-inoculated pieces and halves, regardless of sanitizer concentration or treatment time (up to 20 min). Reductions on surface-inoculated pieces and halves were 0.7 to 2.6 log CFU/g and 1.2 to 3.0 log CFU/g, respectively. Treatment with 2% lactic acid plus SDS (0.05%) and Tsunami (80 μg/ml) was most effective in killing Salmonella on surface-inoculated pieces; treatment of halves with chlorine (1,000 μg/ml) or lactic acid (1 or 2%), with or without SDS, was most efficacious. Exposure of immersion-inoculated pecan pieces to chlorine (200 μg/ml), lactic acid (2%) and levulinic acid (2%) with or without SDS, and Tsunami (80 μg/ml) during intermittent vacuum (18 ± 2 mbar) and ambient atmospheric pressure treatments for up to 20 min reduced Salmonella by only 0.1 to 1.0 log CFU/g. These studies emphasize the importance of preventing contamination of pecan nutmeats with Salmonella. Once nuts are contaminated, the lethality of sanitizers tested in this study is minimal.

Evaluation of sanitizers for inactivating Salmonella on in-shell pecans and pecan nutmeats

Chlorine, organic acids, and water extracts of inedible pecan components were tested for effectiveness in killing Salmonella on pecans. In-shell pecans and nutmeats (U.S. Department of Agriculture medium pieces) were immersion inoculated with a mixture of five Salmonella serotypes, dried to 3.7% moisture, and stored at 4°C for 3 to 6 weeks. In-shell nuts were immersed in chlorinated water (200, 400, and 1,000 μg/ml), lactic acid (0.5, 1, and 2%), and levulinic acid (0.5, 1, and 2%) with and without 0.05% sodium dodecyl sulfate (SDS), and a mixed peroxyacid sanitizer (Tsunami 200, 40 μg/ml) for up to 20 min at 21°C. The rate of reduction of free chlorine in conditioning water decreased as the ratio of in-shell nuts/water was increased. The rate of reduction was more rapid when nuts were not precleaned before treatment. The initial population of Salmonella on in-shell nuts (5.9 to 6.3 log CFU/g) was reduced by 2.8 log CFU/g after treating with chlorinated water (1,000 μg/ml). Treatment with 2% lactic acid plus SDS or 2% levulinic acid plus SDS reduced the pathogen by 3.7 and 3.4 log CFU/g, respectively. Lactic and levulinic acids (2%) without SDS were less effective (3.3- and 2.1-log CFU/g reductions, respectively) than acids with SDS. Treatment with Tsunami 200 resulted in a 2.4-log CFU/g reduction. In-shell nuts and nutmeats were immersed in water extracts of ground pecan shucks (hulls), shells, a mixture of shells and pith, and pith. The general order of lethality of extracts to Salmonella was shuck < shell-pith ≤ shell ≤ pith < chlorine (400 μg/ml) and shuck < shell ≤ pith = shell-pith < chlorine (400 μg/ml). Results emphasize the importance of removing soil and dust on in-shell pecans before conditioning in chlorinated water and the need for sanitizers with increased effectiveness in killing Salmonella on pecans.

Survival of Salmonella enterica, Escherichia coli O157:H7, and Listeria monocytogenes on inoculated walnut kernels during storage

The survival of single strains or cocktails of Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes was evaluated on walnut kernels. Kernels were separately inoculated with an aqueous preparation of the pathogens at 3 to 10 log CFU/g, dried for 7 days, and then stored at 23°C for 3 weeks to more than 1 year. A rapid decrease of 1 to greater than 4 log CFU/g was observed as the inoculum dried. In some cases, the time of storage at 23°C did not influence bacterial levels, and in other cases the calculated rates of decline for Salmonella (0.05 to 0.35 log CFU/g per month) and E. coli O157:H7 (0.21 to 0.86 log CFU/g per month) overlapped and were both lower than the range of calculated declines for L. monocytogenes (1.1 to 1.3 log CFU/g per month). In a separate study, kernels were inoculated with Salmonella Enteritidis PT 30 at 4.2 log CFU/g, dried (final level, 1.9 log CFU/g), and stored at -20, 4, and 23°C for 1 year. Salmonella Enteritidis PT 30 declined at a rate of 0.10 log CFU/g per month at 23°C; storage time did not significantly affect levels on kernels stored at -20 or 4°C. These results indicate the long-term viability of Salmonella, E. coli O157:H7, and L. monocytogenes on walnut kernels and support inclusion of these organisms in hazard assessments.

Inactivation of Salmonella on in-shell pecans during conditioning treatments preceding cracking and shelling

Studies were done to determine the effectiveness of conditioning treatments for killing Salmonella in and on immersion-inoculated and surface-inoculated in-shell pecans. Treatment of immersion-inoculated, dried, stored pecans in chlorinated water (400 μg/ml) reduced Salmonella by not more than 1.6 log CFU/g. Treatment of immersion-inoculated, dried, stored pecans in chlorinated water (200 μg/ml, 1 min) followed by soaking in water for 2 h at 21°C and treating for 10 min in water at 85 to 95°C reduced Salmonella by >5.12 log CFU/g; treatment of nuts containing a low population of Salmonella (<0.60 log CFU/g) for 15 min at 90°C failed to eliminate the pathogen. Reductions of ≥6.42 log CFU/g were achieved by treating surface-inoculated nuts in water at 90 or 95°C for 80 s; treatment of nuts containing 1.78 log CFU/g at 95°C for 10 min did not eliminate the pathogen. Salmonella on surface-inoculated in-shell pecans (kernel moisture, 4.75%; water activity, 0.62) that had been dried and stored at 4°C for 3 to 5 weeks was more resistant to conditioning treatments than was Salmonella on surface-inoculated pecans (kernel moisture, 5.60%; water activity, 0.73) that were not thoroughly dried. Conditioning treatments were less effective for killing Salmonella on immersion-inoculated pecans than on surface-inoculated pecans. Response of Salmonella to conditioning treatments varied, depending on the method of inoculation and whether nuts were dried and stored between the time of inoculation and treatment, which emphasizes the importance of following practices commonly used by commercial pecan shellers when validating the lethality of conditioning treatments.

Survival and growth of salmonella in high-moisture pecan nutmeats, in-shell pecans, inedible nut components, and orchard soil

Outbreaks of salmonellosis associated with almonds have raised interest in better understanding the behavior of Salmonella on other tree nuts. We undertook a study to determine the survival and growth characteristics of Salmonella on high-moisture (water activity of 0.96 to 0.99) pecan nutmeats, in-shell pecans, and inedible components (shuck, shell, and middle septum tissue) of in-shell pecans. Salmonella did not grow on high-moisture nutmeat halves, pieces, or granules stored at 4°C for up to 48 h. Growth did occur, however, at 21, 30, and 37°C. Increases of 1.77 to 5.87 log CFU/g of nutmeats occurred within 48 h at 37°C; the order in which nutmeats supported growth was granules > pieces > halves. Populations of Salmonella on and in high-moisture in-shell pecans (kernel water activity of 0.94) stored at 4, 21, 30, and 37°C for 8 days decreased by 0.52 to 1.19 log CFU/g. The pathogen grew on the surface of high-moisture (water activity of 0.99) pecan shucks and shells but died on middle septum tissue stored at 21, 30, and 37°C for up to 6 days. Salmonella died in water extracts of shucks and in pecan orchard soil saturated with water or shuck extract, but survived well for at least 18 weeks in dry soil. The ability of the pathogen to grow on high-moisture nutmeats and some of the inedible components of pecans emphasizes the importance of controlling or limiting the time pecans are exposed to water in preharvest and postharvest environments.

Factors affecting infiltration and survival of Salmonella on in-shell pecans and pecan nutmeats

A study was done to determine the infiltration and survival characteristics of Salmonella in pecans. The rate of infiltration of water into in-shell nuts varied among six varieties evaluated and was significantly (alpha = 0.05) affected by the extent of shell damage. The rate of infiltration at -20 or 4 degrees C was lower than the rate of infiltration into nuts at 21 or 37 degrees C when nuts were immersed in water at 21 degrees C. In-shell nuts immersed in a suspension of Salmonella (8.66 or 2.82 log CFU/ml) for 1 h contained populations of 6.94 to 6.99 and 1.85 to 1.95 log CFU/g, respectively. Salmonella that infiltrated in-shell nuts reached the kernel and remained viable after drying and during subsequent storage at 4 degrees C. Initially high (5.78 log CFU/g) and low (1.53 log CFU/g) populations of Salmonella did not significantly decrease in in-shell pecans stored at -20 and 4 degrees C for 78 weeks (18 months). Significant reductions of 2.49 and 3.29 log CFU/g occurred in in-shell nuts stored for 78 weeks at 21 and 37 degrees C, respectively. High (6.16 log CFU/g) and low (2.56 log CFU/g) populations on pecan halves and high (7.13 log CFU/g) and low (4.71 log CFU/g) populations on medium pieces stored for 52 weeks at -20 and 4 degrees C decreased slightly, but not always significantly. Significant reductions occurred on nutmeats stored for 52 weeks at 21 and 37 degrees C, but the pathogen was detectable, regardless of the initial inoculum level. Results emphasize the importance of applying process treatments that will inactivate Salmonella.

Prevalence and amounts of Salmonella found on raw California almonds

Data on the prevalence and populations of pathogens in individual foods are critical to the development of product-specific quantitative microbial risk assessments. An outbreak of salmonellosis associated with the consumption of raw almonds in 2000 to 2001 provided an opportunity to evaluate the levels of Salmonella in the recalled product. Duplicate 100-g samples from each of fifty 22.7-kg boxes of recalled almonds were enriched by one of two methods. Salmonella was isolated by at least one method from 42 boxes (84% positive). The levels of Salmonella determined by a three-tube most-probable-number (MPN) method were 8.5+/-1.3 MPN/100 g. In a subsequent study, raw almonds that arrived at almond processors were sampled from 2001 through 2005 to determine the overall prevalence and levels of Salmonella and to characterize the Salmonella isolates obtained. Aerobic plate counts, coliform counts, and MPN levels of Escherichia coli were also determined on positive samples. An isolation frequency for Salmonella of 81 (0.87%+/-0.2%) of 9,274 samples tested (100 g) was determined for raw almonds sampled from throughout California over the 5-year period. Salmonella was not isolated upon retesting in 59 of 65 positive samples. When detected, levels were 1.2 to 2.9 MPN/100 g. Of the 81 total isolates, 35 different serotypes of Salmonella were represented. Aerobic plate counts, coliform counts, and E. coli levels did not correlate with the presence of Salmonella.