Effect of lactose and milk protein on thermal resistance of Enterococcus faecium NRRL B-2354 and Salmonella in dairy powders

Microbial challenge studies using nonpathogenic surrogates provide a practical means for validating thermally based pathogen controls for low-moisture foods. Because the relative thermal resistance, or kill ratio, of Enterococcus faecium NRRL B-2354 (a nonpathogenic surrogate) to Salmonella is greatly influenced by food composition, this study assessed relative thermal resistance of a five-strain Salmonella cocktail and E. faecium in skim milk powder (SMP), lactose-free skim milk powder (LSMP), 90% milk protein isolate (MPI), and lactose powder (LP). The impact of sugar composition (lactose versus glucose-galactose) on resuscitation of bacterial survivors, by using SMP and LSMP, was also determined. Dairy powders were inoculated with agar-grown cultures, mixed, preequilibrated at 0.25 water activity (aw), ground to achieve homogeneity, reequilibrated, and subjected to isothermal treatment. After enumeration on nonselective differential media, log-linear and Bigelow models were fit to the survivor data via one-step global regression. The aw changes and glass transition temperature were assessed at elevated temperatures by using uninoculated, equilibrated powder samples. Estimated D90°C-values were approximately two times higher for E. faecium (P 0.05) in LSMP. Addition of sugars to recovery media did not influence survivor resuscitation from heat-treated SMP and LSMP, confirming that microbial inactivation was impacted primarily by the thermal treatment, not the recovery step. Thermally induced changes in aw were seen only for LP and MPI, with the glass transition temperature observed only for SMP and MPI. In conclusion, rather than always requiring greater lethality of E. faecium than Salmonella, these findings suggest that sufficient pathogen controls for low-moisture foods can also be validated by thoroughly documenting the appropriate kill ratios of E. faecium to Salmonella

Persistence of Silver Nanoparticles Sorbed on Fresh-Cut Lettuce during Flume Washing and Centrifugal Drying

Increased agricultural use of silver nanoparticles (Ag NPs) may potentially lead to residual levels on fresh produce, raising food safety and public health concerns. However, the ability of typical washing practices to remove Ag NPs from fresh produce is poorly understood. This study investigated the removal of Ag NPs from Ag NP-contaminated lettuce during bench-top and pilot-scale washing and drying. Ag NP removal was first assessed by washing lettuce leaves in a 4-L carboy batch system using water containing chlorine (100 mg/L) or peroxyacetic acid (80 mg/L) with and without a 2.5% organic load and water alone as the control. Overall, these treatments removed only 3–7% of the sorbed Ag from the lettuce. Thereafter, Ag NP-contaminated lettuce leaves were flume-washed for 90 s in a pilot-scale processing line using ∼600 L of recirculating water with or without a chlorine-based sanitizer (100 mg/L) and then centrifugally dried. After processing, only 0.3–3% of the sorbed Ag was removed, probably due to the strong binding of Ag with plant organic materials. Centrifugation only removed a minor amount of Ag as compared to flume washing. However, the Ag concentration in the ∼750 mL of centrifugation water was much higher as compared to the flume water, suggesting that the centrifugation water would be preferred when assessing fresh-cut leafy greens for Ag contamination. These findings indicate that Ag NPs may persist on contaminated leafy greens with commercial flume washing systems unable to substantially reduce Ag NP levels

Listeriosis Risk Model for Cancer Patients Who Consume Ready-to-Eat Salad

The foodborne pathogen Listeria monocytogenes generally infects immunocompromised individuals, such as cancer patients, more frequently and with higher morbidity and mortality than the general population. Because of the anticipated risk associated with L. monocytogenes and other pathogens in produce, immunocompromised individuals are often placed on neutropenic diets that exclude fresh produce, though these risks have not been quantified. Therefore, this study developed a data-driven risk model for listeriosis in cancer patients who consume ready-to-eat (RTE) salads, consisting of leafy greens, cucumbers, and tomatoes, as influenced by kitchen-scale treatments and storage practices. Monte Carlo simulations were used to model the risk of invasive listeriosis during one chemotherapy cycle. Refrigerating all salad components decreased the median risk by approximately one-half log. For refrigerated salads with no treatment, the predicted median risk was ≤ 4.3 × 10−08. When salad ingredients were surface blanched with greens rinsed, the predicted risk decreased to 5.4 × 10−10. Predicted risk was lowest (1.4 × 10−13) for a blanched “salad” consisting of solely cucumbers and tomatoes. Interestingly, rinsing, as recommended by FDA, only decreased the median risk by 1 log. A sensitivity analysis revealed that the highly variable dose-response parameter k strongly influenced risk, indicating that reducing uncertainty in this variable may improve model accuracy. Overall, this study demonstrates that kitchen-scale pathogen reduction approaches have high risk reduction efficacy and could be considered as an alternative to diets that exclude produce when making risk management decisions

Uptake and Accumulation of Pharmaceuticals in Overhead- and Surface-Irrigated Greenhouse Lettuce

Understanding the uptake and accumulation of pharmaceuticals in vegetables under typical irrigation practices is critical to risk assessment of crop irrigation with reclaimed water. This study investigated the pharmaceutical residues in greenhouse lettuce under overhead and soil-surface irrigations using pharmaceutical-contaminated water. Compared to soil-surface irrigation, overhead irrigation substantially increased the pharmaceutical residues in lettuce shoots. The increased residue levels persisted even after washing for trimethoprim, monensin sodium, and tylosin, indicating their strong sorption to the shoots. The postwashing concentrations in fresh shoots varied from 0.05 ± 0.04 μg/kg for sulfadiazine to 345 ± 139 μg/kg for carbamazepine. Root concentration factors ranged from 0.04 ± 0.14 for tylosin to 19.2 ± 15.7 for sulfamethoxazole. Translocation factors in surface-irrigated lettuce were low for sulfamethoxalzole, trimethoprim, monensin sodium, and tylosin (0.07–0.15), but high for caffeine (4.28 ± 3.01) and carbamazepine (8.15 ± 2.87). Carbamazepine was persistent in soil and hyperaccumulated in shoots