Surrogates for validation of electron beam irradiation of foods

Integrated proteomics and metabolomics analysis revealed the mechanisms underlying the effect of irradiation on the fat quality of Chinese bacon

Irradiation increases the security and storage period of preserved Chinese bacon; nevertheless, the biological mechanisms underlying the changes in fat quality caused by irradiation are unknown. We investigated the influence of irradiation on Chinese bacon by proteomic and metabolomic. We identified 24 proteins that participated in metabolism and 40 common differential metabolites enriched in 16 signalling pathways. Correlation analysis revealed that irradiation altered 11 pathways shared between the proteome and metabolome, including two lipid metabolism pathways. Acetyl-CoA carboxylase, ACSL, octanoic acid, decanoic acid, palmitic acid, and oleic acid participated in fatty acid biosynthesis. Acyl-CoA thioesterase 1/2/4, enoyl-CoA reductase, acetyl-CoA acyltransferase 1, enoyl-CoA hydratase 2, palmitic acid, and oleic acid participated in unsaturated fatty acid biosynthesis. These findings lay the groundwork for multi-omics research on the effects of irradiation on Chinese bacon quality, assisting in assessing irradiated Chinese bacon quality, and developing effective strategies to standardise quality parameters.

Surrogate for Electron Beam Inactivation of Salmonella on Pumpkin Seeds and Flax Seeds

ABSTRACT

This study aimed to identify a suitable nonpathogenic surrogate for industrial validation of irradiation process by high-energy electron beam (5 MeV) of dried seeds. Pumpkin seeds (Cucurbita pepo var. styriaca) and golden flax seeds (Linum usitatissimum) were contaminated with a five-strain Salmonella cocktail comprising five serovars or a two-strain Escherichia coli cocktail comprising pathogenic strains, including E. coli O157:H7. Comparison of log survival fractions of the E. coli and Salmonella cocktails revealed that on both types of seeds, the Salmonella cocktail exhibited higher tolerance against high-energy electron beam at doses of 4 kGy than the E. coli cocktail, with a log survival fraction of -4.1 ± 0.7 compared with -6.0 ± 0.2 on pumpkin seeds and -4.7 ± 0.7 compared with reduction from 1.8 × 108 CFU/g to below the limit of detection (1 × 102 CFU/g) on flax seeds. For surrogate selection, the Salmonella cocktail and the strains E. coli DSM 18039 (strain MG1655) and Enterococcus faecium NCCB 86023 (strain NRRL B-2354) were subjected to electron beam processing at doses of 2 to 6 kGy. The calculated D10-values of the Salmonella cocktail were not significantly different (P > 0.05) from those of E. coli DSM 18039, i.e., 1.07 ± 0.10 kGy compared with 1.20 ± 0.07 kGy on pumpkin seeds and 0.88 ± 0.04 kGy compared with 1.07 ± 0.03 kGy on flax seeds. E. faecium NCCB 86023 exhibited significantly higher tolerance on pumpkin seeds (3.07 ± 0.18 kGy) and on flax seeds (2.22 ± 0.29 kGy), ∼3 log and 2 log higher than the Salmonella cocktail, respectively. Hence, the nonpathogenic E. coli DSM 18039 is suggested to serve as a surrogate for Salmonella in industrial validation trials. Because on both types of seeds E. faecium NCCB 86023 showed significantly higher tolerance against electron beam than the Salmonella cocktail, this nonpathogenic strain could serve as a process control indicator for the decontamination of dried seeds by electron beam.

HIGHLIGHTS

Survival of outbreak, food, and environmental strains of Listeria monocytogenes on whole apples as affected by cultivar and wax coating

The 2014–2015 U.S. nationwide outbreak of listeriosis linked to apples used in commercially produced, prepackaged caramel apples was the first implication of whole apples in outbreaks of foodborne illnesses. Two case patients of this outbreak didn’t consume caramel apples but did eat whole apples, suggesting that contaminated whole apple may serve as a vehicle for foodborne listeriosis. The current study evaluated the effect of conventional fruit coating with wax and that of apple cultivar on the survival of outbreak-associated and non-outbreak Listeria monocytogenes strains on Red Delicious, Granny Smith and Fuji apples during 160 days under simulated commercial storage. L. monocytogenes survived in calyxes and stem ends of apples of all 3 cultivars through the duration of the experiment. After 2 months of storage, significantly (p < 0.05) larger L. monocytogenes populations were recovered from apples coated with wax than those un-waxed, regardless of the cultivar. No differences in survival amongst L. monocytogenes strains (serotypes 1/2a and 4b) from clinical, food, and environmental sources were observed. The observation that coating with wax facilitates prolonged survival of L. monocytogenes on whole apples is novel and reveals gaps in understanding of microbiological risks associated with postharvest practices of tree fruit production.

Selection of Surrogate Bacteria for Use in Food Safety Challenge Studies: A Review

Nonpathogenic surrogate bacteria are prevalently used in a variety of food challenge studies in place of foodborne pathogens such as Listeria monocytogenes, Salmonella, Escherichia coli O157:H7, and Clostridium botulinum because of safety and sanitary concerns. Surrogate bacteria should have growth characteristics and/or inactivation kinetics similar to those of target pathogens under given conditions in challenge studies. It is of great importance to carefully select and validate potential surrogate bacteria when verifying microbial inactivation processes. A validated surrogate responds similar to the targeted pathogen when tested for inactivation kinetics, growth parameters, or survivability under given conditions in agreement with appropriate statistical analyses. However, a considerable number of food studies involving putative surrogate bacteria lack convincing validation sources or adequate validation processes. Most of the validation information for surrogates in these studies is anecdotal and has been collected from previous publications but may not be sufficient for given conditions in the study at hand. This review is limited to an overview of select studies and discussion of the general criteria and approaches for selecting potential surrogate bacteria under given conditions. The review also includes a list of documented bacterial pathogen surrogates and their corresponding food products and treatments to provide guidance for future studies.

Immunogenicity of an electron beam inactivated Rhodococcus equi vaccine in neonatal foals

Rhodococcus equi is an important pathogen of foals that causes severe pneumonia. To date, there is no licensed vaccine effective against R. equi pneumonia of foals. The objectives of our study were to develop an electron beam (eBeam) inactivated vaccine against R. equi and evaluate its immunogenicity. A dose of eBeam irradiation that inactivated replication of R. equi while maintaining outer cell wall integrity was identified. Enteral administration of eBeam inactivated R. equi increased interferon-γ production by peripheral blood mononuclear cells in response to stimulation with virulent R. equi and generated naso-pharyngeal R. equi-specific IgA in newborn foals. Our results indicate that eBeam irradiated R. equi administered enterally produce cell-mediated and upper respiratory mucosal immune responses, in the face of passively transferred maternal antibodies, similar to those produced in response to enteral administration of live organisms (a strategy which previously has been documented to protect foals against intrabronchial infection with virulent R. equi). No evidence of adverse effects was noted among vaccinated foals.

Factors affecting radiation D-values (D₁₀) of an Escherichia coli cocktail and Salmonella Typhimurium LT2 inoculated in fresh produce

This study evaluated the effect of produce type, resuspension medium, dose uniformity ratio (DUR), and sample preparation conditions (tissue exposure, MAP, anoxia) on the D₁₀ -value of an Escherichia coli cocktail (BAA-1427, BAA-1428, and BAA-1430) and Salmonella Typhimurium LT2 inoculated on the surfaces of tomato, cantaloupe, romaine lettuce, and baby spinach. Produce at room temperature were irradiated using a 1.35 MeV Van de Graaf electron beam accelerator at 0.2 to 0.9 kGy. The D₁₀-values for E. coli and Salmonella were 0.20 ± 0.01 kGy and 0.14 ± 0.01 kGy, respectively. Bacterial inactivation was not affected by produce type as long as the samples were irradiated in unsealed bags, the bacteria were suspended in broth, and the sample tissue was exposed. Sample location in front of the e-beam source during exposure is crucial. A 20% increase in DUR yielded a 53% change in the D₁₀- values. Variations in sample preparation, microbiological methods and irradiation set-up, result in variable D₁₀-values for different microorganisms on fresh produce.

PRACTICAL APPLICATIONS

Most irradiation studies disregard the effect of sample handling and processing parameters on the determination of the D₁₀-value of different microorganisms in fresh and fresh-cut produce. This study shows the importance of exposure of sample, resuspension medium, available oxygen, and dose uniformity ratio. D₁₀-values can differ by 35% to 53% based on these factors, leading to considerable under- or over-estimation of the irradiation treatment. Results from this study will help to lay firm groundwork for future studies on D₁₀-values determination for different pathogens on fruits and vegetables.

Inactivation of Escherichia coli inoculated onto fresh-cut chopped cabbage using electron-beam processing

During the past decade there were more than 50 reported outbreaks involving leafy green vegetables contaminated with foodborne pathogens. Leafy greens, including cabbage, are fresh foods rarely heated before consumption, which enables foodborne illness. The need for improved safety of fresh food drives the demand for nonthermal food processes to decrease the risk of pathogens while maintaining fresh quality. This study examines the efficacy of electron-beam (e-beam) irradiation in decreasing indigenous microflora on fresh-cut cabbage and determines the optimal dosage to pasteurize fresh-cut cabbage inoculated with Escherichia coli K-12. Fresh-cut cabbage (100 g) was inoculated with ∼8 log E. coli K-12 and e-beam irradiated at doses of 0, 1.0, 2.3, or 4.0 kGy. At 2.3 kGy there was <1.0 log indigenous microflora remaining, indicating greater than a 4.0-log reduction by e-beam. At a 4.0-kGy dose there was >7-log reduction of E. coli K-12 in the fresh-cut cabbage. The D(10)-value for E. coli K-12 in fresh-cut cabbage was 0.564 kGy. E-beam irradiation is thus a viable nonthermal treatment that extends the shelf life and increases the safety of fresh cabbage by reducing or eliminating indigenous microflora and unwanted pathogens.

Inactivation kinetics of Escherichia coli by pulsed electron beam

A novel and compact low-energy (keV) high-power pulsed electron beam (e-beam) that utilizes a secondary emission electron gun (SEEG) was designed and constructed. Escherichia coli JM 109 at a concentration of 10(6) CFU/mL was spread-plated on Luria-Bertani (LB) medium and subjected to the SEEG e-beam. The e-beam was administered as 1 or 5 pulses. The duration of a single pulse was constant at 5 micros, e-beam current density was constant at 25 mA/cm2, and e-beam energy varied between 60 and 82.5 keV. Following treatment with the SEEG e-beam, survivors of the irradiated E. coli samples were enumerated by a standard 10-fold dilution and spread-plated. The survivor curves were plotted on logarithmic scale as a function of e-beam dose. The D10-values were calculated as a negative reciprocal of the slope of the survivor curves. The D10-values for E. coli inactivated with 1- and 5-pulse SEEG e-beam were 0.0026 and 0.0217 Gy, respectively. These D10-values were considerably lower than published D10-values for E. coli inactivated with conventional high-energy continuous e-beam, likely due to shorter exposure time (t), greater current density (J), and a pulse mode of the SEEG e-beam. The SEEG e-beam showed promising results for microbial inactivation in a nonthermal manner; however, due to low energy of the SEEG e-beam, current applications are limited to surface decontamination. The SEEG e-beam may be an efficient processing step for surface inactivation of food-borne pathogens on ready-to-eat products, including fresh and leafy vegetables.