Radio-Frequency Processing for Inactivation of Salmonella enterica and Enterococcus faecium NRRL B-2354 in Black Peppercorn

Radiofrequency Inactivation of Salmonella in Black Pepper and Dried Basil Leaves Using In-package Steaming

Radiofrequency (RF) heating has been extensively studied for pasteurizing low-moisture foods. Currently, bulk foods are treated with radiofrequency; potential cross-contamination may occur during packaging of pasteurized products. As an alternative, in-package RF processing was evaluated for Salmonella inactivation on black peppercorns and dried basil leaves and prevention of cross-contamination during storage postprocessing. In-package steaming refers to the process in which the samples were heated in a steam vent package to generate and retain steam during the treatment. This treatment achieved good heating uniformity which could be because of the circulation of steam within the package. One-way steam vent allowed the release of excess steam once a threshold pressure was achieved and later returned to its original position to seal the package, when the RF energy was removed. In-package RF steaming of black peppercorns and dried basil leaves for 135 s and 40 s, respectively, resulted in more than 5 log reduction of Salmonella. The steam vent remained stable posttreatment and properly sealed the package to protect the product from any external contamination. These results indicate that the use of steam vent could effectively pasteurize black peppercorns and dried basil leaves could be beneficial in preventing the potential cross-contamination postprocessing.

Enhanced heat tolerance of freeze-dried Enterococcus faecium NRRL B-2354 as valid Salmonella surrogate in low-moisture foods

In microbial studies of low-moisture foods (LMFs, water activity less than 0.85), freeze-dried bacteria benefit us to inoculate LMFs without introducing extra water or altering food physiochemical properties. However, the freeze-drying process would bring unavoidable damage to bacterial cells and results in less-resistant inoculum that are unlikely to be qualified in microbial studies. Herein, we enhanced bacterial heat tolerance by subjecting the cells to mild heat (42-50 °C) to counteract the reduced heat tolerance and survivability of freeze-dried bacteria. Enterococcus faecium NRRL B-2354 (E. faecium), a Salmonella surrogate in LMFs, was used as the target microorganism because it was widely accepted in microbial validation of thermal pasteurizing LMFs. Three types of LMFs (peanut powder, protein powder, and onion powder) were used as LMFs models to validate the freeze-dried E. faecium in comparison with Salmonella enterica Enteritidis PT 30 (S. Enteritidis) prepared by the traditional aqueous method. The heat tolerance (D65℃ value) of E. faecium increased at all treatments and peaked (+31.48 ± 0.13%) at temperature-time combinations of 45 °C-60 min and 50 °C-5 min. Survivability of freeze-dried inoculum and its heat tolerance retained well within 50 d storage. The freeze-dried E. faecium was prepared in this study brought equal or higher heat tolerance (D85℃ or D75℃) than S. Enteritidis in tested LMFs models. For instance, the D85℃ of freeze-dried E. faecium (heat-treated at 50 °C for 5 min) and S. Enteritidis in whole egg powder are 35.56 ± 1.52 min and 28.41 ± 0.41 min, respectively. The freeze-dried E. faecium with enhanced heat tolerance appears to be a suitable Salmonella surrogate for dry-inoculating LMFs. Our protocol also enables industry-scale production of freeze-dried inoculum by broth-cultivation method combined with mild-heat treatment.

Trends in Food Pathogens Risk Attenuation

Foodborne pathogens represent one of the most dangerous threats to public health along the food chain all over the world. Over time, many methods were studied for pathogen inhibition in food, such as the development of novel packaging materials with enhanced properties for microorganisms' growth inhibition (coatings, films) and the use of emerging technologies, like ultrasound, radio frequency or microwave. The aim of this study was to evaluate the current trends in the food industry for pathogenic microorganisms' inhibition and food preservation in two directions, namely technology used for food processing and novel packaging materials development. Five technologies were discussed in this study, namely high-voltage atmospheric cold plasma (HVACP), High-Pressure Processing (HPP), microwaves, radio frequency (RF) heating and ultrasound. These technologies proved to be efficient in the reduction of pathogenic microbial loads in different food products. Further, a series of studies were performed, related to novel packaging material development, by using a series of antimicrobial agents such as natural extracts, bacteriocins or antimicrobial nanoparticles. These materials proved to be efficient in the inhibition of a wide range of microorganisms, including Gram-negative and Gram-positive bacteria, fungi and yeasts.

Practice and Progress: Updates on Outbreaks, Advances in Research, and Processing Technologies for Low-moisture Food Safety

Large, renowned outbreaks associated with low-moisture foods (LMFs) bring to light some of the potential, inherent risks that accompany foods with long shelf lives if pathogen contamination occurs. Subsequently, in 2013, Beuchat et al. (2013) noted the increased concern regarding these foods, specifically noting examples of persistence and resistance of pathogens in low-water activity foods (LWAFs), prevalence of pathogens in LWAF processing environments, and sources of and preventive measures for contamination of LWAFs. For the last decade, the body of knowledge related to LMF safety has exponentially expanded. This growing field and interest in LMF safety have led researchers to delve into survival and persistence studies, revealing that some foodborne pathogens can survive in LWAFs for months to years. Research has also uncovered many complications of working with foodborne pathogens in desiccated states, such as inoculation methods and molecular mechanisms that can impact pathogen survival and persistence. Moreover, outbreaks, recalls, and developments in LMF safety research have created a cascading feedback loop of pushing the field forward, which has also led to increased attention on how industry can improve LMF safety and raise safety standards. Scientists across academia, government agencies, and industry have partnered to develop and evaluate innovate thermal and nonthermal technologies to use on LMFs, which are described in the presented review. The objective of this review was to describe aspects of the extensive progress made by researchers and industry members in LMF safety, including lessons-learned about outbreaks and recalls, expansion of knowledge base about pathogens that contaminate LMFs, and mitigation strategies currently employed or in development to reduce food safety risks associated with LMFs.

Interlaboratory Evaluation of Enterococcus faecium NRRL B-2354 as a Salmonella Surrogate for Validating Thermal Treatment of Multiple Low-Moisture Foods

ABSTRACT

This multi-institutional study assessed the efficacy of Enterococcus faecium NRRL B-2354 as a nonpathogenic Salmonella surrogate for thermal processing of nonfat dry milk powder, peanut butter, almond meal, wheat flour, ground black pepper, and date paste. Each product was analyzed by two laboratories (five independent laboratories total), with the lead laboratory inoculating (E. faecium or a five-strain Salmonella enterica serovar cocktail of Agona, Reading, Tennessee, Mbandaka, and Montevideo) and equilibrating the product to the target water activity before shipping. Both laboratories subjected samples to three isothermal treatments (between 65 and 100°C). A log-linear and Bigelow model was fit to survivor data via one-step regression. On the basis of D80°C values estimated from the combined model, E. faecium was more thermally resistant (P < 0.05) than Salmonella in nonfat dry milk powder (DEf-80°C, 100.2 ± 5.8 min; DSal-80°C, 28.9 ± 1.0 min), peanut butter (DEf-80°C, 133.5 ± 3.1 min; DSal-80°C, 57.6 ± 1.5 min), almond meal (DEf-80°C, 34.2 ± 0.4 min; DSal-80°C, 26.1 ± 0.2 min), ground black pepper (DEf-80°C, 3.2 ± 0.8 min; DSal-80°C, 1.5 ± 0.1 min), and date paste (DEf-80°C, 1.5 ± 0.0 min; DSal-80°C, 0.5 ± 0.0 min). Although the combined laboratory D80°C for E. faecium was lower (P < 0.05) than for Salmonella in wheat flour (DEf-80°C, 9.4 ± 0.1 min; DSal-80°C, 10.1 ± 0.2 min), the difference was ∼7%. The zT values for Salmonella in all products and for E. faecium in milk powder, almond meal, and date paste were not different (P > 0.05) between laboratories. Therefore, this study demonstrated the impact of standardized methodologies on repeatability of microbial inactivation results. Overall, E. faecium NRRL B-2354 was more thermally resistant than Salmonella, which provides support for utilizing E. faecium as a surrogate for validating thermal processing of multiple low-moisture products. However, product composition should always be considered before making that decision.

HIGHLIGHTS

Induction of the Viable but Non-Culturable State in Salmonella Contaminating Dried Fruit

Salmonella can become viable but nonculturable (VBNC) in response to environmental stressors, but the induction of the VBNC state in Salmonella contaminating ready-to-eat dried fruit is poorly characterized. Dried apples, strawberries, and raisins were mixed with a five-strain cocktail of Salmonella at 4% volume per weight of dried fruit at 10⁹ CFU/g. The inoculated dried fruit were then dried in desiccators at 25°C until the water activity (a_w) approximated that of the uninoculated dried fruit. However, Salmonella could not be recovered after drying, not even after enrichment, suggesting a population reduction of approximately 8 log CFU/g. To assess the potential impact of storage temperature on survival, dried apples were spot-inoculated with the Salmonella cocktail, dried under ambient atmosphere at 25°C, and stored at 4 and 25°C. Spot inoculation permitted recovery of Salmonella on dried apple after drying, with the population of Salmonella decreasing progressively on dried apples stored at 25°C until it was undetectable after about 46 days, even following enrichment. The population decline was noticeably slower at 4°C, with Salmonella being detected until 82 days. However, fluorescence microscopy and laser scanning confocal microscopy with the LIVE/DEAD BacLight bacterial viability system at time points at which no Salmonella could be recovered on growth media even following enrichment showed that a large proportion (56 to 85%) of the Salmonella cells on the dried fruit were viable. The data suggest that the unique combination of stressors in dried fruit can induce large numbers of VBNC cells of Salmonella.

IMPORTANCESalmonella is a leading foodborne pathogen globally causing numerous outbreaks of foodborne illnesses and remains the leading contributor to deaths attributed to foodborne disease in the United States and other industrialized nations. Therefore, efficient detection methods for Salmonella contaminating food are critical for public health and food safety. Culture-based microbiological methods are considered the gold standard for the detection and enumeration of Salmonella in food. Findings from this study suggest that unique stressors on dried fruit can induce the VBNC state in Salmonella, thus rendering it undetectable with culture-based methods even though the bacteria remain viable. Therefore, strong consideration should be given to using, in addition to culture-based methods, microscopic and molecular methods for the accurate detection of all viable and/or culturable cells of Salmonella contaminating dried fruit, as all of these cells have the potential to cause human illness.

Review of proposed different irradiation methods to inactivate food-processing viruses and microorganisms

Coronaviruses, which have been enveloped nonsegmented positive-sense RNA viruses, were first mentioned in the mid-1960s and can attack people as well as a wide range of animals (including mammals and birds). Three zoonotic coronaviruses have been identified as the cause of large-scale epidemics over the last two decades: Middle East respiratory syndrome (MERS), severe acute respiratory syndrome (SARS), and swine acute diarrhea syndrome (SADS). Epithelial cells in the respiratory and gastrointestinal tract are the principal targeted cells, and viral shedding occurs via these systems in diverse ways such as through fomites, air, or feces. Patients infected with the novel coronavirus (2019-nCoV) reported having visited the Wuhan seafood wholesale market shortly before disease onset. The clinical data on established 2019-nCoV cases reported so far indicate a milder disease course than that described for patients with SARS-CoV or MERS-CoV. This study aimed to review radiation inactivation of these viruses in the food industry in three sections: visible light, ionizing radiation (alpha ray, beta ray, X-ray, gamma ray, neutron, plasma, and ozone), and nonionizing radiation (microwave, ultraviolet, infrared, laser light, and radiofrequency). Due to the obvious possibility of human-to-human and animal-to-human transmission, using at least one of these three methods in food processing and packaging during coronavirus outbreaks will be critical for preventing further outbreaks at the community level.

Thermal Inactivation Kinetics of Salmonella and Enterococcus faecium NRRL-B2354 on Whole Chia Seeds (Salvia hispanica L.)

ABSTRACT

Intervention technologies for inactivating Salmonella on whole chia seeds are currently limited. Determination of the thermal inactivation kinetics of Salmonella on chia seeds and selection of an appropriate nonpathogenic surrogate will provide a foundation for selecting and optimizing thermal pasteurization processes for chia seeds. In this study, chia seed samples from three separate production lots were inoculated with a five-strain Salmonella cocktail or Enterococcus faecium NRRL-B2354 and equilibrated to a water activity of 0.53 at room temperature (25°C). After equilibration for at least 3 days, the inoculated seeds were subjected to isothermal treatments at 80, 85, or 90°C. Samples were removed at six time points, and surviving bacteria were enumerated. Whole chia seeds were diluted in a filter bag at 1:30 because bacterial recovery with this method was similar to that obtained from ground seeds. Survivor data were fitted to consolidated models: one primary model (log linear or Weibull) and one secondary model (Bigelow). E. faecium had higher thermal resistance than did Salmonella, suggesting that E. faecium may be a suitable conservative nonpathogenic surrogate for Salmonella. The Weibull model was a better fit for the survivor data than was the log-linear model for both bacteria based on the lower root mean square error and corrected Akaike's information criterion values. Lipid oxidation measurements and fatty acid concentrations were significantly different from those of the control samples, but the overall magnitude of the differences was relatively small. The thermal inactivation kinetics of Salmonella and E. faecium on chia seeds may be used as a basis for developing thermal pasteurization processes for chia seeds.

HIGHLIGHTS

Validation of process technologies for enhancing the safety of low-moisture foods: A review

The outbreaks linked to foodborne illnesses in low-moisture foods are frequently reported due to the occurrence of pathogenic microorganisms such as Salmonella Spp. Bacillus cereus, Clostridium spp., Cronobacter sakazakii, Escherichia coli, and Staphylococcus aureus. The ability of the pathogens to withstand the dry conditions and to develop resistance to heat is regarded as the major concern for the food industry dealing with low-moisture foods. In this regard, the present review is aimed to discuss the importance and the use of novel thermal and nonthermal technologies such as radiofrequency, steam pasteurization, plasma, and gaseous technologies for decontamination of foodborne pathogens in low-moisture foods and their microbial inactivation mechanisms. The review also summarizes the various sources of contamination and the factors influencing the survival and thermal resistance of pathogenic microorganisms in low-moisture foods. The literature survey indicated that the nonthermal techniques such as CO₂ , high-pressure processing, and so on, may not offer effective microbial inactivation in low-moisture foods due to their insufficient moisture content. On the other hand, gases can penetrate deep inside the commodities and pores due to their higher diffusion properties and are regarded to have an advantage over thermal and other nonthermal processes. Further research is required to evaluate newer intervention strategies and combination treatments to enhance the microbial inactivation in low-moisture foods without significantly altering their organoleptic and nutritional quality.

Heating of milk powders at low water activity to 95°C for 15 minutes using hot air-assisted radio frequency processing achieved pasteurization

Salmonella persistence in milk powders has caused several multistate foodborne disease outbreaks. Therefore, ways to deliver effective thermal treatment need to be identified and validated to ensure the microbial safety of milk powders. In this study, a process of hot air-assisted radio frequency (HARF) followed by holding at high temperatures in a convective oven was developed for pasteurization of milk powders. Heating times were compared between HARF and a convection oven for heating milk powders to a pasteurization temperature, and HARF has been shown to considerably reduce the come-up time. Whole milk powder (WMP) and nonfat dry milk (NFDM) were inoculated with a 5-serotype Salmonella cocktail and equilibrated to a water activity of 0.10 to simulate the worst case for the microbial challenge study. After heating the sample to 95°C using HARF, followed by 10 and 15 min of holding in the oven, more than 5 log reduction of Salmonella was achieved in WMP and NFDM. This study validated a HARF-assisted thermal process for pasteurization of milk powder based on previously collected microbial inactivation kinetics data and provides valuable insights to process developers to ensure microbial safety of milk powder. This HARF process may be implemented in the dairy industry to enhance the microbial safety of milk powders.

Control of Salmonella in low-moisture foods: Enterococcus faecium NRRL B-2354 as a surrogate for thermal and non-thermal validation

Salmonella has been implicated in multiple foodborne outbreaks and recalls associated with low water activity foods (La_wF). To verify the effectiveness of a process against Salmonella in La_wF, validation using a nonpathogenic surrogate strain is essential. Enterococcus faecium NRRL B-2354 strain has been used as a potential surrogate of Salmonella in different processing of La_wF. However, the survival of Salmonella and E. faecium in La_wF during food processing is a dynamic function of a_w, food composition and structure, processing techniques, and other factors. This review assessed pertinent literature on the thermal and non-thermal inactivation of Salmonella and its presumable surrogate E. faecium in various La_wF and provided an overview of its suitibility in different La_wF. Overall, based on the D-values, survival/reduction, temperature/time to obtain 4 or 5-log reductions, most studies concluded that E. faecium is a suitable surrogate of Salmonella during La_wF processing as its magnitude of resistance was slightly greater or equal (i.e., statistical similar) as compared to Salmonella. Studies also showed its unsuitability which either does not provide a proper margin of safety or being overly resistant and may compromise the quality and organoleptic properties of food. This review provides useful information and guidance for future validation studies of La_wF.

A Comparison of Three Methods for Determining Thermal Inactivation Kinetics: A Case Study on Salmonella enterica in Whole Milk Powder

ABSTRACT

Different methods for determining the thermal inactivation kinetics of microorganisms can result in discrepancies in thermal resistance values. In this study, thermal resistance of Salmonella in whole milk powder was determined with three methods: thermal death time (TDT) disk in water bath, pouches in water bath, and the TDT Sandwich system. Samples from three production lots of whole milk powder were inoculated with a five-strain Salmonella cocktail and equilibrated to a water activity of 0.20. The samples were then subjected to three isothermal treatments at 75, 80, or 85°C. Samples were removed at six time points and cultures were enumerated for survivors. The inactivation data were fitted to two consolidated models: two primary models (log linear and Weibull) and one secondary model (Bigelow). Normality testing indicated that all the model parameters were normally distributed. None of the model parameters for both consolidated models were significantly different (α = 0.05). The amount of inactivation during the come-up time was also not significantly different among the methods (α = 0.05). However, the TDT Sandwich resulted in less inactivation during the come-up time and overall less variation in model parameters. The survivor data from all three methods were combined and fitted to both consolidated models. The Weibull had a lower root mean square error and a better fit, according to the corrected Akaike's information criterion. The three thermal treatment methods produced results that were not significantly different; thus, the methods are interchangeable, at least for Salmonella in whole milk powder. Comparisons with more methods, other microorganisms, and larger varieties of food products using the same framework presented in this study could provide guidance for standardizing thermal inactivation kinetics studies for microorganisms in foods.

HIGHLIGHTS

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

Effect of water activity on the thermal inactivation kinetics of Salmonella in milk powders

Persistence of Salmonella in milk powders has caused several foodborne outbreaks. The determination of proper pasteurization processing conditions requires an understanding of the thermal inactivation kinetics of Salmonella in milk powders. However, there is a lack of knowledge related to the effects of water activity (a_w) and fat content on Salmonella inactivation in milk powder during thermal processing. Two types of milk powders, nonfat dry milk and whole milk powder, with different fat contents (0.62 and 29.46% wt/wt, respectively) were inoculated with a 5-strain cocktail of Salmonella and equilibrated to 3 a_w levels (0.10, 0.20, and 0.30) for isothermal treatments at 75, 80, and 85°C to obtain D-values (the time required to achieve a 10-fold reduction of the bacteria at the isothermal treatment temperature) and z-values (the increase in temperature required to achieve a 90% reduction of the decimal reduction time D). Stability tests showed that the inoculation method used in this study provided a high and stable population of Salmonella for thermal inactivation studies. A moisture sorption isotherm was measured to understand the relationship between a_w and moisture content of milk powders. The thermal resistance of Salmonella was found to significantly increase as a_w decreased, which suggested that a higher temperature or longer processing time would be required at low a_w to achieve the desired inactivation of Salmonella. The microbial inactivation kinetics were not significantly different for the 2 milk powders; therefore, data were combined to develop a universal model. A response surface model was compared with a modified Bigelow model. The modified Bigelow model performed well to predict D-values [root mean square error (RMSE) = 1.47 min] and log reductions (RMSE = 0.48 log cfu/g). The modified Bigelow model developed here could be used to estimate D-value as a function of water activity and temperature to design a thermal pasteurization system for milk powders.

A comprehensive review on recent developments of radio frequency treatment for pasteurizing agricultural products

Recent pathogen incidents have forced food industry to seek for alternative processes in postharvest pasteurization of agricultural commodities. Radio frequency (RF) heating has been used as one alternative treatment to replace chemical fumigation and other conventional thermal methods since it is relatively easy to apply and leaves no chemical residues. RF technology transfers electromagnetic energy into large bulk volume of the products to provide a fast and volumetric heating. There are two types of RF technology commonly applied in lab and industry to generate the heat energy: free running oscillator and 50-Ω systems. Several reviews have been published to introduce the application of RF heating in food processing. However, few reviews have a comprehensive summary of RF treatment for pasteurizing agricultural products. The objective of this review was to introduce the developments in the RF pasteurization of agricultural commodities and to present future directions of the RF heating applications. While the recent developments in the RF pasteurization were presented, thermal death kinetics of targeted pathogens as influenced by water activity, pathogen species and heating rates, non-thermal effects of RF heating, combining RF heating with other technologies for pasteurization, RF heating uniformity improvements using computer simulation and development of practical RF pasteurization processes were also focused. This review is expected to provide a comprehensive understanding of RF pasteurization for agricultural products and promote the industrial-scale applications of RF technology with possible process protocol optimization purposes.

Radiofrequency heating for powder pasteurization of barley grass: antioxidant substances, sensory quality, microbial load and energy consumption

BACKGROUND

Young barley grass powder contains abundant nutrition and its antioxidant substances are severely impaired by radiation (⁶⁰ Co) sterilization. To overcome product quality degradation, radiofrequency pasteurization was conducted using pilot-scale radiofrequency equipment (27 MHz, 6 kW) with electrode gaps of 12, 14 and 16 cm, while hot-air (80 °C) pasteurization was used for comparison.

RESULTS

Assessment suggested that uneven radiofrequency heating was improved for the 14 cm electrode gap. With an increase of electrode gap, microbial inactivation needs more energy consumption. A minimum energy consumption of 970 J g^-1 was required for 1 log-reduction of colonies. Radiofrequency pasteurization retained better antioxidant substances, lightness (L*), green color (a*) and odors in barley grass powder, compared with hot-air sterilization. Contents of flavonoid and chlorophyll were 5.82 and 4.87 g kg^-1 respectively, using the 14 cm electrode gap. Additionally, radiofrequency pasteurization led to an improvement in sourness, bitterness and umami tastes.

CONCLUSIONS

Radiofrequency pasteurization would be a superior alternative for the pasteurization of barley grass powder. © 2019 Society of Chemical Industry.