Combination treatment of ohmic heating with various essential oil components for inactivation of food-borne pathogens in buffered peptone water and salsa

Synergistic bactericidal mechanisms of RF energy simultaneously combined with cinnamon essential oil or epsilon-polylysine against Salmonella revealed at cellular and metabolic levels

Radio frequency (RF) heating and antimicrobials are considered to be effective methods for inactivating food pathogens. This study explored the bactericidal effects against Salmonella of RF heating combined with two kinds of natural antimicrobials possessing different hydrophobic properties and their synergistic bactericidal mechanisms. Results showed that RF heating caused sublethal damage to bacterial cells and enhanced the interaction of cells and antimicrobials, leading to synergistic bactericidal effects of the simultaneous combination of RF heating and antimicrobials. The combination of RF heating and ε-polylysine (ε-PL) further promoted cell morphological alteration, raised membrane permeability, intracellular adenosine triphosphate (ATP) leakage and intracellular reactive oxygen species (ROS) accumulation compared to individual treatment. The simultaneous combination of RF heating and cinnamon essential oil nanoemulsion (CEON) also further enhanced membrane permeability and ROS accumulation compared to individual treatment, but impacts were less than those in the combination of RF heating and ε-PL. The major synergistic bactericidal mechanism of RF heating and CEON was significantly inhibiting intracellular ATP synthesis. The untargeted metabolomics analysis revealed that the combined treatments enhanced disturbances to multiple intracellular metabolisms compared to individual treatment, thus leading to synergistic bactericidal effects against Salmonella. These results provide an in-depth understanding of the synergistic bactericidal mechanisms of the combination of RF heating and natural antimicrobials from cellular and metabolic levels.

Perspectives on Novel Technologies of Processing and Monitoring the Safety and Quality of Prepared Food Products

With the changes of lifestyles and rapid growth of prepared food industry, prepared fried rice that meets the consumption patterns of contemporary young people has become popular in China. Although prepared fried rice is convenient and nutritious, it has the following concerns in the supply chain: (1) susceptible to contamination by microorganisms; (2) rich in starch and prone to stall; and (3) vegetables in the ingredients have the issues of water loss and discoloration, and meat substances are vulnerable to oxidation and deterioration. As different ingredients are used in prepared fried rice, their food processing and quality monitoring techniques are also different. This paper reviews the key factors that cause changes in the quality of prepared fried rice, and the advantages and limitations of technologies in the processing and monitoring processes. The processing technologies for prepared fried rice include irradiation, high-voltage electric field, microwave, radio frequency, and ohmic heating, while the quality monitoring technologies include Raman spectral imaging, near-infrared spectral imaging, and low-field nuclear magnetic resonance technology. These technologies will serve as the foundation for enhancing the quality and safety of prepared fried rice and are essential to the further development of prepared fried rice in the emerging market.

Assessment of trans-cinnamaldehyde and eugenol assisted heat treatment against Salmonella Typhimurium in low moisture food components

Increased thermal resistance of Salmonella at low water activity (a_w) is a significant food safety concern in low-moisture foods (LMFs). We evaluated whether trans-cinnamaldehyde (CA, 1000 ppm) and eugenol (EG, 1000 ppm), which can accelerate thermal inactivation of Salmonella Typhimurium in water, can show similar effect in bacteria adapted to low a_w in different LMF components. Although CA and EG significantly accelerated thermal inactivation (55 °C) of S. Typhimurium in whey protein (WP), corn starch (CS) and peanut oil (PO) at 0.9 a_w, such effect was not observed in bacteria adapted to lower a_w (0.4). The matrix effect on bacterial thermal resistance was observed at 0.9 a_w, which was ranked as WP > PO > CS. The effect of heat treatment with CA or EG on bacterial metabolic activity was also partially dependent on the food matrix. Bacteria adapted to lower a_w had lower membrane fluidity and unsaturated to saturated fatty acids ratio, suggesting that bacteria at low a_w can change its membrane composition to increase its rigidity, thus increasing resistance against the combined treatments. This study demonstrates the effect of a_w and food components on the antimicrobials-assisted heat treatment in LMF and provides an insight into the resistance mechanism.

Synergistic effects of 915 MHz microwave heating and essential oils on inactivation of foodborne pathogen in hot-chili sauce

Essential oil is a food additive with antimicrobial properties but with limitations due to strong organoleptic properties. However, thermal treatments can be applied to reduce essential oil content while ensuring antimicrobial activities in food matrices. In this study, the inactivation efficiency of essential oils on E. coli O157:H7, Salmonella Typhimurium and Listeria monocytogenes in buffered peptone water (BPW) and hot-chili sauce was evaluated when coupled with 915 MHz microwave heating. Essential oils used in this study did not affect the dielectric properties and further heating rate of BPW and hot-chili sauce. The dielectric constant of BPW was 76.3 and dielectric loss factor was 30.9. In addition, it took 85 s to reach 100 °C for all samples. Among essential oils, synergistic microbial inactivation with microwave heating was observed from carvacrol (CL) and citral (CI), but not from eugenol (EU) and Carvone (CN). Specifically, CL and microwave heating (M) for 45 s showed the most effective inactivation (ca. 6 log reduction) for the pathogens in BPW. Similar trends were shown in hot-chili sauce. However, M + CI inactivation did not show synergistic effects in hot-chili sauce. Microwave heating time for hot-chilis sauce was 40 s. In propidium iodide uptake study, M + CL was found to cause most severe damage to cell membrane (758.5 of PI value for E. coli O157:H7) while M + CU and M + CN had little impact. In DiBAC₄(3) test, CL resulted in the largest value (2.09 for E. coli O157:H7). These observations highlight that CL induces synergistic effects as it caused severe membrane damage along with destruction of membrane potential. The combined treatment did not show any significant difference in quality change compared to untreated hot-chili sauce (p > 0.05). The result indicates the potential application of CL and M combination for hot-chili sauce processes to ensure microbiological safety with acceptable quality.

Descriptive study of foodborne disease using disease monitoring data in Zhejiang Province, China, 2016-2020

Background

This study aimed to identify the epidemiology, seasonality, aetiology and clinical characteristics of sporadic foodborne diseases in Zhejiang province during 2016–2020.

Methods

Descriptive statistical methods were used to analyze the data from surveillance network established by the Zhejiang Provincial Center for Disease Control and Prevention. There were 31 designated hospitals in all 11 cities which were selected using probability proportionate to size sampling method.

Results

During the study period, the surveillance system received 75,124 cases with 4826 (6.42%) hospitalizations from 31 hospitals. The most common cause was Norovirus, 6120 cases (42.56%), followed by Salmonella, 3351 cases (23.30%). A significant seasonal trend was observed for the V. parahaemolyticus, with the highest rates over the summer period, peaking in August, 1171 cases (38.75%), a similar trend was also observed with Salmonella and Diarrheagenic E. coli. Norovirus infections showed the highest rate in November (904, 14.77%) and March (660,10.78%), the lowest in August, 215 cases (3.51%). Patients between 19 ~ 40 years were more likely to infected by Norovirus, V. parahaemolyticus and Diarrheagenic E. coli, patients below 1 year were the highest among patients with Salmonella infection, 881 cases (26.3%). The Norovirus, V. parahaemolyticus and Diarrheagenic E. coli infection with the highest positive detection rates among the workers were observed. The largest number cases of food categories were from aquatic product infection. The private home was the most common exposure setting.

Conclusion

Our study highlighted the necessity for conducting an active, comprehensive surveillance for pathogens in all age groups, to monitor the changing dynamics in the epidemiology and aetiology of foodborne diseases to guide policies that would reduce related illnesses.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12889-022-14226-1.

Inactivation of Salmonella enterica Serovar Typhimurium and Staphylococcus aureus in Rice by Radio Frequency Heating

ABSTRACT

The objectives of this study were to determine the effect of the milling degree (MD) of rice (Oryza sativa L.) on the heating rate, pathogen inactivation (Salmonella Typhimurium and Staphylococcus aureus), and color change resulting from radio frequency (RF) heating. Rice samples inoculated with pathogens were placed in a polypropylene jar and subjected to RF heating for 0 to 75 s. The heating rate of rice with a 2% MD was the highest during RF heating, followed by those with a 0, 8, and 10% MD; the reduction of pathogens showed the same trend. The reductions of pathogen levels in rice with MDs of 0 and 2% were significantly higher than those observed for rice with MDs of 8 and 10% under the same treatment conditions. For example, log reductions of Salmonella Typhimurium in rice by 55-s RF heating were 3.64, 5.19, 2.18, and 1.80 for MDs of 0, 2, 8, and 10%, respectively. At the same treatment conditions, log reductions of S. aureus were 2.77, 5.08, 1.15, and 0.90 for MDs of 0, 2, 8, and 10%, respectively. The color of rice measured according to L*, a*, and b* was not significantly altered after RF heating, regardless of the MD. Therefore, the MD of rice should be considered before RF heating is applied to inactivate foodborne pathogens.

HIGHLIGHTS

Recent (2011-2017) foodborne outbreak cases in the Republic of Korea compared to the United States: a review

This study analyzes and compares foodborne disease outbreaks reported in the Republic of Korea (KR) and the United States (US) during 2011-2017. The foodborne outbreaks data in the KR and the US were collected from the Ministry of Food and Drug Safety and from the Surveillance for Foodborne Disease Outbreaks United States, respectively. The average number of outbreaks and illness population were higher in the US than in the KR, but the KR's illness ratio considering population size was 2.4 times higher than that of the US. When the sites of outbreaks compared, the number of illness was the highest at schools in the KR whereas outbreaks at restaurants were more frequent in the US. In the KR, bacterial infections were the primary cause of outbreaks while bacterial and viral infections accounted for the largest share of outbreaks in the US. Specifically, pathogenic E. coli presented a significant risk in the KR whereas Salmonella was the most prevalent in the US. These results indicate that the main microbiological targets for detection and control in the KR should differ from the US, which should be considered for developing food safety related policies.

Comparative transcriptomic study of Escherichia coli O157:H7 in response to ohmic heating and conventional heating

In this study, the high-throughput Illumina HiSeq 2000 mRNA sequencing technique was used to investigate the transcriptome response of Escherichia coli O157:H7 exposed to ohmic heating (OH) and water bath heating (WB). Compared to untreated samples, a total of 293, 516, and 498 genes showed differential expression after HVOH (high voltage short time ohmic heating), LVOH (low voltage long time ohmic heating), and WB, respectively. Therefore, LVOH had the potential to cause comparable effects on the transcriptome of E. coli O157:H7 as compared to WB, but not HVOH. These results indicated that additional non-thermal effects were not reflected on transcriptome of E. coli O157:H7 using both HVOH and LVOH, in particular the HVOH. Most of differentially expressed genes involved in information storage and processing, and cellular processes and signaling showed up-regulation whereas most of genes related to the metabolism were down-regulated after HVOH, LVOH, and WB. In addition, more attention needs to be paid to the up-regulation of a large number of virulence genes, which might increase the ability of surviving E. coli O157:H7 to infect host cells after HVOH, LVOH, and WB. This transcriptomic study on the response of E. coli O157:H7 to OH protomes the understanding of inactivation mechanism of OH on the molecular level and opens the door to future studies for OH.

Bactericidal and synergistic effects of X-ray irradiation and gallic acid against foodborne pathogens on lettuce

The objectives of this study were to evaluate the bactericidal effects of X-ray irradiation and gallic acid (GA) against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on lettuce leaves and in phosphate-buffered saline (PBS). Inoculated PBS and lettuce were exposed to X-rays (0.05, 0.1, and 0.15; 0.1, 0.2, and 0.3 kGy, respectively), and GA was applied to lettuce leaves as a solution and in PBS at concentrations of 0.5% (w/v). Combined treatment with 0.3 kGy and 0.5% GA reduced E. coli O157:H7, S. Typhimurium, and L. monocytogenes cell counts 5.41, 2.57, and 1.36 log CFU/cm2 on lettuce, respectively. Combined treatment with 0.15 kGy X-ray and 0.5% GA reduced counts for the same species by 6.54, 4.24, and 1.51 log CFU/mL in PBS. The combined treatments exerted a synergistic antibacterial effect against E. coli O157:H7 on lettuce, but not against S. Typhimurium or L. monocytogenes. In PBS, the synergistic effect was confirmed in both E. coli O157:H7 and S. Typhimurium cells. Mechanistic investigations indicated that the synergistic antibacterial effect was associated with intracellular reactive oxygen species (ROS) generation and bacterial cell membrane damage. Additionally, the X-ray and GA combination treatment did not adversely affect the color, total phenol content, and texture of lettuce. These findings demonstrate that treatment with X-ray radiation and GA can enhance the microbiological safety of fresh produce.

Inactivation Kinetics and Membrane Potential of Pathogens in Soybean Curd Subjected to Pulsed Ohmic Heating Depending on Applied Voltage and Duty Ratio

The aim of this research was to investigate the efficacy of the duty ratio and applied voltage in the inactivation of pathogens in soybean curd by pulsed ohmic heating (POH). The heating rate of soybean curd increased rapidly as the applied voltage increased, although the duty ratio did not affect the temperature profile. We supported this result by verifying that electrical conductivity increased with the applied voltage. Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, and Listeria monocytogenes in soybean curd were significantly (P < 0.05) inactivated by more than 1 log unit at 80 Vrms (root mean square voltage). To elucidate the mechanism underlying these results, the membrane potential of the pathogens was examined using DiBAC4(3) [bis-(1,3-dibutylbarbituric acid)trimethine oxonol] on the basis of a previous study showing that the electric field generated by ohmic heating affected the membrane potential of cells. The values of DiBAC4(3) accumulation increased under increasing applied voltage, and they were significantly (P < 0.05) higher at 80 Vrms, while the duty ratio had no effect. In addition, morphological analysis via transmission electron microscopy showed that electroporation and expulsion of intracellular materials were predominant at 80 Vrms Moreover, electrode corrosion was overcome by the POH technique, and the textural and color properties of soybean curd were preserved. These results substantiate the idea that the applied voltage has a profound effect on the microbial inactivation of POH as a consequence of not only the thermal effect, but also the nonthermal effect, of the electric field, whereas the duty ratio does not have such an effect.IMPORTANCE High-water-activity food products, such as soybean curd, are vulnerable to microbial contamination, which causes fatal foodborne diseases and food spoilage. Inactivating microorganisms inside food is difficult because the transfer of thermal energy is slower inside than it is outside the food. POH is an adequate sterilization technique because of its rapid and uniform heating without causing electrode corrosion. To elucidate the electrical factors associated with POH performance in the inactivation of pathogens, the effects of the applied voltage and duty ratio on POH were investigated. In this study, we verified that a high applied voltage (80 Vrms) at a duty ratio of 0.1 caused thermal and nonthermal effects on pathogens that led to an approximately 4-log-unit reduction in a significantly short time. Therefore, the results of this research corroborate database predictions of the inactivation efficiency of POH based on pathogen control strategy modeling.

Inactivation of foodborne pathogens by the synergistic combinations of food processing technologies and food-grade compounds

There is a need to develop food processing technologies with enhanced antimicrobial capacity against foodborne pathogens. While considering the challenges of adequate inactivation of pathogenic microorganisms in different food matrices, the emerging technologies are also expected to be sustainable and have a minimum impact on food quality and nutrients. Synergistic combinations of food processing technologies and food-grade compounds have a great potential to address these needs. During these combined treatments, food processes directly or indirectly interact with added chemicals, intensifying the overall antimicrobial effect. This review provides an overview of the combinations of different thermal or nonthermal processes with a variety of food-grade compounds that show synergistic antimicrobial effect against pathogenic microorganisms in foods and model systems. Further, we summarize the underlying mechanisms for representative combined treatments that are responsible for the enhanced microbial inactivation. Finally, regulatory issues and challenges for further development and technical transfer of these new approaches at the industrial level are also discussed.

Mathematical modeling of ohmic heating for inactivation of acid-adapted foodborne pathogens in tomato juice

The objective of the present study was to predict the inactivation trends of acid-adapted foodborne pathogens in tomato juice by ohmic heating through a numerical analysis method. The mathematical model based on finite element method (FEM) was used to simulate the multiphysics phenomena including electric heating, heat transfer, fluid dynamics, and pathogen inactivation. A cold spot was observed in the corner part of the ohmic heating chamber, where some pathogens survived even though all pathogens were inactivated elsewhere. Challenges of this study were how to reflect the increased resistance of pathogen by acid-adaptation. After simulation, we verified that inactivation level of acid-adapted foodborne pathogens by 25 Vrms/cm ohmic heating (1 kHz), predicted with the developed mathematical model, had no significant differences with experimental results (p > 0.05). Therefore, the mathematical approaches described in the present study will help juice processors determine the processing conditions necessary to ensure microbial safety at the cold point of a rectangular type batch ohmic heater.

Combination of mild heat and plant essential oil constituents to inactivate resistant variants of Escherichia coli in buffer and in coconut water

The growing demand for minimally processed foods with clean labels has stimulated research into mild processing methods and natural antimicrobials to replace intensive heating and conventional preservatives, respectively. However, we have previously demonstrated that repetitive exposure of some bacteria to mild heat or subinhibitory concentrations of essential oil constituents (EOCs) may induce the emergence of mutants with increased resistance to these treatments. Since the combination of mild heat with some EOCs has a synergistic effect on microbial inactivation, we evaluated the potential of such combinations against our resistant E. coli mutants. While citral, carvacrol and t-cinnamaldehyde synergistically increased heat inactivation (53.0 °C, 10 min) of the wild-type MG1655 suspended in buffer, only the combination with carvacrol (200 μl/l) was able to mitigate the increased resistance of all the mutants. Moreover, the combination of heat and carvacrol acted synergistically inactivating heat-resistant variants of E. coli O157:H7 (ATCC 43888). This combined treatment could synergistically achieve more than 5 log₁₀ reductions of the most resistant mutants in coconut water, although the temperature had to be raised to 57.0 °C. Therefore, the combination of mild heat with carvacrol appears to hold promise for mild processing, and it is expected to counteract the development of heat resistance.

Influence of Antimicrobial Agents on the Thermal Sensitivity of Foodborne Pathogens: A Review

HIGHLIGHTS

Consumers are demanding more natural and organic foods and ingredients. Many additives alone are insufficient to inactivate pathogens or prevent growth. Mild heat combined with antimicrobial agents synergistically inactivate pathogens. Here we review studies using thermally assisted antimicrobial inactivation. Many opportunities exist for the application of this hurdle technology in foods.

Combination effect of saturated or superheated steam and lactic acid on the inactivation of Escherichia coli O157:H7, Salmonella Typhimurium and Listeria monocytogenes on cantaloupe surfaces

The purpose of this study was to evaluate the effectiveness of the combination treatment of lactic acid immersion and saturated or superheated steam (SHS) on inactivation of foodborne pathogens on cantaloupes. Saturated steam (SS) treatments were performed at 100 °C, while SHS treatments were delivered at either 150 or 200 °C. Escherichia coli O157:H7, Salmonella Typhimurium and Listeria monocytogenes-inoculated cantaloupes were exposed to 2% lactic acid or sterile distilled water for 1 min followed by a maximum of 20 s of SS or SHS. Populations of each of the three pathogens on cantaloupes were reduced to under the detection limit (1.0 log CFU/cm²) after the combination treatment of 2% lactic acid and 200 °C steam for 20 s. To compare the effect of the lactic acid treatment method, we conducted spray application with 2% lactic acid combined with SS or SHS treatment; however, no significant log reduction differences were found between immersion and spraying techniques. After combination treatment of cantaloupes for 20 s, color and maximum load values (a characteristic of texture) were not significantly different from those of untreated controls. The results of this study suggest that the combination treatment of lactic acid and SHS can be used as an antimicrobial intervention for cantaloupes without inducing quality deterioration.

New Insights Into the Response of Metabolome of Escherichia coli O157:H7 to Ohmic Heating

The objective of this study was to investigate the effects of ohmic heating and water bath heating (WB) on the metabolome of Escherichia coli O157:H7 cells at the same inactivation levels. Compared to low voltage long time ohmic heating (5 V/cm, 8.50 min, LVLT) and WB (5.50 min), the high voltage short time ohmic heating (10 V/cm, 1.75 min, HVST) had much shorter heating time. Compared to the samples of control (CT), there were a total of 213 differential metabolites identified, among them, 73, 78, and 62 were presented in HVST, LVLT, and WB samples, revealing a stronger metabolomic response of E. coli cells to HVST and LVLT than WB. KEGG enrichment analysis indicated that the significantly enriched pathways were biosynthesis and metabolism of amino acids (alanine, arginine, aspartate, and glutamate, etc.), followed by aminoacyl-tRNA biosynthesis among the three treatments. This is the first metabolomic study of E. coli cells in response to ohmic heating and presents an important step toward understanding the mechanism of ohmic heating on microbial inactivation, and can serve as a theoretical basis for better application of ohmic heating in food products.

Flow Cytometric Assessment of the Morphological and Physiological Changes of Listeria monocytogenes and Escherichia coli in Response to Natural Antimicrobial Exposure

Essential oils (EOs) or their components represent one of the most promising natural, safe, and feasible alternatives to prevent the growth of food-borne pathogens like Listeria monocytogenes and Escherichia coli in food matrices. Although antimicrobial properties of EOs and their components are well-documented, limited and fragmented information is available on the changes induced by these compounds, even at sub-lethal concentrations, in the physiological properties of microbial cells. The aim of this study was to explore the morpho-physiological changes of L. monocytogenes Scott A and E. coli MG 1655 induced after 1 h exposure to different sub-lethal and lethal concentrations of citral, carvacrol, (E)-2-hexenal, and thyme EO. For this purpose, different cell viability parameters such as membrane integrity, esterase activity, and cytoplasmic cell membrane potential were measured by flow cytometry. Flow cytometric data revealed specific response patterns in relation to the strain, the natural antimicrobial and its concentrations. Both the target microbial strains showed an increased cell membrane permeabilization without a loss of esterase activity and cell membrane potential with increasing citral, carvacrol and thyme EO concentrations. By contrast, (E)-2-hexenal did not significantly affect the measured physiological properties of L. monocytogenes Scott A and E. coli MG 1655. The used approach allowed identifying the most effective natural antimicrobials in relation to the microbial target.

Inactivation of Microorganisms in Foods by Ohmic Heating: A Review

Ohmic heating (OH) is an alternative food processing technology for effectively inactivating microorganisms that depends on the heat that has been generated when electrical current passes directly through food material. The advantages of OH for microbial inactivation include shorter heating time, more uniform heat distribution inside food, reduced nutrition losses, and higher energy efficiency. This review presents some published information regarding the inactivation of microorganisms by OH, including the major factors that influence the inactivation effectiveness of OH, the inactivation of vegetative cells and spores in foods by OH, the inactivation mechanisms of OH, and the challenges and prospects of OH for food processing. This information will improve the understanding of OH for inactivation of microorganisms and promote the application of OH in the food industry.

Linalool, citral, eugenol and thymol: control of planktonic and sessile cells of Shigella flexneri

The antimicrobial activity of linalool, citral, eugenol and thymol was determined in growth studies of both planktonic (PC) and biofilm cells (BC) Shigella flexneri. These components were evaluated either in isolation or in combinations using a sequential experimental strategy with Plackett & Burman and central composite rotational designs totaling 47 treatments. The minimum inhibitory concentration for PC was 0.125% (v v⁻¹) for linalool and 0.5% (v v⁻¹) for citral, eugenol and thymol. The biofilm minimum bactericidal concentration was 3 and 1% (v v⁻¹) for linalool and citral, respectively, and 2% (v v⁻¹) for eugenol and thymol. In the mixtures, the minimum concentrations in the efficient assays for PC growth inhibition were 0.0003, 0.0443 and 0.0443% (v v⁻¹), for linalool, citral and thymol, respectively. In the BC, only two assays with concentrations of 0.0558, 0.0558 and 0.319% (v v⁻¹) and 0.035, 0.035 and 0.3999% (v v⁻¹) for linalool, citral and thymol, respectively, inhibited Shigella growth. Synergism was observed among the components, where PC and BC growth inhibition occurred at lower concentrations than those noted individually. The bactericidal effect of the components in microplate was different from the observed in stain steel coupons. Therefore, the obtained model can describe and predict the PC count of S. flexneri in medium with the tested compounds and they could be an alternative for the use in microbiological control in food industry.