Inactivation of Listeria monocytogenes on and within Apples Destined for Caramel Apple Production by Using Sequential Forced Air Ozone Gas Followed by a Continuous Advanced Oxidative Process Treatment

Listeria monocytogenes Outbreaks Related to Commercially Produced Caramel Apples: Developments in Sanitation, Product Formulation, and Packaging: A Review

ABSTRACT

Prior to a deadly 2014 listeriosis outbreak, caramel apples were not thought to be vehicles for the foodborne pathogen Listeria monocytogenes. The purpose of this review article is to summarize what has been learned from research prompted by this outbreak. This overview includes descriptions of the two L. monocytogenes infection outbreaks related to prepackaged caramel apples and a brief discussion of apple sanitation, the production processes used to make caramel apples, and research on ways to prevent future outbreaks associated with caramel apples. A qualitative analysis of the literature and interviews with current caramel apple manufacturers were conducted. Sanitation, packaging, and storage procedures used by manufacturers in the past may not effectively inactivate L. monocytogenes from contaminated product. Novel apple sanitation methods and product formulations to control L. monocytogenes on caramel apples have been developed and, in some cases, implemented in commercial production.

HIGHLIGHTS

A Review: Gaseous Interventions for Listeria monocytogenes Control in Fresh Apple Cold Storage

Listeria monocytogenes (L. monocytogenes) causes an estimated 1600 foodborne illnesses and 260 deaths annually in the U.S. These outbreaks are a major concern for the apple industry since fresh produce cannot be treated with thermal technologies for pathogen control before human consumption. Recent caramel apple outbreaks indicate that the current non-thermal sanitizing protocol may not be sufficient for pathogen decontamination. Federal regulations provide guidance to apple processors on sanitizer residue limits, organic production, and good manufacturing practices (GMPs). However, optimal methods to control L. monocytogenes on fresh apples still need to be determined. This review discusses L. monocytogenes outbreaks associated with caramel apples and the pathogen’s persistence in the environment. In addition, this review identifies and analyzes possible sources of contaminant for apples during cold storage and packing. Gaseous interventions are evaluated for their feasibility for L. monocytogenes decontamination on apples. For example, apple cold storage, which requires waterless interventions, may benefit from gaseous antimicrobials like chlorine dioxide (ClO₂) and ozone (O₃). In order to reduce the contamination risk during cold storage, significant research is still needed to develop effective methods to reduce microbial loads on fresh apples. This requires commercial-scale validation of gaseous interventions and intervention integration to the current existing apple cold storage. Additionally, the impact of the interventions on final apple quality should be taken into consideration. Therefore, this review intends to provide the apple industry suggestions to minimize the contamination risk of L. monocytogenes during cold storage and hence prevent outbreaks and reduce economic losses.

Effectiveness of Low-Dose Continuous Gaseous Ozone in Controlling Listeria innocua on Red Delicious Apples During 9-Month Commercial Cold Storage

This study aimed to investigate the effects of low-dose continuous ozone gas in controlling Listeria innocua and quality attributes and disorders of Red Delicious apples during long-term commercial cold storage. Red Delicious apples were inoculated with a three-strain L. innocua cocktail at ∼6.2 log10 CFU/apple, treated with or without 1-methylcyclopropene, and then subjected to controlled atmosphere (CA) storage with or without continuous gaseous ozone in a commercial facility for 36 weeks. Uninoculated Red Delicious apples subjected to the above storage conditions were used for yeast/mold counts and quality attributes evaluation. The 36 weeks of refrigerated air (RA) or CA storage caused ∼2.2 log10 CFU/apple reduction of L. innocua. Ozone gas application caused an additional > 3 log10 CFU/apple reduction of L. innocua compared to RA and CA storage alone. During the 36-week CA storage, low-dose continuous gaseous ozone application significantly retarded the growth of yeast/mold, delayed apple firmness loss, and had no negative influence on ozone burn, lenticel decay, russet, CO2 damage, superficial scald, and soft scald of Red Delicious apples compared to CA-alone storage. In summary, the application of continuous low-dose gaseous ozone has the potential to control Listeria on Red Delicious apples without negatively influencing apple quality attributes.

Development of Strategies to Minimize the Risk of Listeria monocytogenes Contamination in Radish, Oriental Melon, and Carrots

Contamination by Listeria monocytogenes in packaged produce is a major concern. The purpose of this study was to find natural and affordable sanitizers to reduce L. monocytogenes contamination in agricultural products. Organic acids, ultraviolet-C (UV-C), and ethanol were analyzed either alone or in combination to assess their ability to reduce L. monocytogenes population in radish, oriental melon, and carrot samples. In radish samples, 3% malic acid combined with UV-C at a dosage of 144 mj/cm² significantly reduced (>4 log CFU/g) the population of L. monocytogenes (1.44 ± 0.5) compared to the control sample (5.14 ± 0.09). In the case of the melon samples, exposure to UV-C at a dosage of 144 mj/cm² combined with 3% lactic acid (2.73 ± 0.75) or 50% ethanol (2.30 ± 0.01) was effective against L. monocytogenes compared to the control sample (5.10 ± 0.19). In carrot samples, 3% lactic acid combined with 144 mj/cm² dosage UV-C reduced L. monocytogenes population (4.48 ± 0.25) more than in the control sample (5.85 ± 0.08). These results reveal that sanitizers that are effective for one crop are less effective for another crop indicating that effective prevention methods should be customized for each crop to prevent pathogen cross contamination during postharvest washing.

Gaseous ozone to preserve quality and enhance microbial safety of fresh produce: Recent developments and research needs

Fresh fruits and vegetables are highly perishable and are subject to large postharvest losses due to physiological (senescence), pathologic (decay), and physical (mechanical damage) factors. In addition, contamination of fresh produce with foodborne human pathogens has become a concern. Gaseous ozone has multiple benefits including destruction of ethylene, inactivation of foodborne and spoilage microorganisms, and degradation of chemical residues. This article reviews the beneficial effects of gaseous ozone, its influence on quality and biochemical changes, foodborne human pathogens, and spoilage microorganisms, and discusses research needs with an emphasis on fruits. Ozone may induce synthesis of a number of antioxidants and bioactive compounds by activating secondary metabolisms involving a wide range of enzymes. Disparities exist in the literature regarding the impact of gaseous ozone on quality and physiological processes of fresh produce, such as weight loss, ascorbic acid, and fruit ripening. The disparities are complicated by incomplete reporting of the necessary information, such as relative humidity and temperatures at which ozone measurement and treatment were performed, which is needed for accurate comparison of results among studies. In order to fully realize the benefits of gaseous ozone, research is needed to evaluate the molecular mechanisms of gaseous ozone in inhibiting ripening, influence of relative humidity on the antimicrobial efficacy, interaction between ozone and the cuticle of fresh produce, ozone signaling pathways in the cells and tissues, and so forth. Possible adverse effects of gaseous ozone on quality of fresh produce also need to be carefully evaluated for the purpose of enhancing microbial and chemical safety of fresh produce.

Decontamination of N95 and surgical masks using a treatment based on a continuous gas phase-Advanced Oxidation Process

A gas-phase Advanced Oxidation Process (gAOP) was evaluated for decontaminating N95 and surgical masks. The continuous process was based on the generation of hydroxyl-radicals via the UV-C (254 nm) photo-degradation of hydrogen peroxide and ozone. The decontamination efficacy of the gAOP was dependent on the orientation of the N95 mask passing through the gAOP unit with those positioned horizontally enabling greater exposure to hydroxyl-radicals compared to when arranged vertically. The lethality of gAOP was independent of the applied hydrogen peroxide concentration (2-6% v/v) but was significantly (P<0.05) higher when H2O2 was introduced into the unit at 40 ml/min compared to 20 ml/min. A suitable treatment for N95 masks was identified as 3% v/v hydrogen peroxide delivered into the gAOP reactor at 40 ml/min with continuous introduction of ozone gas and a UV-C dose of 113 mJ/cm2 (30 s processing time). The treatment supported >6 log CFU decrease in Geobacillus stearothermophilus endospores, > 8 log reduction of human coronavirus 229E, and no detection of Escherichia coli K12 on the interior and exterior of masks. There was no negative effect on the N95 mask fitting or particulate efficacy after 20 passes through the gAOP system. No visual changes or hydrogen peroxide residues were detected (<1 ppm) in gAOP treated masks. The optimized gAOP treatment could also support >6 log CFU reduction of endospores inoculated on the interior or exterior of surgical masks. G. stearothermophilus Apex spore strips could be applied as a biological indicator to verify the performance of gAOP treatment. Also, a chemical indicator based on the oxidative polymerization of pyrrole was found suitable for reporting the generation of hydroxyl-radicals. In conclusion, gAOP is a verifiable treatment that can be applied to decontaminate N95 and surgical masks without any negative effects on functionality.

Ozone Treatments for Preserving Fresh Vegetables Quality: A Critical Review

Ozone is recognized as an antimicrobial agent for vegetables storage, washing, and processing. This strong disinfectant is now being used in the food industry. In this review, the chemical and physical properties of ozone, its generation, and factors affecting ozone processing efficiency were explained as well as recent regulatory developments in the food industry. By then selecting three vegetables, we show that ozone avoids and controls biological growth on vegetables, keeping their attractive appearance and sensorial qualities, assuring nutritional characteristics' retention and maintaining and increasing the shelf-life. In liquid solution, ozone can be used to disinfect processing water and vegetables, and in gaseous form, ozone helps to sanitize and preserve vegetables during storage. The multifunctionality of ozone makes it a promising food processing agent. However, if ozone is improperly used, it causes some deleterious effects on products, such as losses in their sensory quality. For an effective and a safe use of ozone, specific treatment conditions should be determined for all kinds of vegetables. In a last step, we propose highlighting the different essential characteristics of ozone treatment in order to internationally harmonize the data relating to the treatments carried-out.

Survival and Inactivation by Advanced Oxidative Process of Foodborne Viruses in Model Low-Moisture Foods

Enteric viruses, such as human norovirus (NoV) and hepatitis A virus (HAV), are the major causes of foodborne illnesses worldwide. These viruses have low infectious dose, and may remain infectious for weeks in the environment and food. Limited information is available regarding viral survival and transmission in low-moisture foods (LMF). LMFs are generally considered as ready-to-eat products, which undergo no or minimal pathogen reduction steps. However, numerous foodborne viral outbreaks associated with LMFs have been reported in recent years. The objective of this study was to examine the survival of foodborne viruses in LMFs during 4-week storage at ambient temperature and to evaluate the efficacy of advanced oxidative process (AOP) treatment in the inactivation of these viruses. For this purpose, select LMFs such as pistachios, chocolate, and cereal were inoculated with HAV and the norovirus surrogates, murine norovirus (MNV) and feline calicivirus (FCV), then viral survival on these food matrices was measured over a four-week incubation at ambient temperature, by both plaque assay and droplet-digital RT-PCR (ddRT-PCR) using the modified ISO-15216 method as well as the magnetic bead assay for viral recovery. We observed an approximately 0.5 log reduction in viral genome copies, and 1 log reduction in viral infectivity for all three tested viruses following storage of select inoculated LMFs for 4 weeks. Therefore, the present study shows that the examined foodborne viruses can persist for a long time in LMFs. Next, we examined the inactivation efficacy of AOP treatment, which combines UV-C, ozone, and hydrogen peroxide vapor, and observed that while approximately 100% (4 log) inactivation can be achieved for FCV, and MNV in chocolate, the inactivation efficiency diminishes to approximately 90% (1 log) in pistachios and 70% (< 1 log) in cereal. AOP treatment could therefore be a good candidate for risk reduction of foodborne viruses from certain LMFs depending on the food matrix and surface of treatment.

Advanced Oxidation Process as a Postharvest Decontamination Technology To Improve Microbial Safety of Fresh Produce

Fresh produce is frequently associated with outbreaks of foodborne diseases; thus, there is a need to develop effective intervention technologies and antimicrobial treatments to improve the microbial safety of fresh produce. Washing with chemical sanitizers, commonly used by the industry, is limited in its effectiveness and is viewed as a possible cross-contamination opportunity. This review discuses the advanced oxidation process (AOP), which involves generating highly reactive hydroxyl radicals to inactivate human pathogens. Ionizing irradiation, ultraviolet (UV) light, and cold plasma can be regarded as AOP; however, AOPs employing combinations of UV, H₂O₂, cold plasma, and ozone may be more promising because higher amounts of hydroxyl radicals are produced in comparison to the individual treatments and the combinative AOPs may be more consumer friendly than ionizing irradiation. When applied as a gaseous/aerosolized treatment, AOPs may have advantages over immersion treatments, considering the reactivity of hydroxyl radicals and presence of organic materials in wash water. Gaseous/aerosolized AOPs achieve up to 5 log reductions of pathogenic bacteria on fresh produce compared to reductions of 1-2 logs with aqueous sanitizers. Further research needs to be conducted on specific AOPs before being considered for commercialization, such as reduced formation of undesirable chemical byproducts, impact on quality, and scaled up studies.

Validation of a vapor-phase advanced oxidation process for inactivating Listeria monocytogenes, its surrogate Lactobacillus fructivorans, and spoilage molds associated with green or red table grapes

A method based on vapor-phase advanced oxidation process (AOP) for decontaminating red or green grapes was validated for inactivating Listeria monocytogenes and spoilage molds. A Central Composite Design (CCD) and Response Surface Methodology (RSM) were applied to determine the contribution of UV-C (254 nm) dose, hydrogen peroxide, and ozone concentration on the lethality toward Aspergillus niger spores (biodensiometer) and changes to the grape quality (firmness and color over 14-day post-treatment storage at 4 °C). A high UV-C dose (>129 mJ/cm² ) or >4.0 % v/v hydrogen peroxide induced-blistering and darkening of grapes at the end of the storage period. Yet, an optimized AOP treatment (with regards to preserving grape quality) was derived to be 1.3% v/v hydrogen peroxide (5 mL/10 berries) with 9-mg ozone gas and a UV-C dose of 123 mJ/cm² (10 s at UV-C intensity of 12 mW/cm² ). A predictive model was constructed and verified based on the log reduction of A. niger spores and changes in quality characteristics of red grapes. The optimal AOP treatment supported a 1.6-log CFU/g reduction of Aspergillus spores and decreased L. monocytogenes counts by 3.92 ± 0.17 and 4.77 ± 0.30 log CFU/g on green and red grapes, respectively, that were not significantly different to the surrogate, Lactobacillus fructivorans. There was no significant difference in the reduction of L. monocytogenes with grapes arranged in a single or double layer. Botrytis cinerea counts were reduced by 1.08 to 1.35 log CFU/g using the optimized AOP treatment with no change in grape color or firmness during storage. A sensory panel could not differentiate AOP-treated grapes from nontreated controls although 3 of 15 panelists did note subtle flavor notes. PRACTICAL APPLICATION: Postharvest washing of fresh produce has limited efficacy in removing foodborne pathogens and spoilage microbes. This is especially relevant to berries, such as grapes, that are susceptible to spoilage following washing. The vapor-phase AOP treatment provides a supplemental or alternative approach for produce decontamination. However, the operating parameters need to be optimized to ensure that decontamination of grapes is not at the expense of quality. In the current study, this was achieved by ensuring a balance between hydrogen peroxide, ozone, and UV-C dose that form the elements of an AOP treatment.

Advanced oxidation process for the inactivation of Salmonella typhimurium on tomatoes by combination of gaseous ozone and aerosolized hydrogen peroxide

Fresh produce-associated outbreaks of foodborne illnesses continue to occur every year in the U.S., suggesting limitations of current practices and the need for effective intervention technologies. Advanced oxidation process involves production of hydrogen radicals, which are the strongest oxidant. The objective of the present study was to evaluate the effectiveness of advanced oxidation process by combining gaseous ozone and aerosolized hydrogen peroxide. Grape tomatoes were inoculated with a 2-strain cocktail of Salmonella typhimurium on both stem scar and smooth surface. Gaseous ozone (800 and 1600 ppm) and aerosolized hydrogen peroxide (2.5, 5 and 10%) were separately or simultaneously introduced into a treatment chamber where the inoculated tomatoes were placed. During the 30 min treatments, hydrogen peroxide was aerosolized using an atomizer operated in two modes: continuously or 15 s on/50 s off. After the treatments, surviving Salmonella on the smooth surface and stem scar were enumerated. Results showed that ozone alone reduced Salmonella populations by <0.6 log CFU/fruit on both the smooth surface and the stem scar area, and aerosolized hydrogen peroxide alone reduced the populations by up to 2.1 log CFU/fruit on the smooth surface and 0.8 log CFU/fruit on stem scar area. However, the combination treatments reduced the populations by up to 5.2 log CFU/fruit on smooth surface and 4.2 log CFU/fruit on the stem scar. Overall, our results demonstrate that gaseous ozone and aerosolized hydrogen peroxide have synergistic effects on the reduction of Salmonella populations on tomatoes.

Aqueous ozone controls decay and maintains quality attributes of strawberry (Fragaria × ananassa Duch.)

Investigations were made on the changes in physical and biochemical attributes, fruit decay and storage life of 'Winter Dawn' strawberry fruits in response to aqueous ozone dip treatment for different exposure times. Fruits were subjected to 0.1 ppm aqueous ozone for different time intervals (1-4 min). The treated strawberries were air dried and stored under ambient (25 ± 2 °C and 45-50% RH) and low temperature (2 ± 1 °C and 90% RH) conditions. Results revealed that treatment of strawberry fruits with aqueous ozone @ 0.1 ppm for 2 min resulted in 21% lower weight loss, 16% higher firmness and 15% lesser change in fruit colour during 2 days in ambient storage. Under low temperature storage, 2 min ozone treated fruits exhibited ~ 21% lower PLW, 19% higher firmness and 46% lesser colour change as compared to control fruits during 14 days of storage. Fruit decay reduced significantly under both low and cold storage conditions. Thus, it can be concluded that application of aqueous ozone for 2 min was able to retain the strawberry fruit quality and extend its storage life till 14 days under low temperature storage and 2 days under ambient storage conditions.

Control of Listeria monocytogenes in Caramel Apples by Use of Sticks Pretreated with Potassium Sorbate

A multistate listeriosis outbreak associated with caramel apples from 2014 to 2015 prompted research on the survival of Listeria monocytogenes in fresh apples and caramel apples. Research indicated that stem end-inoculated caramel apples with stick insertion allowed for the survival and growth of L. monocytogenes at both refrigeration and ambient temperatures. This study aimed to assess the effectiveness of chemical preservatives as pretreatments for the wooden stick component to reduce L. monocytogenes loads in stem end-inoculated caramel apples during storage. Wooden sticks were pretreated with 1, 3, or 5% ascorbic acid (vitamin C), Nisaplin (2.5% nisin), potassium sorbate, and sodium benzoate and then inoculated with L. monocytogenes at 7 log CFU per stick. After storage at 25°C, the pathogen was reduced most effectively by the ascorbic acid pretreatments. At all three ascorbic acid concentrations tested, L. monocytogenes levels were reduced below the level of enumeration (2.5 log CFU per apple) at 24 h and were no longer detectable by enrichment after 72 h. Ascorbic acid (5, 10, and 20%) and potassium sorbate (10, 20, 30, and 40%) were further tested as wooden stick pretreatments for pathogen reduction on stem end-inoculated caramel apples stored at 5 and 25°C. The 40% potassium sorbate solution at 25°C was the most effective pretreatment condition in caramel apples and demonstrated a 3.1-log CFU per apple overall decrease in L. monocytogenes population levels after 216 h. Pretreatment of the wooden stick component of a caramel apple with potassium sorbate may be a viable preventive measure to reduce postprocess L. monocytogenes population levels and hence reduce consumer risk associated with caramel apple consumption.