Decontamination of Listeria monocytogenes in enoki mushrooms using a 405-nm light-emitting diode illumination combined with organic acid dipping

Radical species generating technologies for decontamination of Listeria species in food: a recent review report

Foodborne illnesses occur due to the contamination of fresh, frozen, or processed food products by some pathogens. Among several pathogens responsible for the illnesses, Listeria monocytogenes is one of the lethal bacteria that endangers public health. Several preexisting and novel technologies, especially non-thermal technologies are being studied for their antimicrobial effects, particularly toward L. monocytogenes. Some noteworthy emerging technologies include ultraviolet (UV) or light-emitting diode (LED), pulsed light, cold plasma, and ozonation. These technologies are gaining popularity since no heat is employed and undesirable deterioration of food quality, especially texture, and taste is devoided. This review aims to summarize the most recent advances in non-thermal processing technologies and their effect on inactivating L. monocytogenes in food products and on sanitizing packaging materials. These technologies use varying mechanisms, such as photoinactivation, photosensitization, disruption of bacterial membrane and cytoplasm, etc. This review can help food processing industries select the appropriate processing techniques for optimal benefits, in which the structural integrity of food can be preserved while simultaneously destroying L. monocytogenes present in foods. To eliminate Listeria spp., different technologies possess varying mechanisms such as rupturing the cell wall, formation of pyrimidine dimers in the DNA through photochemical effect, excitation of endogenous porphyrins by photosensitizers, generating reactive species, causing leakage of cellular contents and oxidizing proteins and lipids. These technologies provide an alternative to heat-based sterilization technologies and further development is still required to minimize the drawbacks associated with some technologies.

Escherichia coli O157:H7 had a high degree of acid resistance in the presence of osmolytes (glycerol, glycine or fructose) by altering its lipid membrane composition

This study aims to investigate the resistance of E. coli O157:H7 to acetic acid (AA) or malic acid (MA) by adding osmolytes, such as glycerol, glycine, glucose, and fructose, in Luria-Bertani broth without NaCl (LBW/S) or phosphate buffer (PB) stored at 25 °C. In LBW/S, a significantly (p < 0.05) higher D-value of E. coli O157:H7 was observed when treated with AA and 20% glycine (D-value: 1.18-3.44) or 40% glucose (D-value: 1.05-2.52) compared to that of AA alone (D-value: 0.40-0.47). In contrast, the addition of osmolytes (i.e. 3-40% glucose, 3-40% fructose or 20% glycine) to LBW/S acidified by MA significantly decreased D-values of E. coli O157:H7, which was enumerated by using a selective medium. Furthermore, when E. coli O157:H7 was incubated in LBW/S containing AA and osmolytes at 25 °C for 3 d, this bacterium had an increased proportion of C16:0 and C17:0 cyclo (cyclopropane acid) compared to its AA-treated counterparts. Along with the altered shift in membrane phospholipids, the addition of osmolytes into a laboratory medium in the presence of nutritive substrates may increase the resistance of E. coli O157:H7 to AA.

Efficacy of combined caprylic acid and thymol treatments for inactivation of Listeria monocytogenes on enoki mushrooms in household and food-service establishments

Raw enoki mushroom is a high-risk vector for listeriosis, which led to foodborne outbreaks resulting in four deaths in the United States in 2020. This study aimed to investigate the washing method for the inactivation of L. monocytogenes in enoki mushrooms for household and food service establishments. Five methods of washing fresh agricultural products without using disinfectants were selected: (1) rinsing under running water (2 L/min, 10 min), (2-3) dipping in water (200 ml/20 g) at 22 or 40 °C for 10 min, and using (4) 10% NaCl or (5) 5% vinegar at 22 °C for 10 min. The antibacterial efficacy of each washing method along with the final rinse was tested with enoki mushrooms inoculated with a 3-strain cocktail of L. monocytogenes (ATCC 19111, 19115, 19117; ca. 6 log CFU/g). The 5% vinegar showed a significant difference in antibacterial effect compared to the other treatments except 10% NaCl (P < 0.05), with the maximum elimination of L. monocytogenes by 1.23 log CFU/g. Therefore, a disinfectant for enoki mushrooms that can complement the commonly used washing method was developed using antimicrobials (caprylic acid, CA: 0, 0.20, 0.40%; thymol, TM: 0, 0.075, 0.15%). By combined treatment of 0.40% CA and 0.15% TM at 22 °C for 10 min, L. monocytogenes was completely inactivated (>5.55 log reduction CFU/g) and did not recover after enrichment, although individual treatments of antimicrobials showed low bactericidal effects of <1.50 log reduction CFU/g. The bacterial membrane disintegration induced by the disinfectant was analyzed through flow cytometry. Additionally, the sensory scores (odor and appearance) and color parameters (L*, a*, and b*) of enoki mushrooms treated with the disinfectant were not significantly different from those of enoki mushrooms washed with water (P > 0.05). Our findings suggest a washing disinfectant consisting of low concentrations of CA and TM with synergistic antibacterial effects without quality deterioration that can ensure the safe consumption of raw enoki mushrooms in homes and food service establishments.

Vapor-Phase Hydroxyl or Chlorine Radical Treatment for Inactivating Listeria monocytogenes on Mushrooms (Agaricus bisporus) without Negatively Affecting Quality or Shelf Life

ABSTRACT

Processes based on generating vapor-phase hydroxyl radicals or chlorine radicals were developed for inactivating Listeria monocytogenes on mushrooms without negatively affecting quality. Antimicrobial radicals were generated from the UV-C degradation of hydrogen peroxide or hypochlorite and ozone gas. Response surface modeling was used to identify the interaction among the operating parameters for the hydroxyl radical process: UV-C254nm intensity, hydrogen peroxide concentration, and ozone delivered. There was an inverse relationship between hydrogen peroxide concentration and UV-C intensity in terms of the log reduction of L. monocytogenes. The independent parameters for the chlorine radical process were hypochlorite concentration, pH, and UV-C intensity. From predictive models, the optimal hydroxyl radical treatment was found to be 5% (v/v) H2O2, 2.86 mW/cm2 UV-C intensity (total UV-C dose 144 mJ/cm2), and 16.5 mg of ozone. The optimal parameters for the chlorine radical process were 10 ppm of hypochlorite (pH 3.0), 11.0 mg of ozone, and 4.60 mW/cm2 UV-C intensity. When inoculated mushrooms were treated with the optimal hydroxyl radical and chlorine radical processes, the reduction of L. monocytogenes was found to be 2.42 ± 0.42 and 2.61 ± 0.30 log CFU, respectively, without any negative effects on mushroom quality (weight loss and Browning index during 14 days of storage at 4°C). These reductions were significantly greater than those from application of the individual elements of the radical processes and those in the control process, which used a 90-s dip in 1% (v/v) hydrogen peroxide. The study has demonstrated that hydroxyl radical and chlorine radical vapor-phase treatments are equally effective at inactivating L. monocytogenes on mushrooms and can be considered as a preventative control step.

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