Effect of Filling Type and Heating Method on Prevalence of Listeria species and Listeria monocytogenes in Dumplings Produced in Poland

The count of Listeria monocytogenes was determined, before and after heat treatment, in 200 samples of dumplings of 9 brands and with different types of stuffing. Analyses were conducted according to ISO 11290–1 standard and with real‐time PCR method. The highest count of L. monocytogenes was found in meat dumplings (10² to 10⁴ CFU/g), whereas products with white cheese‐potato stuffing and vegetable‐mushroom stuffing contained significantly less Listeria, 20 to 80 and 5 to 32 CFU/g, respectively. In cooled meat dumplings the extent of contamination depended significantly on the producer. In addition, a significant (P < 0.05) correlation was determined between contamination level and meat content in the stuffing (rho = 0.418), especially in stuffing containing pork meat (0.464), contrary to beef‐containing stuffing (0.284). Heating dumplings in boiling water for 2 min completely eliminated L. monocytogenes in meat dumplings. In contrast, the microwave heating applied for 2 min at 600 W only reduced the count of L. monocytogenes by 1 to 2 logs. Hence, the microwave heating failed to reduce the risk of infection with this pathogen below the level permissible in the EU regulation, especially in the most contaminated samples. In this case, the efficacy of microwave heating was significantly (P < 0.05) affected by the initial count of L. monocytogenes (rho = 0.626), then by meat content in the stuffing (0.476), and to the lowest extent—by the type of meat (0.415 to 0.425). However, no Listeria sp. and L. monocytogenes were isolated from cooked dumplings with fruits (strawberries or blueberries).     

Efficacy of pulsed light for shelf-life extension and inactivation of Listeria monocytogenes on ready-to-eat cooked meat products

Pulsed light (PL) was tested for its utility to improve the microbial quality and safety of ready-to-eat cooked meat products. Vacuum-packaged ham and bologna slices were superficially inoculated with Listeria monocytogenes and treated with 0.7, 2.1, 4.2 and 8.4 J/cm². PL treatment at 8.4 J/cm² reduced L. monocytogenes by 1.78 cfu/cm² in cooked ham and by 1.11 cfu/cm² in bologna. The effect of PL on lipid oxidation and sensory properties was also investigated. The 2-thiobarbituric acid values were very low and chromaticity parameters were within the normal values reported for cooked meat products. PL at 8.4 J/cm² did not affect the sensory quality of cooked ham, while treatments above 2.1 J/cm² negatively influenced the sensory properties of bologna. The combination of PL and vacuum packaging provided ham with an additional shelf-life extension of 30 days compared with only vacuum packaging. The shelf-life of bologna was not extended by PL.

Industrial relevance

The efficacy of pulsed light for the decontamination of surfaces offers excellent possibilities to ensure food safety and to extend shelf-life of ready-to-eat (RTE) products. The results of this study indicate that Listeria monocytogenes can be reduced by approximately 2 log cfu/cm² in RTE cooked ham and 1 log cfu/cm² in bologna using a fluence of 8.4 J/cm². This dose does not affect the sensory properties of ham and triples its shelf-life when compared with conventional RTE products. On the contrary, fluences above 2.1 J/cm² are not suitable for the treatment of bologna since sensory quality is modified.     

丹东口岸2003-2005年进口海产品中3种致病菌的检验

目的掌握丹东口岸进口海产品中单核细胞增生李斯特菌、沙门菌和副溶血性弧菌污染情况。方法对丹东口岸2003-2005年进口的海产品3种致病菌的检验结果进行了分析。结果1965批进口海产品中检出不合格产品44批(2.24%)。其中32批(1.63%)检出单核细胞增生李斯特菌、6批(0.31%)检出沙门菌和6批(0.31%)检出副溶血性弧菌。不合格产品主要为冻章鱼、冻海螺、冻紫石房蛤、活河螺、冻河螺肉等。结论从丹东口岸进口的海产品3种致病菌的污染比较严重,特别是被单核细胞增生李斯特菌的污染最严重,海产品中冻章鱼和冻海螺被这3种致病菌的污染比较普遍。     

Inhibition of Listeria monocytogenes on Hot Dogs Using Antimicrobial Whey Protein-based Edible Casings

ABSTRACT: Whey protein isolate (WPI) films (pH 5.2) containing p‐aminobenzoic acid (PABA) were heat‐sealed to form casings. Hot dogs prepared with WPI, collagen, or natural casings were cooked, surface‐inoculated to contain 103 Listeria monocytogenes CFU/g, and examined for numbers of L. monocytogenes, mesophilic aerobic bacteria (MAB), lactic acid bacteria (LAB), and yeast/mold during 42 d of storage at 4 °C. Listeria populations on hot dogs prepared with WPI‐1.0%‐PABA casings remained relatively unchanged; however, numbers of Listeria on hot dogs prepared with WPI‐0.0%‐PABA, collagen, and natural casings increased about 2.5 logs during 42 d of refrigerated storage. Populations of MAB, LAB, and mold on WPI‐1.0%‐PABA casings were 1 to 3 logs lower compared to others casings.     

Characterization of Listeria monocytogenes strains isolated from Gorgonzola cheese rinds

Listeria monocytogenes is a foodborne pathogen frequently present in ripened soft cheeses. Forty-three strains of L. monocytogenes isolated from the rind of ripened Gorgonzola cheeses produced in 24 different dairy plants were characterized by biotyping, serotyping, and molecular typing. Biotyping was performed by studying two phenotypes closely associated with virulence, such as hemolytic and phospholipase C activities. Traditional typing techniques did not allow a discrimination among the 43 strains studied. All strains showed a good hemolytic activity on blood agar, and only slight differences were observed when titration of hemolytic activity of culture supernatants was performed. Also phospholipase activities were quite similar for all the strains. Concerning serotyping, all strains belonged to serotype 1/2a. The molecular characterization was performed by RAPD-PCR. Combined cluster analysis following PCR amplification experiments allowed to group L. monocytogenes strains into few distinguishable profiles. At a level of similarity of 80%, the 43 strains were grouped into only 5 composite profile groups. Although isolated in 24 different plants, the presence of a few closely related strains demonstrated a possible relationship between these cheese isolates; a special ability of these strains to adapt to Gorgonzola cheese processing environment could be suggested.     

Effect of Salts of Organic Acids on Listeria monocytogenes,Shelf Life,Meat Quality,and Consumer Acceptability of Beef Frankfurters

The objective of this study was to evaluate anti‐listerial efficacy of salts of organic acids, and their impact on the quality of frankfurters. Beef frankfurters were manufactured by incorporating organic acids in 5 different combinations: (1) control (no marinade addition; C); (2) sodium lactate (2% wt/wt; SL); (3) potassium lactate (2% wt/wt; PL); (4) sodium citrate (0.75% wt/wt; SC); and (5) sodium lactate (2% wt/wt)/sodium diacetate (0.25% wt/wt; SL/SD). Cooked frankfurters were inoculated with streptomycin‐resistant (1500 μg/mL) L. monocytogenes (7 log₁₀ CFU/frank). Inoculated and noninoculated frankfurters were vacuum packaged and stored at 4 °C. Samples were taken weekly up to 10 wk for estimation of L. monocytogenes as well as aerobic plate count (APC) and psychrotrophs (PSY), respectively. Total of 2 independent trials of the entire experiment were conducted. Noninoculated beef frankfurters were evaluated weekly by untrained sensory panelists for 7 wk. SL, PL, and SC treatments did not (P > 0.05) adversely affect consumer acceptability through 8 wk although, SL/SD treatment was significantly (P ≤ 0.05) less preferred across all sensory attributes. SL/SD treatment negatively affected product quality, but was able to control APC, PSY, and L. monocytogenes levels. SC performed similar to the control throughout the 8, 9, and 10 wk storage periods, providing no benefit for inhibiting L. monocytogenes (increasing from 7 logs CFU/frank to 10 logs CFU/frank throughout storage) or extending shelf life of the beef frankfurters. In conclusion, 2% SL and PL, and 2% SL/0.25% SD may be effective L. monocytogenes inhibitors (maintaining inoculation levels of 7 logs CFU/frank during storage), but changes in SL/SD treatment formulation should be studied to improve product quality.     

Satureja horvatii essential oil: In vitro antimicrobial and antiradical properties and in situ control of Listeria monocytogenes in pork meat

The dominant compounds in Satureja horvatii oil were p-cymene (33.14%), thymol (26.11%) and thymol methyl ether (15.08%). The minimum inhibitory concentration (MIC) varied from 0.03 to 0.57 mg/mL for bacteria, and from 0.56 to 2.23 mg/mL for yeast strains, while minimum bactericidal/yeast-cidal concentration (MBC/MYC) varied from 0.07 to 1.15 mg/mL and 1.11 to 5.57 mg/mL for bacteria and yeasts, respectively. The antiradical potential of the essential oil was evaluated using hydroxyl radical (•OH) generated in Fenton reaction. The meat preserving potential of essential oil from Satureja horvatii was investigated against L. monocytogenes. Essential oil successfully inhibited development of L. monocytogenes in pork meat. Sensorial evaluation on flavor and color of meat was performed. The color and flavor of meat treated with essential oil improved after 4 days of storage. S. horvatii essential oil can act as a potent inhibitor of food spoiling microorganisms, in meat products and also can be a useful source of natural antioxidants.     

Use of organic acids to inactivate Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on organic fresh apples and lettuce

Abstract: This study was undertaken to investigate the antimicrobial effect of organic acids against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on whole red organic apples and lettuce. Several studies have been conducted to evaluate organic acids as sanitizers. However, no studies have compared antimicrobial effects of various organic acids on organic fresh produce, including evaluation of color changes of produce. Apples and lettuce were inoculated with a cocktail of 3 strains each of 3 foodborne pathogens provided above and treated with 1% and 2% organic acids (propionic, acetic, lactic, malic, and citric acid) for 0, 0.5, 1, 5, and 10 min. With increasing treatment time and acid concentration, organic acid treatments showed significant reduction compared to the control treatment (distilled water), and differences in antimicrobial effects between organic acids were observed. After 10 min of treatment with 1% and 2% organic acids in apples, propionic (0.92 to 2.75 log reduction), acetic (0.52 to 2.78 log reduction), lactic (1.69 to >3.42 log reduction), malic (1.48 to >3.42 log reduction), and citric acid (1.52 to >3.42 log reduction) exhibited significant (P < 0.05) antibacterial effects against 3 foodborne pathogens compared to the control treatment. In lettuce, propionic (0.93 to 1.52 log reduction), acetic (1.13 to 1.74 log reduction), lactic (1.87 to 2.54 log reduction), malic (2.32 to 2.98 log reduction), and citric acid (1.85 to 2.86 log reduction) showed significant (P < 0.05) effects compared to the control treatment. Changes in sample color subjected to organic acids treatment were not significant during storage. Practical Application: It is suggested that organic acids have a potential as sanitizers for organic fresh produce. These data may help the organic produce industry provide safe fresh produce for consumers.     

SigB plays a major role in Listeria monocytogenes tolerance to bile stress

The ability of Listeria monocytogenes to tolerate high levels of bile stress is critical to its successful infection and colonization in the human gastrointestinal tract. L. monocytogenes encodes bile salt hydrolase by a bsh gene which plays a significant role in hydrolyzing high concentrations of bile salt when L. monocytogenes grows under hypoxemic condition. As the bsh promoter contains consensus SigB and PrfA binding sites, we investigated the role of SigB (σ^B) and PrfA in L. monocytogenes tolerance against bile stress by comparing the survival of isogenic deletion mutants of L. monocytogenes EGD_ΔsigB, EGD_ΔprfA and EGD_ΔprfAΔsigB with their parent strain EGD at high levels of bile salt. Our results show that the sigB deletion significantly reduced the MICs of bile salt for EGD_ΔsigB and EGD_ΔprfAΔsigB (2.6% and 2.2% vs 3.5% in wild type strain EGD), while the growth rates of these two sigB deletion mutants (EGD_ΔsigB and EGD_ΔprfAΔsigB) were affected the most in the presence of 3% bile salt. Pre-exposure to alkali (pH 9.0) and osmotic (0.3 M NaCl) stresses for a short period of time (30 min) resulted in improved growth of L. monocytogenes as well as its prfA-sigB isogenic mutants even under sublethal concentrations of bile salt, while pre-exposure to acid pH (pH 4.5) failed to provide cross-protection against subsequent bile stress. Furthermore, the sigB gene had more remarkable influence than that of prfA on bsh expression, as much lower levels of bsh transciption were observed in EGD_ΔsigB and EGD_ΔprfAΔsigB. Meanwhile, bsh expression in the deletion mutants did not respond to elevated levels of bile salt. These data indicate that σ^B might play a crucial role in Listeria survival under bile salt environment in the gastrointestinal tract before its successful colonization, invasion and intracellular propagation.     

Predictive Modeling for Growth of Non‐ and Cold‐adapted Listeria monocytogenes on Fresh‐cut Cantaloupe at Different Storage Temperatures

The aim of this study was to determine the growth kinetics of Listeria monocytogenes, with and without cold‐adaption, on fresh‐cut cantaloupe under different storage temperatures. Fresh‐cut samples, spot inoculated with a 4‐strain cocktail of L. monocytogenes (～3.2 log CFU/g), were exposed to constant storage temperatures held at 10, 15, 20, 25, or 30 °C. All growth curves of L. monocytogenes were fitted to the Baranyi, modified Gompertz, and Huang models. Regardless of conditions under which cells grew, the time needed to reach 5 log CFU/g decreased with the elevated storage temperature. Experimental results showed that there were no significant differences (P > 0.05) in the maximum growth rate k (log CFU/g h^?1) and lag phase duration λ (h) between the cultures of L. monocytogenes with or without previous cold‐adaption treatments. No distinct difference was observed in the growth pattern among 3 primary models at various storage temperatures. The growth curves of secondary modeling were fitted on an Arrhenius‐type model for describing the relationship between k and temperature of the L. monocytogenes on fresh‐cut cantaloupe from 10 to 30 °C. The root mean square error values of secondary models for non‐ and cold‐adapted cells were 0.018, 0.021, and 0.024, and 0.039, 0.026, and 0.017 at the modified Gompertz, Baranyi, and Huang model, respectively, indicating that these 3 models presented the good statistical fit. This study may provide valuable information to predict the growth of L. monocytogenes on fresh‐cut cantaloupes at different storage conditions.