Detection of Listeria monocytogenes in Fresh Produce Using Molecular Beacon—Real‐time PCR Technology

ABSTRACT: The capability of an assay to detect Listeria monocytogenes from artificially inoculated fresh‐cut produce such as cantaloupe and mixed salad was demonstrated. An oligonucleotide probe that becomes fluorescent upon hybridization to the target DNA (Molecular Beacon, MB) was used in a real‐time polymerase chain reaction (PCR) assay. As few as 4 to 7 colony‐forming units (CFU) of L. monocytogenes per 25 g of artificially contaminated produce could be detected. A comparison of 2 commercially available kits using MB‐PCR (iQ‐Check, Bio‐Rad Laboratories) and conventional PCR (BAX, Dupont Qualicon) was performed on artificially inoculated produce. The time required to detect L. monocytogenes (from produce to PCR) was considerably shorter for the iQ‐Check protocol (approximately 26 h) compared with the BAX‐PCR (approximately 52 h). The iQ‐Check protocol was also used to confirm the identity of the L. monocytogenes isolates obtained during a microbiological screen of conventional and organic leaf lettuce and alfalfa sprout samples from local supermarkets. The iQ check protocol was successful in differentiating L. monocytogenes isolates from other Listeria spp. such as L. welshimeri, L. innocua, and L. ivanovii. This is the 1st report of the application of the MB probe being used for real‐time detection of L. monocytogenes in whole and fresh‐cut produce.     

Growth Potential of Listeria Monocytogenes and Staphylococcus Aureus on Fresh‐Cut Tropical Fruits

The objective of this study was to evaluate the fate of Staphylococcus aureus, Listeria monocytogenes, and natural microbiota on fresh‐cut tropical fruits (pitaya, mango, papaya and pineapple) with commercial PVC film at different storage temperature (5, 13, and 25 °C). The results showed that S. aureus, L. monocytogenes, and natural microbiota increased significantly on fresh‐cut tropical fruits at 25 °C. Both pathogen and natural microbiota were able to grow on fresh‐cut tropical fruits at 13 °C. The maximum population of L. monocytogenes was higher than that of S. aureus on fresh‐cut tropical fruits. L. monocytogenes and S. aureus could survive without growth on fresh‐cut pitaya, mango, and papaya at 5 °C. The population of L. monocytogenes declined significantly on fresh‐cut pineapple at all temperature, indicating composition of fresh‐cut pineapple could inhibit growth of L. monocytogenes. However, S. aureus was still able to grow on fresh‐cut pineapple at storage temperature. Thus, this study suggests that 4 kinds of fresh‐cut tropical fruits (pitaya, mango, papaya, and pineapple) should be stored at low temperature to extend shelf life as well as to ensure the safety of fresh‐cut fruits.     

Predictive Modeling of the Effect of ε‐Polylysine Hydrochloride on Growth and Thermal Inactivation of Listeria monocytogenes in Fish Balls

This study aimed to evaluate the effect of ε‐polylysine hydrochloride (ε‐PLH) on the growth and thermal inactivation kinetics of Listeria monocytogenes in fish balls. Samples, supplemented with ε‐PLH (0, 150, or 300 ppm, w/w), were inoculated with a three‐strain cocktail of L. monocytogenes and incubated at constant temperatures of 3.4, 8, 12, or 16 °C for growth studies, or heated at 60, 62.5, 65, or 67.5 °C for thermal inactivation tests. The growth curves were fitted to the Huang primary model, and the Huang and Ratkowsky square‐root models (SRM) were used as the secondary models to evaluate the effect of temperature on bacterial growth. The survival during heating was analyzed with a log‐linear model. The results showed that, while the lag time of L. monocytogenes was affected by both ε‐PLH concentration and temperature, the specific growth rate was unaffected by ε‐PLH. Under the same temperature, a 10‐time in increase of the lag time would be expected for every 565 ppm in the increase of ε‐PLH concentration. Using the Ratkowsky SRM, the estimated nominal minimum growth temperature was –2.04 °C, while the minimum growth temperature was 0.29 °C when estimated with the Huang SRM. Validation at 10 °C showed that the Huang primary model, in combination with either the Huang or Ratkowsky SRM, could accurately predict the growth of L. monocytogenes. On the other hand, the thermal resistance of the pathogen was significantly reduced by increase in temperature or ε‐PLH. The thermal z value of L. monocytogenes was 5.78 °C, and the ε‐PLH z value was 1642 ppm. The results of this study showed that the combined application of ε‐PLH and temperature can be used to control L. monocytogenes in fish balls and to improve food safety and reduce risks to public health.     

Combined treatment of ultraviolet‐C and L. plantarum on Salmonella enteritidis and quality control of fresh‐cut apple

Outbreaks of foodborne illnesses are big problems associated with the consumption of fresh‐cut fruits and vegetables. To enhance the safety of fresh‐cut apples, this study combined ultraviolet (UV) irradiation and lactic acid bacteria (LAB) to eliminate and inhibit the growth of Salmonella enteritidis on fresh‐cut apples. The results showed that the combination treatment of 0.96 kJ/m² UV‐C irradiation and subsequently dipping in about 5 log CFU/mL LAB suspension could reduce the population of Salmonella enteritidis by 2.67 ± 0.13 log CFU/g, which was about 2 log more reduction than distilled water treatment (the reduction was 0.36 ± 0.28 log CFU/g). During storage, Salmonella enteritidis remained stable in the combination treatment group while increased up to 5 log CFU/g in the control group. For the quality impact, the combination treatment did not decrease the color, firmness, and flavor of fresh‐cut apples but slightly deteriorated the taste attributes.

Practical applications

The markets of fresh‐cut fruits and vegetables have increased rapidly during recent decades. However, the rise of consumption of fresh‐cut fruits and vegetables also increases outbreaks of foodborne illnesses. For the health concerns of chlorine, a commonly used disinfectant, healthier, and efficient methods should to be invented to replace the chlorine treatment. Ultraviolet (UV) light possesses high bactericidal ability and lactic acid bacteria (LAB) can inhibit the growth of other microorganisms during storage. The combination can significantly decrease and inhibit other microorganisms before and during storage. It has a promising prospect to be applied in the fresh‐cut industry.     

MODEL OF MICROBIAL GROWTH ON FRESH-CUT LETTUCE TREATED WITH CHLORINATED WATER DURING STORAGE UNDER DIFFERENT TEMPERATURES

Various Gompertz models of the growth of bacteria on fresh‐cut lettuce were established in order to predict the shelf life of fresh‐cut lettuce untreated and treated with chlorinated water in storage under different temperatures. The sensory quality of fresh‐cut lettuce treated with water containing 75 µg/L of available chlorine was also examined during storage at 0, 4 and 25C, respectively. Bacteria growth on fresh‐cut lettuce in storage was analyzed to evaluate the effect of temperature, and the established model of predictive bacteria growth on fresh‐cut lettuce fitted the Gompertz modified model best. The lag time λ, the asymptote A and maximum specific growth rate μ_m of the bacteria were calculated, according to Gompertz models. The value of μ_m and A at 0C were lower than that at 4 and 25C, and the lag time was longer at a low temperature than at a higher temperature, indicating that the lower the storage temperature, the slower the growth of bacteria on lettuce, and the smaller the maximum bacteria number reached on the lettuce. The efficacy of preservation of fresh‐cut lettuce treated with chlorinated water was significantly better than that of nontreated lettuce under the same storage temperature. It was found that the final bacteria number predicted (S) on lettuce treated with chlorinated water was lower than that on nontreated lettuce, and that the higher the storage temperature, the higher the final bacteria number predicted (S). The established growth models at different temperature were able to predict the shelf life of fresh‐cut lettuce.     

Effect of Hops Beta Acids on the Survival of Unstressed‐ or Acid‐Stress‐Adapted‐Listeria Monocytogenes and on the Quality and Sensory Attributes of Commercially Cured Ham Slices

This study evaluated the antilisterial activity of hops beta acids (HBA) and their impact on the quality and sensory attributes of ham. Commercially cured ham slices were inoculated with unstressed‐ and acid‐stress‐adapted (ASA)‐L. monocytogenes (2.2 to 2.5 log CFU/cm²), followed by no dipping (control), dipping in deionized (DI) water, or dipping in a 0.11% HBA solution. This was followed by vacuum or aerobic packaging and storage (7.2 °C, 35 or 20 d). Samples were taken periodically during storage to check for pH changes and analyze the microbial populations. Color measurements were obtained by dipping noninoculated ham slices in a 0.11% HBA solution, followed by vacuum packaging and storage (4.0 °C, 42 d). Sensory evaluations were performed on ham slices treated with 0.05% to 0.23% HBA solutions, followed by vacuum packaging and storage (4.0 °C, 30 d). HBA caused immediate reductions of 1.2 to 1.5 log CFU/cm² (P < 0.05) in unstressed‐ and ASA‐L. monocytogenes populations on ham slices. During storage, the unstressed‐L. monocytogenes populations on HBA‐treated samples were 0.5 to 2.0 log CFU/cm² lower (P < 0.05) than control samples and those dipped in DI water. The lag‐phase of the unstressed‐L. monocytogenes population was extended from 3.396 to 7.125 d (control) to 7.194 to 10.920 d in the HBA‐treated samples. However, the ASA‐L. monocytogenes population showed resistance to HBA because they had a higher growth rate than control samples and had similar growth variables to DI water‐treated samples during storage. Dipping in HBA solution did not adversely affect the color or sensory attributes of the ham slices stored in vacuum packages. These results are useful for helping ready‐to‐eat meat processors develop operational procedures for applying HBA on ham slices.     

1‐Methylcyclopropene effects on temporal changes of aroma volatiles and phytochemicals of fresh‐cut cantaloupe

BACKGROUND: Orange‐fleshed cantaloupe melons have intense aroma and flavor but are very perishable during storage life. Fresh‐cut processing enhances ethylene‐mediated quality losses. Post‐cutting 1‐methylcyclopene (1‐MCP) application to fresh‐cut cantaloupe was evaluated for its effects on quality attributes, phytochemical content and aroma volatiles. RESULTS: Fresh‐cut cantaloupe (Cucumis melo var. cantalupensis ‘Fiesta’) cubes treated with 1.0 µL L^?1 of 1‐MCP for 24 h at 5 °C, packaged in vented plastic clamshells and stored under normal atmosphere at 5 °C for 9 days, preserved their soluble solids, total phenolics, total carotenoids and β‐carotene contents, but significant softening occurred. A significant increase of non‐acetate esters and a decrease of aldehydes occurred during storage. Most quality attributes of fresh‐cut cantaloupe were unaffected by the treatment with 1‐MCP. 1‐MCP‐treated fresh‐cut cantaloupe accumulated higher levels of propyl acetate, 2‐methylbutyl acetate, methyl butanoate, methyl 2‐methyl butanoate, methyl hexanoate, 2‐methylbutyl alcohol and phenethyl alcohol, and lower levels of benzyl alcohol and heptanal than untreated controls. CONCLUSION: Post‐cutting treatment with 1‐MCP affected nine of the flavor‐important volatiles, particularly those derived from the amino acids isoleucine and phenylalanine, but had no practical effect on phytochemicals or other quality attributes. © 2012 Society of Chemical Industry     

INHIBITION OF LISTERIA MONOCYTOGENES BY NATIVE MICROFLORA OF WHOLE CANTALOUPE1

Fresh‐cut cantalcupe has been recalled due to the possible presence of Listeria monocytogenes. Several studies have reported that naturally occurring microflora of vegetable surfaces may be antagonistic to pathogen attachment, growth or survival. To test this possibility for L. monocytogenes and cantaloupes, whole melon were treated with water, ethanol (70%) or chlorine (200 ppm) to reduce the native microflora on the melon surfaces. Treated or untreated melons were immersed in a six strain cocktail of L. monocytogenes (10⁷ CFU/mL) for 10 min and then allowed to dry for 1 h inside a biosafety cabinet followed by storage at 5, 10 and 20C for 15 days. Fresh‐cut pieces prepared from the treated or untreated melons and directly inoculated with L. monocytogenes (3.48 log CFU/g) were stored under the same conditions listed above. Populations of L. monocytogenes and five classes of native microflora were investigated. Growth of L. monocytogenes in sterile or nonsterile rind and fresh‐cut homogenates was also studied. The population of L. monocytogenes recovered from inoculated (10³ to 10⁸ CFU/mL) whole melons given no disinfection treatment or washed with water was significantly less (P < 0.05) than that recovered from melons treated with chlorine or EtOH. In general, populations of L. monocytogenes declined on the surface of treated and untreated whole melons and on fresh‐cut pieces over the 15 days storage period at the temperatures tested. However, the decline in pathogen populations was less rapid in the presence of reduced populations of native microflora. Higher populations of L. monocytogenes were attained in sterile tissue homogenates than in nonsterile homogenates. Addition of yeast and mold to sterile rind homogenates was highly inhibitory to growth and survival of the pathogen. The results of this study indicate that native microflora of whole cantaloupe inhibited attachment to rind surfaces as well as survival and growth of L. monocytogenes on cantaloupe surfaces and homogenized fresh‐cut pieces. Thus, L. monocytogenes recontamination of melons having a reduced level of native microflora following application of a disinfection treatment may be a food safety concern.     

Antilisterial Properties of Marinades during Refrigerated Storage and Microwave Oven Reheating against Post‐Cooking Inoculated Chicken Breast Meat

This study evaluated growth of Listeria monocytogenes inoculated on cooked chicken meat with different marinades and survival of the pathogen as affected by microwave oven reheating. During aerobic storage at 7 °C, on days 0, 1, 2, 4, and 7, samples were reheated by microwave oven (1100 W) for 45 or 90 s and analyzed microbiologically. L. monocytogenes counts on nonmarinated (control) samples increased (P < 0.05) from 2.7 ± 0.1 (day‐0) to 6.9 ± 0.1 (day‐7) log CFU/g during storage. Initial (day‐0) pathogen counts of marinated samples were <0.5 log CFU/g lower than those of the control, irrespective of marinating treatment. At 7 d of storage, pathogen levels on samples marinated with tomato juice were not different (P ≥ 0.05; 6.9 ± 0.1 log CFU/g) from those of the control, whereas for samples treated with the remaining marinades, pathogen counts were 0.7 (soy sauce) to 2.0 (lemon juice) log CFU/g lower (P < 0.05) than those of the control. Microwave oven reheating reduced L. monocytogenes counts by 1.9 to 4.1 (45 s) and >2.4 to 5.0 (90 s) log CFU/g. With similar trends across different marinates, the high levels of L. monocytogenes survivors found after microwave reheating, especially after storage for more than 2 d, indicate that length of storage and reheating time need to be considered for safe consumption of leftover cooked chicken.     

Probabilistic models for the prediction of target growth interfaces of Listeria monocytogenes on ham and turkey breast products

Abstract: This study developed growth/no growth models for predicting growth boundaries of Listeria monocytogenes on ready‐to‐eat cured ham and uncured turkey breast slices as a function of lactic acid concentration (0% to 4%), dipping time (0 to 4 min), and storage temperature (4 to 10 °C). A 10‐strain composite of L. monocytogenes was inoculated (2 to 3 log CFU/cm²) on slices, followed by dipping into lactic acid and storage in vacuum packages for up to 30 d. Total bacterial (tryptic soy agar plus 0.6% yeast extract) and L. monocytogenes (PALCAM agar) populations were determined on day 0 and at the endpoint of storage. The combinations of parameters that allowed increases in cell counts of L. monocytogenes of at least l log CFU/cm² were assigned the value of 1, while those limiting growth to <1 log CFU/cm² were given the value of 0. The binary data were used in logistic regression analysis for development of models to predict boundaries between growth and no growth of the pathogen at desired probabilities. Indices of model performance and validation with limited available data indicated that the models developed had acceptable goodness of fit. Thus, the described procedures using bacterial growth data from studies with food products may be appropriate in developing growth/no growth models to predict growth and to select lactic acid concentrations and dipping times for control of L. monocytogenes. Practical Application: The models developed in this study may be useful in selecting lactic acid concentrations and dipping times to control growth of Listeria monocytogenes on cured ham and uncured turkey breast during product storage, and in determining probabilities of growth under selected conditions. The modeling procedures followed may also be used for application in model development for other products, conditions, or pathogens.     

Combined Effect of Thermosonication and Slightly Acidic Electrolyzed Water to Reduce Foodborne Pathogens and Spoilage Microorganisms on Fresh‐cut Kale

This study evaluated the efficacy of individual treatments (thermosonication [TS+DW] and slightly acidic electrolyzed water [SAcEW]) and their combination on reducing Escherichia coli O157:H7, Listeria monocytogenes, and spoilage microorganisms (total bacterial counts [TBC], Enterobacteriaceae, Pseudomonas spp., and yeast and mold counts [YMC]) on fresh‐cut kale. For comparison, the antimicrobial efficacies of sodium chlorite (SC; 100 mg/L) and sodium hypochlorite (SH; 100 mg/L) were also evaluated. Each 10 g sample of kale leaves was inoculated to contain approximately 6 log CFU/g of E. coli O157:H7 or L. monocytogenes. Each inoculated or uninoculated samples was then dip treated with deionized water (DW; control), TS+DW, and SAcEW at various treatment conditions (temperature, physicochemical properties, and time) to assess the efficacy of each individual treatment. The efficacy of TS+DW or SAcEW was enhanced at 40 °C for 3 min, with an acoustic energy density of 400 W/L for TS+DW and available chlorine concentration of 5 mg/L for SAcEW. At 40 °C for 3 min, combined treatment of thermosonication 400 W/L and SAcEW 5 mg/L (TS+SAcEW) was more effective in reducing microorganisms compared to the individual treatments (SAcEW, SC, SH, and TS+DW) and combined treatments (TS+SC and TS+SH), which significantly (P < 0.05) reduced E. coli O157:H7, L. monocytogenes, TBC, Enterobacteriaceae, Pseudomonas spp., and YMC by 3.32, 3.11, 3.97, 3.66, 3.62, and >3.24 log CFU/g, respectively. The results suggest that the combined treatment of TS+SAcEW has the potential as a decontamination process in fresh‐cut industry.     

Development of a predictive model describing the growth of Yersinia enterocolitica in Camembert‐type cheese

Behaviour of Yersinia enterocolitica in mould‐ripened Camembert‐type cheese during storage at temperature range 3–15 °C was evaluated and mathematically described. The Baranyi and Gompertz models were adjusted to the results of the study to calculate the growth rate (GR) and lag time (LT) for Y. enterocolitica at each temperature. Goodness of fit was assessed by calculating the Akaike information criterion (AIC) and mean square error (MSE). Square root models were constructed which described the relations between GR, LT and applied storage temperature. The secondary models were mathematically validated based on the results generated by ComBase Predictor. Moreover, generated models were validated using external, independent data from ComBase database. Based on this, it was found that the square root models of Ratkowsky constructed on GR that were determined based on the Baranyi and Roberts model most accurately described the behaviour of Y. enterocolitica in Camembert‐type cheese during storage under refrigerated conditions.     

Growth or Survival of Listeria monocytogenes in Ready-to-Eat Meat Products and Combination Deli Salads During Refrigerated Storage

Growth or survival of Listeria monocytogenes in cold‐smoked salmon; sliced, cooked ham; sliced, roasted turkey; shrimp salad; and coleslaw obtained at retail supermarkets stored at 5 °C, 7 °C, or 10 °C (41 °F, 45 °F, or 50 °F, respectively) for up to 14 d was evaluated. Cold‐smoked salmon, ham, and turkey were obtained in case‐ready, vacuum packages. All food products were stored aerobically to reflect additional handling within the retail supermarket. Cold‐smoked salmon, ham, and turkey supported the growth of L. monocytogenes at all 3 storage temperatures. Fitted growth curves of initial populations (about 3 log₁₀ colony‐forming units [CFU]/g) in cold‐smoked salmon, ham, and turkey stored at 5 °C achieved maximal growth rates of 0.29, 0.45, and 0.42 log₁₀ CFU/g growth per day, respectively. Storage at 10 °C increased the estimated maximal growth rate of the pathogen by 0.56 to 1.08 log₁₀ CFU/ g growth per day compared with storage at 5 °C. A decline in populations of L. monocytogenes was observed in shrimp salad and coleslaw, and the rate of decline was influenced by storage temperature. Retention of viability was higher in shrimp salad than in coleslaw, where populations fell 1.2, 1.8, and 2.5 log₁₀ CFU/g at 5 °C, 7 °C, and 10 °C, respectively, after 14 d of storage. Inability of shrimp salad and coleslaw to support the growth of L. monocytogenes may be attributed to the acidic pH (4.8 and 4.5, respectively) of the formulations used in this study. Results show that the behavior of L. monocytogenes in potentially hazardous ready‐to‐eat foods is dependent upon the composition of individual food products as well as storage temperature.     

PREDICTIVE MODELING AND GROWTH MODELS OF AEROBIC MESOPHILIC BACTERIA ON FRESH-CUT LETTUCE BY HYPOCHLORITE-WASHING

Response surface methodology was used to evaluate the effect of three selected factors (chlorine concentration, washing time and water‐to‐lettuce ratio) on reducing aerobic mesophilic bacteria on fresh‐cut lettuce. According to statistical analysis, the model established was effective in predicting the reduction of aerobic mesophilic bacteria on fresh‐cut lettuce by washing with chlorine. In addition, best‐fit Gompertz‐modified models were described to evaluate aerobic mesophilic bacteria growth on fresh‐cut lettuce during storage at 0, 4 and 25C, respectively. The final load of aerobic mesophilic bacteria and shelf life of fresh‐cut lettuce could be predicted in various storing temperatures with the growth models.     

EFFECTIVENESS OF CHLORINE AND NISIN‐EDTA TREATMENTS OF WHOLE MELONS AND FRESH‐CUT PIECES FOR REDUCING NATIVE MICROFLORA AND EXTENDING SHELF‐LIFE1

Efficacy of nisin‐EDTA treatments as a sanitizing treatment for reducing native microflora of whole melons and extending shelf‐life of fresh‐cut pieces was compared to chlorine treatments. Whole cantaloupe and honeydew melons were washed with water, nisin (10 μg/mL)‐EDTA (0.02 M), or 200 ppm chlorine for 5 min at ～ 20C before fresh‐cut preparation and storage at 5C for 15 days with periodic microbiological sampling. In addition, some fresh‐cut pieces were washed with 10 μg/mL nisin‐EDTA or 50 ppm chlorine for 1 min before storage. Changes in appearance, odor, overall acceptability and the shelf‐life of the minimally processed fresh‐cut melons were investigated. Preliminary studies indicated that water washes, EDTA (0.002 to 0.2 M) or nisin (5 to 10 μg/mL) were not effective in reducing the microflora of whole melon when used individually. Nisin‐EDTA and chlorine treatments were significantly (P < 0.05) more effective in reducing native microflora than water washes. Nisin‐EDTA treatments were significantly (P < 0.05) more effective than chlorine in reducing populations of yeast and mold and Pseudomonas spp. on whole melon surfaces but were not as effective as chlorine treatments for reducing aerobic mesophilic bacteria, lactic acid bacteria and total gram‐negative bacteria. Microbial contaminants on fresh‐cut pieces washed with 50 ppm chlorine or nisin‐EDTA were further reduced. However, microbial populations increased throughout refrigerated storage irrespective of treatments. Odor, appearance, and overall acceptability ratings for cantaloupe and honeydew fresh‐cut pieces treated with nisin‐EDTA or chlorine were not significantly (P > 0.05) different from each other throughout the storage period (15 to 21 days). However, both treatments led to significantly (P < 0.05) improved ratings compared to the controls for the fresh‐cut pieces at 9 to 12 days of storage and thereafter. The results of this study suggest that treatments with nisin‐EDTA before and after fresh‐cut processing would improve the quality and extend the shelf‐life of fresh‐cut melon.     

A MODEL FOR PREDICTING THE GROWTH OF LISTERIA MONOCYTOGENES IN PACKAGED WHOLE MILK

Experiments were conducted to determine growth characteristics of Listeria monocytogenes in sterilized whole milk at nine temperatures in the range of 277.15 to 308.15K (4 to 35C). Based on these data, the parameter values of the Baranyi dynamic growth model were statistically determined. Finite element software, ANSYS, was used to determine temperature distributions in milk cartons subject to a time‐varying ambient temperature profile. The space‐time‐temperature data were input to the Baranyi dynamic growth model, to predict the microbial population density distribution and the average population density in the milk carton. The Baranyi dynamic growth model and the finite element model were integrated and validated using experimental results from inoculated sterilized whole milk in half‐gallon laminated paper cartons. In all experiments, the milk cartons were subjected to the same temperature profile as the Baranyi dynamic growth model. Experimental microbial counts were within predicted upper and lower bounds obtained using the integrated Baranyi dynamic growth and finite element models. In addition, the growth curve at the mean value of initial physiological state parameter for L. monocytogenes underpredicted the microbial growth (standard error = 0.54 log (cfu/mL) and maximum relative difference = 15.49%).     

Survival and Metabolic Activity of Listeria monocytogenes on Ready‐to‐Eat Roast Beef Stored at 4 °C

Three brands of commercial roast beef were purchased and artificially inoculated with a 5‐strain Listeria monocytogenes cocktail at 2 inoculation levels (approximately 3 and 6 Log CFU/g). Although all 3 brands contained sodium diacetate and sodium lactate, inoculated Listeria cocktail survived for 16 d in all 3 brands; significant increases in L. monocytogenes numbers were seen on inoculated Brand B roast beef on days 12 and 16. Numbers of L. monocytogenes increased to 4.14 Log CFU/g for the 3 Log CFU/g inoculation level and increased to 7.99 Log CFU/g for the 6 Log CFU/g inoculation level by day 16, with the pH values being 5.4 and 5.8 respectively. To measure the cell viability in potential biofilms formed, an Alamar blue assay was conducted. Brand B meat homogenate had the highest metabolic activities (P < 0.05). By comparing its metabolic activities to Brands A and C and the inoculated autoclaved meat homogenates, results indicated that the microflora present in Brand B may be the reason for high metabolic activities. Based on the denaturing gradient gel electrophoresis and the Shannon–Wiener diversity index analysis, the “Brand” factor significantly impacted the diversity index (P = 0.012) and Brand B had the highest microflora diversity (Shannon index 1.636 ± 0.011). Based on this study, results showed that antimicrobials cannot completely inhibit the growth of L. monocytogenes in ready‐to‐eat roast beef. Native microflora (both diversity and abundance), together with product formula, pH, antimicrobial concentrations, and storage conditions may all impact the survival and growth of L. monocytogenes.     

Growth modelling of Listeria monocytogenes in packaged fresh green asparagus

Growth of Listeria monocytogenes in packaged fresh green asparagus was investigated at different temperatures. The present study monitored the growth of L. monocytogenes in packaged fresh green asparagus stored at 2, 4, 8, 12 and 20°C. Changes in carbon dioxide, oxygen concentrations and pH were determined. Growth data were fitted to the Baranyi equation. The influence of storage temperature was very apparent, at higher temperatures (8, 12 and 20°C) an increase of two logarithmic units of L. monocytogenes took place, criteria used for the determination of the sanitary risk. The recommended storage temperatures for the optimal and safe conservation of asparagus are the lower ones studied (2 and 4°C), because growth of L. monocytogenes is not allowed, although it can survive, so in cases of a high contamination of this micro-organism it could be a sanitary risk. Predictive modelling appears to be a useful tool in estimating growth rates ofL. monocytogenes in these products.     

Influence of soil type and storage conditions on sensory qualities of fresh‐cut cantaloupe (Cucumis melo)

On‐farm cantaloupe (Cucumis melo) production as well as fresh‐cut storage duration can affect postharvest fruit sensory attributes. Both effects of soil type during production of cantaloupe fruits and storage temperature after fresh‐cut processing on sensory flavour and texture attributes were determined. Melons grown in sandy loam vs heavy clay soil were lower in sweet aromatic and sweet taste and higher in moisture release and fermented flavour. Fruity/melon, sweet aromatic, surface wetness, hardness and moisture release attributes decreased while fermented and sour flavour increased during storage regardless of soil type. During storage an increase in peroxidase activity occurred in fruits produced in sandy loam soil but decreased in fruits produced in clay soil. Clay soil appeared to have some advantages over sandy loam soil in producing cantaloupe fruits with better sensory quality attributes. Storage temperature conditions in this experiment (4 °C for 10 days or 4 °C for 4 days plus 10 °C for 6 days) did not have a statistically significant effect on these sensory attributes. Copyright © 2004 Society of Chemical Industry     

Antimicrobial activity of buckwheat starch films containing zinc oxide nanoparticles against Listeria monocytogenes on mushrooms

Buckwheat starch (BS) films containing zinc oxide nanoparticles (ZnO‐N; 0%, 1.5%, 3% and 4.5%) were prepared, and their physical, optical and antimicrobial properties were examined. As ZnO‐N content increased from 0% to 4.5%, TS increased from 14.99 to 19.09 MPa and E decreased from 25.60% to 20.65%. In addition, L* and a* values decreased, whereas b*, ΔE and opacity increased. Regarding antimicrobial activity, the BS/ZnO‐N films had the reductions of 2.96–3.74 log CFU mL^?1 against Listeria monocytogenes after 8 h based on viable cell count assay. The BS film containing 3% ZnO‐N, an optimal concentration chosen in this study, was applied to fresh‐cut mushroom packaging, and the film exhibited antimicrobial activity against L. monocytogenes, resulting in a reduction of 0.86 log CFU g^?1 after 6 days of storage. Thus, these results indicate that the BS/ZnO‐N film can be used as a biodegradable packaging material.