Modeling the Growth Characteristics of Listeria monocytogenes and Native Microflora in Smoked Salmon

ABSTRACT:  Smoked salmon contaminated with Listeria monocytogenes has been implicated in foodborne listeriosis. The objectives of this study were to model the growth characteristics and examine the growth relationship of L. monocytogenes and native microflora in smoked salmon. Smoked salmon samples with a native microflora count of 2.9 log₁₀ CFU/g were inoculated with a 6-strain mixture of L. monocytogenes to levels of log₁₀ 1.6 and log₁₀ 2.8 CFU/g, and stored at 4, 8, 12, and 16 °C. Growth characteristics (lag phase duration [LPD, h], growth rate [GR, log₁₀ CFU/h], and maximum population density [MPD, log₁₀ CFU/g]) of L. monocytogenes and native microflora were determined. At 4 to 16 °C, the LPD, GR, and MPD were 254 to 35 h, 0.0109 to 0.0538 log₁₀ CFU/h, and 4.9 to 6.9 log₁₀ CFU/g for L. monocytogenes , respectively, and were 257 to 29 h, 0.0102 to 0.0565 log₁₀ CFU/h, and 8.5 to 8.8 log₁₀ CFU/g for native microflora. The growth characteristics of L. monocytogenes or the native microflora were not significantly different ( P > 0.05), regardless the initial levels of L. monocytogenes . Mathematical equations were developed to describe the LPD, GR, and MPD of L. monocytogenes and native microflora as a function of storage temperature. The growth relationship between L. monocytogenes and native microflora was modeled and showed that the LPD and GR of L. monocytogenes were similar to those of native microflora. These models can be used to estimate the growth characteristics of L. monocytogenes in smoked salmon, and thereby enhance the microbiological safety of the product.     

SPOILAGE AND HISTAMINE IN WHOLE PACIFIC HERRING (CLUPEA HARENGUS PALLASI) AND PINK SALMON (ONCORHYNCHUS GORBUSCHA) FILLETS

Fresh pink salmon (Oncorhynchus gorbuscha) fillets and whole Pacific herring (Clupea harengus pallasi) were stored for two weeks at 10C to determine if significant amounts of histamine were produced before the fish spoiled. Spoilage odors in salmon were moderate by day 4 and intense by day 7, while herring had detectable spoilage by day 4 and became potent by day 6. Aerobic bacterial counts increased from 4.0 × 10²/g initially to 3.6 × 10⁸/g in salmon fillets by day 14 and from 2.3 × 10³/g initially to 2.7 × 10⁷/g in whole herring by day 14. Total volatile nitrogen increased from 1.8 to 78.5 mg N/100 g in salmon and 2.2 to 23.6 mg N/100 g in herring. Histamine was not detected in salmon, while concentrations reached 54.9 ppm in herring at day 14. However, herring were considered spoiled by day 6.     

Combining Antimicrobial and Steam Treatments in a Vacuum-packaging System to Control Listeria monocytogenes on Ready-to-eat Franks

ABSTRACT: This study was to evaluate the effectiveness of steam or steam in combination with an antimicrobial agent to control Listeria monocytogenes on ready-to-eat (RTE) franks. The franks were surface-inoculated to contain 6 or 3 log₁₀(colony-forming units [CFU])/cm² of L. monocytogenes and treated with steam or steam in combination with an antimicrobial agent, immediately followed by vacuum-sealing the top films of frank packages (6 franks per package in a single layer). Three log (CFU) /cm² of reductions were achieved at the both inoculation levels for L. monocytogenes on franks. At an inoculation level of 3 logs, no outgrowth of L. monocytogenes was obtained on the treated franks after storing at 4.4°C or 16°C for a combined 47 d. This study provided an alternative approach for controlling L. monocytogenes in packaged franks.     

Development of Biogenic Amines in Yellowfin Tuna (Thunnus albacares): Effect of Storage and Correlation with Decarboxylase-Positive Bacterial Flora

ABSTRACT: The effects of storage at 0,4,10, and 22°C for 0,1,3,5, and 9 d on the quality of yellowfin tuna fillets as determined by microbiological assessment, development of some biogenic amines, and sensory analysis were studied. Tuna fillets stored at 22 °C for 3 d, 10 °C for 5 d, and 4 °C for 9 d were rated unacceptable for consumption. Those stored at 22 °C for 3 d had total aerobic bacterial count of > 8 log₁₀ CFU/g, a histamine-producing bacterial population of 7 log₁₀ CFU/g, and 832 ppm of histamine, 35.8 ppm of putrescine, and 147 ppm of cadaverine. A comparison of the capillary electrophoresis, AOAC fluorometric method, and gas chromatography showed a very good correlation (r² > 0.99) among these 3 methods for histamine quantitation in tuna samples. Morganella morganii, Enterobacter agglomerans, Enterobacter intermedium, Pseudomonas fluorescens, Proteins vulgaris , and Serratia liquefaciens were the decarboxylase-positive bacterial species isolated by using the Niven's medium and identified during storage, which were responsible for histamine production in test tuna fillets.     

Reduction of Escherichia Coli O157:H7 and Lactobacillus Plantarum Numbers on Fresh Beef by Polylactic Acid and Vacuum Packaging

ABSTRACT: The efficacy of 2% molecular weight 240, 2% molecular weight 360 polylactic acid (PLA), and an equal mix of both at reducing numbers of Escherichia coli O157:H7 and Lactobacillus plantarum on raw beef was determined. Fresh beef cubes inoculated with either organism were dipped in PLA solutions or wrapped in PLA-sprayed films. Samples were vacuum packaged and stored at 4°C for 42 d. Treated samples maintained a significantly lower pH than controls. Growth of E. coli O157:H7 was totally inhibited by both PLA treatments by up to 7.29 log₁₀ CFU/cm² when the spray method was used. However, PLA treatments against L. plantarum were not very effective.     

INFLUENCE OF WASHING TREATMENT ON NATIVE MICROFLORA AND ESCHERICHIA COLI POPULATION OF INOCULATED CANTALOUPES

The influence of chlorine or hydrogen peroxide treatment on populations of Escherichia coli 25922 on the external surface of inoculated cantaloupe was investigated. Surface treatment with 70% EtOH, followed by immersion in 10⁸ CFU/mL E. coli inoculum deposited an average of 4.4 log₁₀CFU/cm² cell population on the cantaloupe surface. The efficncy of washing inoculated cantaloupe was dependent on storage interval between inoculation and treatment. Dipping the cantaloupes in solutions containing 1000 mg/L chlorine or 5% peroxide for 5 min, within 24 h of inoculation, caused a 2 log₁₀ CFU/cm² reduction of the indigenous surface microflora and a 3–4.0 log₁₀ CFU/cm² reduction in E. coli. The efficacy was less when the interval between inoculation and treatment exceeded 24 h. Chlorine appeared in be a better antimicrobial agent than hydrogen peroxide against F. coli ATCC 25922 inoculated on cantaloupe surfaces while hydrogen peroxide was better in reducing surface microflora of cantaloupe.     

DRY ICE IN VARIOUS SHIPPING BOXES FOR CHILLED POULTRY: EFFECT ON MICROBIOLOGICAL AND ORGANOLEPTIC QUALITY

SUMMARY– Comparisons were made of dry ice and weter ice in shipping boxes for chilled chickens. Three types of boxes were tested: wax-resin-coated corrugated fiberboard, expanded polystyrene foam, and wirebound wood-veneer. Microbial counts, CO₂ concentration, and off-odor development were determined. Microbial counts on poultry stored at 0.5°C for up to 9 days were not significantly different as a function of box type or coolant. Counts on poultry stored at 4.4°C were significantly greater at 9 days on poultry stored in fiberboard boxes with dry ice than on poultry with water ice in fiberboard boxes or polystyrene boxes with dry ice; at 3 and 6 days there were no significant differences. At 5.2°C, counts were significantly smaller in polystyrene boxes with dry ice than in either wirebound boxes with water ice or fiberboard boxes with dry ice. An off-odor not characteristic of spoilage odor could develop in CO₂ atmosphere storage earlier than spoilage odor in an air atmosphere if storage temperature was low (0.5°C). At higher temperature (5.2°C), spoilage odor in air occurred earlier than Co₂-related off-odor in Co₂ atmosphere.     

EARLY DETECTION OF CLOSTRIDIUM PERFRINGENS ENTEROTOXIN AND ITS RELATIONSHIP TO SENSORY QUALITY OF COOKED CHICKEN

Growth, sporulation, and enterotoxin formation by Clostridium perfringens NCTC 8239 were determined in chicken thigh meat incubated at 45°C for 1.5 h and 37°C for up to 12.5 h. With an inoculum of 10⁶ vegetative cells per g, the cell counts reached mean log ₁₀ 7.32/g after 6 h of incubation and remained in that range through 14 h. Heat-resistant spores (log₁₀ 2.48/g) were first detected at 4 h, and the number increased to log₁₀ 5.19/g at 14 h. Enterotoxin (0.19 μg/g) was first detected after 2 h of incubation (1.5 h at 45°C and 0.5 h at 37°C) in the absence of detectable sporulation, and the enterotoxin concentration increased to 0.76 μg/g after 14 h. Significant differences (p < 0.01) in the odor, color, and texture scores for inoculated versus uninoculated cooked chicken following 2 h incubation correlated with the production of enterotoxin and suggested that these parameters could be used as indices of chicken spoilage by C. perfringens.     

Low-level heat-treatment to extend shelf-life of fresh fish

The application of heat to reduce the microbial load and extend the lag phase was studied on whole fish and with bacterial isolates from fresh fish in mixed culture. Bacterial isolates obtained from fresh horse-mackerel ( Trachurus trachurus ) were heat treated at 60°C for 20s and stored in nutrient broth on ice. the flora were shown to be heat sensitive and the initial numbers were reduced by over 2.0 log₁₀ cycles. Heating at 60°C extended the lag phase but subsequent growth rates were increased, giving a shelf-life extension of a day and a half. There were no significant effects on visual EC grades of whole fish or on the flavour of cooked fillets.     

Effect of Salt, Smoke Compound, and Temperature on the Survival of Listeria monocytogenes in Salmon during Simulated Smoking Processes

ABSTRACT:  The objectives of this study were to examine and develop a model to describe the survival of  Listeria monocytogenes  in salmon as affected by salt, smoke compound (phenol), and smoking process temperature. Cooked minced salmon containing selected levels of salt (0%, 2%, 4%, and 6%) and smoke compound (0, 5, 10, and 15 ppm phenol) were inoculated with a 6-strain mixture of  L. monocytogenes  to an inoculum level of 6.0 log₁₀ CFU/g. The populations of  L. monocytogenes  in salmon during processing at 40, 45, 50, and 55 °C that simulated cold- and hot-smoking process temperatures were determined, and the effects of salt, phenol, and temperature on the survival of  L. monocytogenes  in salmon were analyzed and described with an exponential regression. At 40 °C, the populations of  L. monocytogenes  in salmon decreased slightly with inactivation rates of <0.01 log₁₀ CFU/h, and at 45, 50, and 55 °C, the inactivation rates were 0.01 to 0.03, 0.15 to 0.30, and 2.8 to 3.5 log₁₀ CFU/h, respectively. An exponential regression model was developed and was shown to closely describe the inactivation rates of  L. monocytogenes  as affected by the individual and combined effects of salt, phenol, and smoking process temperature. Temperature was the main effector in inactivating  L. monocytogenes  while salt and phenol contributed additional inactivation effects. This study demonstrated the inactivation effects of salt, smoke compound, and temperature on  L. monocytogenes  in salmon under a smoking process. The data and model can be used by manufacturers of smoked seafood to select concentrations of salt and smoke compound and alternative smoking process temperatures at 40 to 55 °C to minimize the presence of  L. monocytogenes  in smoked seafood.     

Microwave Oven Heating for Inactivation of Listeria Monocytogenes on Frankfurters before Consumption

ABSTRACT:  Microwave oven heating was evaluated for inactivation of  Listeria monocytogenes  on inoculated and stored frankfurters. Frankfurters formulated without/with 1.5% potassium lactate and 0.1% sodium diacetate were inoculated with  L. monocytogenes  (1.9 ± 0.2 log CFU/cm²), vacuum-packaged, and stored (4 °C) to simulate conditions prior to purchase by consumers. At storage days 18, 36, and 54, packages were opened and placed at 7 °C, simulating aerobic storage in a household refrigerator. At 0, 3, and 7 d of aerobic storage, 2 frankfurters were placed in a bowl with water (250 mL) and treated in a household microwave oven at high (1100 W) power for 30, 45, 60, or 75 s, or medium (550 W) power for 60 or 75 s. Frankfurters and the heating water were analyzed for total microbial counts and  L. monocytogenes  populations. Exposure to high power for 75 s reduced pathogen levels (0.7 ± 0.0 to 1.0 ± 0.1 log CFU/cm²) to below the detection limit (<−0.4 log CFU/cm²) on frankfurters with lactate/diacetate, even after 54 d of vacuum-packaged storage followed by 7 d of aerobic storage. For frankfurters without lactate/diacetate, high power for 75 s caused reductions between > 1.5 and 5.9 log CFU/cm² from control levels of 1.5 ± 0.1 to 7.2 ± 0.5 log CFU/cm². Depending on treatment and storage time, the water used to reheat the frankfurters had viable  L. monocytogenes  counts of <−2.4 to 5.5 ± 0.5 log CFU/mL. The results indicated that frankfurters should be reheated in a microwave oven at high power for 75 s to inactivate up to 3.7 log CFU/cm² of  L. monocytogenes  contamination.     

MODIFIED-ATMOSPHERE PACKAGING OF BONE-IN PORK LOINS

Paired bone-in pork loins (n = 110 pairs) trimmed to 3.2 and 6.4 mm subcutaneous fat were placed five per box in modified-atmosphere packages (MAP) and stored for up to 19 days at 1°C. Using commercial conditions and a single flush of 0.61 1/kg of meat of carbon dioxide (CO₂), the resulting CO₂ concentration was 78.5%, which then decreased to 55.1% (day 3) and 41% (day 19). Oxygen, 3.1% initially, increased to 7.2% (day 3), decreased to 3% (day 15), then increased to 7.1%. Although microbial counts increased from 3 to 19 days, those through 9 days of MAP (331 CFU/cm²) were lower than prior to MAP (653 CFU/cm²). Longer storage increased (P < 0.05) loin weight loss; off-odor; and discoloration of the blade and sirloin lean, subcutaneous fat, and bone surface. Off-odor intensity and microbial levels were acceptable at day 19, thus discoloration was the limiting factor in loin shelf-life and retail chop display color stability.     

Efficacy of pulsed UV-light for the decontamination of Escherichia coli O157:H7 and Salmonella spp. on raspberries and strawberries

ABSTRACT:  Small fruits are increasingly being implicated in outbreaks of foodborne illness, and fresh produce is now the 2nd leading cause of foodborne illness in the United States. Conventional methods of decontamination are not effective, and there is a need to evaluate novel technologies. Pulsed ultraviolet (UV)-light is one such technology. In this study, pulsed UV-light was applied to strawberries and raspberries at varying UV doses and times. On raspberries, maximum reductions of Escherichia coli O157:H7 and Salmonella were 3.9 and 3.4 log₁₀ CFU/g at 72 and 59.2 J/cm², respectively. On the surfaces of strawberries, maximum reductions were 2.1 and 2.8 log₁₀ CFU/g at 25.7 and 34.2 J/cm², respectively. There was no observable damage to the fruits at these UV doses. The results obtained in this study indicate that pulsed UV-light has the potential to be used as a decontamination method for raspberries and strawberries.     

Chitosan Protects Cooked Ground Beef and Turkey Against Clostridium perfringens Spores During Chilling

ABSTRACT:  We investigated the inhibition of Clostridium perfringens spore germination and outgrowth by the biopolymer chitosan during abusive chilling of cooked ground beef (25% fat) and turkey (7% fat) obtained from a retail store. Chitosan was mixed into the thawed beef or turkey at concentrations of 0.5%, 1.0%, 2.0%, or 3.0% (w/w) along with a heat-activated 3-strain spore cocktail to obtain a final spore concentration of 2 to 3 log₁₀ CFU/g. Samples (5 g) of the ground beef or turkey mixtures were then vacuum-packaged and cooked to 60 °C in 1 h in a temperature-controlled water bath. Thereafter, the products were cooled from 54.4 to 7.2 °C in 12, 15, 18, or 21 h, resulting in 4.21, 4.51, 5.03, and 4.70 log₁₀ CFU/g increases, respectively, in C. perfringens populations in the ground beef control samples without chitosan. The corresponding increases for ground turkey were 5.27, 4.52, 5.11, and 5.38 log₁₀ CFU/g. Addition of chitosan to beef or turkey resulted in concentration- and time-dependent inhibition in the C. perfringens spore germination and outgrowth. At 3%, chitosan reduced by 4 to 5 log₁₀ CFU/g C. perfringens spore germination and outgrowth ( P ≤ 0.05) during exponential cooling of the cooked beef or turkey in 12, 15, or 18 h. The reduction was significantly lower ( P < 0.05) at a chilling time of 21 h, about 2 log₁₀ CFU/g, that is, 7.56 log₁₀ CFU/g (unsupplemented) compared with 5.59 log₁₀ CFU/g (3% chitosan). The results suggest that incorporation of 3% chitosan into ground beef or turkey may reduce the potential risk of C. perfringens spore germination and outgrowth during abusive cooling from 54.4 to 7.2 °C in 12, 15, or 18 h.     

TVB-N Correlation with Odor Evaluation and Aerobic Plate Count in Mahi-Mahi (Coryphaena hippurus)

ABSTRACT: Two groups of 6 mahi-mahi fillets, one untreated and the other dipped in a suspension of Morganella morganii , were refrigerated at 7°C and sampled after 0, 2, 4, 6, 8, and 10 d of storage. Results were similar for both groups of fish. Total volatile base-nitrogen (TVB-N) reached 30 mg-N/100 g on d 3, at which time aerobic plate count (APC) reached 10⁶ colony forming units (CFU)/g and odor scores reached 6 on a scale of 1 to 10 (10 very fresh and 1 very spoiled). Good correlation existed between TVB-N and odor intensity (r = 0.84), between TVB-N and the log₁₀ APC (r = 0.71), and between odor and the log₁₀ APC (r = 0.91). These results show that TVB-N can serve as a good indicator of chilled mahi-mahi quality.     

MICROBIOLOGICAL STATUS OF BEEF CARCASS MEAT IN INDONESIA

The microbiological status of beef carcass meat was investigated at a traditional and a semi-modern abattoir in Indonesia. Carcasses from the traditional abattoir were generally less contaminated than those from the semi-modern abattoir. The average total aerobic viable bacterial count of carcasses produced at the traditional abattoir, (log₁₀ 3.62/cm²) was found to be significantly lower (p = 0.02) at the semi-modern abattoir (log₁₀ 4.02/cm²). More than 70% of carcasses at both abattoirs were found to be positive for Escherichia coli (> 10 organisms/cm²), while Salmonellae were found on approximately 15% of carcasses at both locations.     

EFFECT OF MARINADE AND DRYING TEMPERATURE ON INACTIVATION OF ESCHERICHIA COLI O157:H7 ON INOCULATED HOME DRIED BEEF JERKY

Beef slices were inoculated (5.7–7.5 log CFU/cm²) with a 4-strain composite of E. coli O157:H7, stored (4C, 24 h), marinated (4C, 24 h), dried for 10 h at 62.5C or 68.3C, and stored for 90 days at 21C. Unmarinated beef slices dried for 10 h at 62.5C were used to determine the relative contribution of the marinate versus temperature treatment in the 62.5C trials. Samples were analyzed (bacterial enumeration with selective and nonselective agar media, pH, and a_w) following inoculation, marinating, at 4, 6, 8 and 10 h of drying, and after 30, 60 and 90 days of storage. Marination resulted in slight changes in bacterial populations (−0.3 to + 0.6 log CFU/cm²), but did not enhance bacterial reduction during drying. For all treatments, most bacterial reductions occurred in the first 4 h of drying, with little reduction thereafter. After 10 h of drying, bacterial reductions were 3.2–3.4 log CFU/cm² for unmarinated beef slices dried at 62.5C. Reductions of 2.2 and 3.0–4.6 log CFU/cm² were achieved in marinated jerky slices dried at 62.5C and 68.3C, respectively. No treatment resulted in the recommended 5-log reduction at the end of 10 h drying. However, bacteria did become undetectable by direct plating (<10 CFU/cm²) following 30 days of storage in all treatments except the unmarinated beef slices plated on tryptic soy agar (TSA). Additional work is needed to develop procedures for adequate destruction of E. coli O157:H7 during drying of beef jerky.     

Effect of irradiation and modified atmosphere packaging on the microbiological safety of minced pork stored under temperature abuse conditions

The safety of irradiated pork packed in 25% CO₂:75% N₂ and stored at abuse temperature (10 or 15°C) was assessed by inoculation studies involving Salmonella typhimurium, Listeria monocytogenes, Escherichia coli, Yersinia enterocolitica and Clostridium perfringens . Irradiation to a dose of 1.75 kGy reduced pathogen numbers to below the detection limit of 10² cells g^-1. When higher inoculum levels were used (10⁶ cells g^-1) irradiation at 1.75 kGy reduced pathogen numbers by 1 –>5 log₁₀ cycles depending on strain. Clostridium perfringens was the most resistant, and Y. enterocolitica the most sensitive of the pathogens studied.
In all cases when high numbers (10⁶ to 10⁷g^-1) of spoilage and/or pathogenic bacteria were present initially on the pork the meat appeared spoiled, and although irradiation reduced the number of microorganisms, the meat was still unacceptable from a sensory viewpoint after treatment.
It was concluded that the microbiological safety of irradiated, modified atmosphere packaged (MAP) pork is better than that of unirradiated MAP pork.     

SURVIVAL OF SALMONELLA TYPHIMURIUM, LISTERIA MONOCYTOGENES AND INDICATOR BACTERIA ON COOKED UNCURED TURKEY LOAF STORED UNDER VACUUM AT 3°C

Sterile slices of cooked uncured turkey loaf were inoculated with 10⁶ CFU of either Salmonella typhimurium, Listeria monocytogenes, Escherichia coli, Enterococcus faecalis, Citrobacter freundii, Klebsiella pneumoniae, or Enterobacter cloacae. Inoculated samples were vacuum-packaged and stored at 3 ± 1°C. Microorganisms were enumerated at 0, 3, 6, 9, 12, and 15 days on nonselective media . K. pneumoniae exhibited the least cold-tolerance with a log₁₀ 1.70 decrease in numbers. The coliforms E. cloacae, E. coli, and C. freundii had a survival pattern similar to that of S. typhimurium, with population decreases of log₁₀ 0.65, 0.82, 1.13, and 0.79, respectively . E. faecalis and L. monocytogenes were significantly more cold-resistant, with a decrease of log₁₀ 0.20 and no significant change in numbers, respectively. Survival of E. faecalis was not significantly (p < 0.01) different than that of L. monocytogenes, suggesting the use of enterococci as indicators of L. monocytogenes contamination of processed meats .     

INHIBITION OF STAPHYLOCOCCUS AUREUS AND BACILLUS CEREUS ON A VEGETARIAN FOOD TREATED WITH NISIN COMBINED WITH EITHER POTASSIUM SORBATE OR SODIUM BENZOATE

Various amounts of nisin (0, 10³ and 5 × 10³ IU/g) in combination with either potassium sorbate (0, 2, and 3%) or sodium benzoate (0, 0.06 and 0.12%) were tested for effectiveness in inhibiting growth of Staphylococcus aureus C10 and Bacillus cereus B7 inoculated on a vegetarian food. The strains used were isolated from vegetarian foods obtained commercially in Taiwan, and the test food, spice and dried bean curd, was selected for the study based on ability to support the growth of these organisms. After treatment with a preservative combination, the surfaces of sterilized food samples were inoculated, samples were stored in vacuum or nonvacuum packages at either 4C or 30C, and at appropriate times, tested for microbial growth. Growth of both isolates was unaffected by vacuum-packaging treatment; however, a bacteriostatic effect was found at 4C. Data indicated that during the 14-day storage at 4C, vacuum-packaged samples treated with 5 × 10³ IU/g nisin and 0.12% sodium benzoate significantly (p < 0.05) decreased the counts of S. aureus C10 and B. cereus B7 by 2.61 and 3.02 log₁₀ CFU/g, respectively. In the vacuum-packaged samples treated with 5 × 10³ IU/g nisin and 3% potassium sorbate, counts for C10 and B7 were decreased by 2.35 and 2.64 log₁₀ CFU/g, respectively. Thus, the combined treatment extended the shelf-life of the vegetarian food .