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.     

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 .     

Growth of Salmonella enterica serovar Enteritidis in albumen and yolk contents of eggs inoculated with this organism onto the vitelline membrane

By using an in vitro model simulating the potential opportunities for Salmonella enterica serovar Enteritidis (SE) to proliferate within eggs contaminated with this organism following oviposition, we investigated growth of SE in eggs. Seventy to 140 CFU of one of three SE strains originating either from egg contents, chicken meat, or a human infection were experimentally inoculated onto the vitelline membrane of eggs collected from specific-pathogen-free flocks of chickens and incubated at 25 degrees C. SE organisms were detected in 6 of 71 yolk contents of the eggs inoculated with any of the test strains attaining levels ranging from 2.0 x 10(2) to 4.2 x 10(8) CFU/ml by day 6. The organisms were also detected in the albumen from 38 of 55 eggs tested, growing to levels ranging from 1.0 x 10(2) to 4.3 x 10(8) CFU/ml by day 6 after inoculation. An additional three yolk contents and 15 albumen samples were culture positive for SE following enrichment. There was no correlation between the number of the organisms in the yolk contents and that in the albumen from each of the eggs. When 73 to 91 CFU of the egg strain were inoculated into samples of separated albumen obtained from eggs that were stored at 4 degrees C for 1 to 4 weeks or at 25 degrees C for 1 week, slight growth (3.0 x 10(2) to 7.4 x 10(3) CFU/ml) was found in only 3 of the 60 albumen samples by day 6 after inoculation, but the organisms were recovered from 52 samples following enrichment. The results suggest that the environment on or near the vitelline membrane can be conducive to SE proliferation over time.     

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.     

POTENTIAL FOR BACTERIAL GROWTH ON THE FRESH CUT TROPICAL SQUASH, CALABAZA (CURCUBITA MOSCHATA), DURING STORAGE

Calabaza (Curcubita moschata) is a tropical squash which is gaining popularity as a specialty crop for agricultural producers in the Northeast United States. It is commonly marketed by being cut in half, wrapped in plastic and may be held unrefrigerated until sold. This method of display is essential for consumer acceptance, yet unrefrigerated storage means that some potential for food safety problems exists. Experiments were conducted to determine the potential for bacterial growth during storage of cut calabaza. Freshly cut calabaza contained between 1.3 and 4.7 log₁₀CFU/g aerobic mesophiles. By 10 h, duplicate counts from some samples exceeded 4 log₁₀ CFU/g. After 24 h of room temperature storage, total aerobic plate counts ranged from 5.2 to 7.7 log₁₀ CFU/g. Rapid bacterial growth on cut calabaza stored at room temperature indicates that these products are highly perishable, and may be able to support the growth of pathogenic bacteria, should they be introduced during the slicing process.     

Multiplication of Salmonella enteritidis in egg yolks after inoculation outside, on, and inside vitelline membranes and storage at different temperatures

Prompt refrigeration to restrict bacterial growth is important for reducing eggborne transmission of Salmonella enterica serovar Enteritidis (SE). The nutrient-rich yolk interior is a relatively infrequent location for initial SE deposition in eggs, but migration across the vitelline membrane can result in rapid bacterial multiplication during storage at warm temperatures. The objective of the present study was to measure the multiplication of SE in yolks after introduction at three different locations and subsequent storage at a range of temperatures. Using an in vitro egg contamination model, approximately 100 CFU of SE was inoculated either inside yolks, onto the exterior surface of vitelline membranes, or into the adjacent albumen. After storage of samples from each inoculation group at 10, 15, 20, and 25°C for 24 h, SE was enumerated in yolks. For all three inoculation locations, the final SE levels in yolks increased significantly with increasing storage temperatures. At all storage temperatures, significant differences in SE multiplication were observed between inoculation sites (yolk inoculation>vitelline membrane inoculation>albumen inoculation). At 25°C, final log concentrations of 7.759 CFU of SE per ml (yolk inoculation), 2.014 CFU/ml (vitelline membrane inoculation), and 0.757 CFU/ml (albumen inoculation) were attained in yolks after storage. These results demonstrate that, even when the initial site of SE deposition is outside the egg yolk, substantial multiplication supported by yolk nutrients can occur during the first day of storage and the risk of bacterial growth increases at higher ambient storage temperatures.     

Sensitivity analysis of Salmonella enteritidis levels in contaminated shell eggs using a biphasic growth model

Salmonella enteritidis (SE) is a common foodbome pathogen, the transmission of which is primarily associated with the consumption of contaminated Grade A shell eggs. In order to estimate the level of SE present in raw shell eggs, it is necessary to consider the protective effects of the egg albumin, which effectively inhibits SE growth in a time- and temperature-dependent manner. In this study, a SE growth model was produced by combining two mathematical equations that described both the extended lag phase of SE growth (food component) and a SE growth model (pathogen component). This biphasic growth model was then applied to various egg handling scenarios based on the farm-to-table continuum, including in-line and off-line processing facilities with consideration of key events in production, processing, transportation, and storage. Seasonal effects were also studied. Monte Carlo simulation was used to characterize variability in temperature and time parameter values influencing the level of SE to which individuals are exposed. The total level of SE consumed was estimated under best, most likely, and time-temperature abusive handling scenarios. The model estimated that, in most cases, there was no SE growth in contaminated eggs handled under most likely practices, because 10-70% of the yolk membrane remained intact. Under abusive handling scenarios, complete loss of yolk membrane integrity frequently occurred by the time eggs reach the distribution phase, followed by subsequent SE growth, which was often quite rapid. In general, the effect of season and processing method (in-line vs. off-line) was minimal. Further sensitivity analysis demonstrated that the initial SE contamination level significantly influenced the final exposure levels only under no-abuse or mildly abusive conditions. The results of our study suggest that, for maximum reduction of SE exposure level, cooling strategies should not only focus on the on-farm or processing phases, but should emphasize the importance of cooling strategies at the distribution and consumer phases of the farm-to-fork continuum.     

Effects of Shelf-Life Enhancers on E. coli K12 Survival in Solutions Used to Enhance Beef Strip Steaks

ABSTRACT:  The objective of this study was to evaluate the effects of enhancement solutions containing sodium lactate or sodium lactate/sodium diacetate on E. coli K12 transmission to beef strip steaks and in purge. Solutions containing salt, phosphate, and shelf-life enhancers were injected to 10% over initial weight of the steaks. Lactate or lactate/diacetate addition to a solution inoculated with 6 log₁₀ E. coli K12 CFU/mL was equally effective in limiting growth resulting in a 1-2 log₁₀ CFU/g reduction when compared to salt/phosphate in steaks. When inoculation level was 3 log₁₀ CFU/mL in the enhancement solution, microbial growth was detectable only in the purge of steaks enhanced with salt/phosphate only. Lactate increased CIE L* value and b* and decreased a* values when compared to the control. Lactate-/diacetate-containing solutions decreased L* values when compared to lactate alone. Lactate/diacetate also reduced purge loss from steaks compared to lactate and salt/phosphate solutions.     

Portable Electronic Nose for Detection of Spoiling Alaska Pink Salmon (Oncorhynchus gorbuscha)

ABSTRACT:  The ability of a portable hand-held electronic nose (EN) in detecting spoilage of whole Alaska pink salmon ( Oncorhynchus gorbuscha ) stored at 14 °C and in slush ice (1 °C) was investigated. Fish were sampled daily at 14 °C for up to 3 d, while fish stored in slush ice were sampled at various intervals up to 16 d. Sensory evaluations indicated that fish were rejected at day 3 when stored at 14 °C and at day 12 when stored in slush ice. Aerobic bacteria counts for fish skin at 14 °C ranged from 3.4 log₁₀ colony-forming units (CFU)/cm² (day 0) to 4.8 log₁₀ CFU/cm² (day 3) and for fish stored in slush ice ranged from 3.4 log₁₀ CFU/cm² (day 0) to 5.5 log₁₀ CFU/cm² (day 16). The correct classification rate using forward stepwise general discriminate analysis was 85% and 92% for EN analysis of belly cavity volatiles for fish held at 14 °C and in slush ice, respectively. A predictive model may be developed for spoilage of whole Alaska pink salmon by analyzing belly cavity odors using the EN.     

Super Chilling Enhances Preservation of the Freshness of Salted Egg Yolk During Long-Term Storage

ABSTRACT:  Pasteurized egg yolk with 10% (w/w) salt was stored at 5, –5, –15, –20, and –30 °C for 1 to 6 mo, respectively. Changes in generation of volatiles of the stored samples (5 and −5 °C for 6 mo) were analyzed by SPME-GC-MS. Emulsifying properties of egg yolk stored at −5, −15, −20, and −30 °C for 1 mo, respectively, were also evaluated by measurement of emulsion particle diameters in model emulsions prepared with the yolk samples. In addition, structural changes in low-density lipoprotein (LDL) in the egg yolks dependent on storage conditions for 6 mo were evaluated by ³¹P–NMR. Volatile compounds such as hexanal, 2-methylbutanal, and 3-methylbutanal increased in egg yolk during storage at 5 °C; however, volatile compounds hardly increased in any samples stored at −5 °C (super chilling). The mean emulsion particle diameter in super chilled egg yolk was significantly smaller than that in egg yolk stored at the other lower temperatures. In addition, the results of ³¹P–NMR evaluation suggested that prevention of structural changes of LDL resulted in maintenance of emulsifying properties of egg yolk. Thus, these results indicate that super chilling is an effective means of preserving salted egg yolk during long-term storage.     

Dynamic predictive model for the growth of Salmonella spp. in liquid whole egg

Abstract: A dynamic model for the growth of Salmonella spp. in liquid whole egg (LWE) (approximately pH 7.8) under continuously varying temperature was developed. The model was validated using 2 (5 to 15 °C; 600 h and 10 to 40 °C; 52 h) sinusoidal, continuously varying temperature profiles. LWE adjusted to pH 7.8 was inoculated with approximately 2.5–3.0 log CFU/mL of Salmonella spp., and the growth data at several isothermal conditions (5, 7, 10, 15, 20, 25, 30, 35, 37, 39, 41, 43, 45, and 47 °C) was collected. A primary model (Baranyi model) was fitted for each temperature growth data and corresponding maximum growth rates were estimated. Pseudo‐R² values were greater than 0.97 for primary models. Modified Ratkowsky model was used to fit the secondary model. The pseudo‐R² and root mean square error were 0.99 and 0.06 log CFU/mL, respectively, for the secondary model. A dynamic model for the prediction of Salmonella spp. growth under varying temperature conditions was developed using 4th‐order Runge–Kutta method. The developed dynamic model was validated for 2 sinusoidal temperature profiles, 5 to 15 °C (for 600 h) and 10 to 40 °C (for 52 h) with corresponding root mean squared error values of 0.28 and 0.23 log CFU/mL, respectively, between predicted and observed Salmonella spp. populations. The developed dynamic model can be used to predict the growth of Salmonella spp. in LWE under varying temperature conditions. Practical Application: Liquid egg and egg products are widely used in food processing and in restaurant operations. These products can be contaminated with Salmonella spp. during breaking and other unit operations during processing. The raw, liquid egg products are stored under refrigeration prior to pasteurization. However, process deviations can occur such as refrigeration failure, leading to temperature fluctuations above the required temperatures as specified in the critical limits within hazard analysis and critical control point plans for the operations. The processors are required to evaluate the potential growth of Salmonella spp. in such products before the product can be used, or further processed. Dynamic predictive models are excellent tools for regulators as well as the processing plant personnel to evaluate the microbiological safety of the product under such conditions.     

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.     

Effectiveness of Cross-Flow Microfiltration for Removal of Microorganisms Associated with Unpasteurized Liquid Egg White from Process Plant

ABSTRACT:  Thermal preservation is used by the egg industry to ensure the microbiological safety of liquid egg white (LEW); however, it does not eliminate all microorganisms and impairs some of the delicate functional properties of LEW. In this study, a pilot-scale cross-flow microfiltration (MF) process was designed to remove the natural microflora present in commercial LEW, obtained from a local egg-breaking plant, while maintaining the nutritional and functional properties of the LEW. LEW, containing approximately 10^{6 ± 1.7} colony forming units (CFU) per milliliter of total aerobic bacteria, was microfiltered using a ceramic membrane with a nominal pore size of 1.4 μm, at a cross-flow velocity of 6 m/s. To facilitate MF, LEW was screened, homogenized, and then diluted (1 : 2, w/w) with distilled water containing 0.5% sodium chloride. Homogenized LEW was found to have a threefold lower viscosity than unhomogenized LEW. Influence of MF temperature (25 and 40 °C) and pH (6 and 9) on permeate flux, transmission of egg white nutrients across the membrane, and microbial removal efficiency were evaluated. The pH had a significantly greater influence on permeate flux than temperature. Permeate flux increased by almost 148% when pH of LEW was adjusted from pH 9 to pH 6 at 40 °C. Influence of temperature on permeate flux, at a constant pH, however, was found to be inconclusive. Microbial removal efficiency was at least 5 log₁₀ CFU/mL. Total protein and SDS-PAGE analysis indicated that this MF process did not alter the protein composition of the permeate, compared to that of the feed LEW, and that the foaming properties of LEW were retained in the postfiltered samples.     

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.     

Evaluation of Listeria innocua as a suitable indicator for replacing Listeria monocytogenes during ripening of Camembert cheese

The suitability of Listeria innocua for use as an indicator for replacing Listeria monocytogenes during the cheese-making and ripening of Camembert cheese was evaluated. Pasteurized whole milk inoculated with either L. innocua or L. monocytogenes was used to make Camembert cheese, which were ripened in three stages. All cheese was ripened in three stages: room temperature (∼20 °C) and relative humidity of 60% for 36 h; 12 °C and relative humidity of 93% for 2 weeks; and 7 °C and relative humidity of 85% for 3 weeks. Results showed that population values of L. innocua and L. monocytogenes on day 1 were 7.16 and 6.11 log₁₀ CFU/g, respectively, which declined to 6.54 and 5.45 log₁₀ CFU/g, respectively, during subsequent 20 days. Thereafter, L. innocua and L. monocytogenes populations increased to 7.38 and 6.06 log₁₀ CFU/g on day 35 of ripening, respectively. During ripening, surface and interior of cheeses were analysed for populations of L. innocua and L. monocytogenes , respectively. The data were collected on day 1, 5, 10, 15, 20, 25, 30, and 35 of ripening. Generally, the growth of L. innocua and L. monocytogenes is faster in surface than in centre. Top centre, bottom centre and bottom surface locations had similar population values during ripening. There were no significant differences ( P  > 0.05) between batch and section of cheese. The ripening time and locations had significant effect ( P  < 0.05) on the survival and growth of L. innocua and L. monocytogenes . The trends of survival and growth of L. innocua and L. monocytogenes were similar. These results indicated that L. innocua can be considered as an indicator for L. monocytogenes during ripening of Camembert cheese.     

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.     

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.     

EFFECT OF CONCURRENT HIGH HYDROSTATIC PRESSURE, ACIDITY AND HEAT ON THE INJURY AND DESTRUCTION OF LISTERIA MONOCYTOGENES

The effect of concurrent use of high hydrostatic pressure, heat and acidity on Listeria monocytogenes Scott A and CA was investigated. In general, lethality was enhanced when cells were pressurized at higher temperatures or lower pH. Strain CA demonstrated an additional 3-log₁₀ reduction when pressurized at pH 4.0 as compared with pH 6.0 at 353 MPa, 45C for 10 min. Scott A was reduced an additional 1 log₁₀ by increasing the temperature from 25C to 45C with pressurization at 252 MPa, pH 6.0 for 30 min. Exposure to 404 MPa at 45C for 30 min demonstrated complete injury or death of CA cells with an initial concentration of >10⁸ CFU/mL. At least an 8-log₁₀ reduction was observed for both L. monocytogenes strains Scott A and CA when exposed to the combined treatments of 252 MPa, 45C, pH 4.0 for 30 min .     

Effect of temperature on the growth response of Salmonella enteritidis inoculated onto the vitelline membranes of fresh eggs

The growth response of Salmonella Enteritidis (SE) on the vitelline membrane in vitro was studied with the use of a special tube devised specifically for the inoculation of SE onto the vitelline membrane and for the sampling of the yolk near the inoculation site. This latter ability allowed the detection of the movement of SE into the yolk. The growth of SE on the membrane was compared with that of SE inoculated into yolk and albumen in vitro and in ovo in fresh in-shell eggs. The incubation time was 2 days, and the incubation temperatures were 4, 8, 15, 27, and 37 degrees C. Comparison of the results obtained for in vitro growth showed that at 4, 8, and 15 degrees C, SE behaved as if it were in the albumen, with its numbers decreasing over time. At 27 and 37 degrees C, SE grew as if it were in yolk, with a maximum increase of 4.5 log CFU after 2 days at 37 degrees C. In no experiments involving growth on the vitelline membrane did SE appear in the yolk. Comparisons between in vitro and in ovo growth responses of SE in yolk and albumen indicate that SE growth on the membrane parallels that in the in-shell egg.     

Empirical Distribution Models for Escherichia coli 57:H7 in Ground Beef Produced by a Mid-size Commercial Grinder

ABSTRACT: The purpose of this research was to develop empirical models that describe the amount and distribution of ground beef contaminated with Escherichia coli O157:H7 when a contaminated beef trim is introduced into a batch of uncontaminated beef before processing in a mid-size commercial grinder (34 g/s). A beef trim was inoculated with a rifampacin-resistant strain of E. coli O157:H7 and added to a batch of noncontaminated trims at the grinding step. To study the distribution of the E. coli O157:H7^rif in the ground beef, 6 treatments with different inoculum levels (1 to 6 log₁₀ colony-forming units [CFU]) were tested. Removal or pick up of the residual contamination with E. coli O157:H7^rif left in the grinder was evaluated. E. coli O157:H7^rif was detected in 9% to 86% of the total ground beef for the 1 to 6 log₁₀ CFU inoculum levels, respectively. E. coli O157:H7^rif contamination was detected in the collar that fixes the grinder's die and blade to the hub. An exponential algorithm described the relationship between the quantities of ground beef containing E. coli O157:H7^rif and the inoculum level ( R ²= 0.82). Distribution models based on a Chi-squared algorithm were developed for each inoculum level describing the contamination level as a function of the batch fraction processed ( R ²= 0.81 to 0.99). The results of this study corroborate that when beef processors test for pathogenic contamination in a mid-scale grinder, they should test the beef residues in the collar that fixes the grinder's die and blade to the hub.